[v9,3/4] seccomp: add a return code to trap to userspace

Submitted by Tycho Andersen on Dec. 3, 2018, 3:28 a.m.

Details

Message ID 20181203032827.27978-4-tycho@tycho.ws
State New
Series "seccomp trap to userspace"
Headers show

Commit Message

Tycho Andersen Dec. 3, 2018, 3:28 a.m.
This patch introduces a means for syscalls matched in seccomp to notify
some other task that a particular filter has been triggered.

The motivation for this is primarily for use with containers. For example,
if a container does an init_module(), we obviously don't want to load this
untrusted code, which may be compiled for the wrong version of the kernel
anyway. Instead, we could parse the module image, figure out which module
the container is trying to load and load it on the host.

As another example, containers cannot mount() in general since various
filesystems assume a trusted image. However, if an orchestrator knows that
e.g. a particular block device has not been exposed to a container for
writing, it want to allow the container to mount that block device (that
is, handle the mount for it).

This patch adds functionality that is already possible via at least two
other means that I know about, both of which involve ptrace(): first, one
could ptrace attach, and then iterate through syscalls via PTRACE_SYSCALL.
Unfortunately this is slow, so a faster version would be to install a
filter that does SECCOMP_RET_TRACE, which triggers a PTRACE_EVENT_SECCOMP.
Since ptrace allows only one tracer, if the container runtime is that
tracer, users inside the container (or outside) trying to debug it will not
be able to use ptrace, which is annoying. It also means that older
distributions based on Upstart cannot boot inside containers using ptrace,
since upstart itself uses ptrace to monitor services while starting.

The actual implementation of this is fairly small, although getting the
synchronization right was/is slightly complex.

Finally, it's worth noting that the classic seccomp TOCTOU of reading
memory data from the task still applies here, but can be avoided with
careful design of the userspace handler: if the userspace handler reads all
of the task memory that is necessary before applying its security policy,
the tracee's subsequent memory edits will not be read by the tracer.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
CC: Kees Cook <keescook@chromium.org>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Eric W. Biederman <ebiederm@xmission.com>
CC: "Serge E. Hallyn" <serge@hallyn.com>
CC: Christian Brauner <christian@brauner.io>
CC: Tyler Hicks <tyhicks@canonical.com>
CC: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
---
v2: * make id a u64; the idea here being that it will never overflow,
      because 64 is huge (one syscall every nanosecond => wrap every 584
      years) (Andy)
    * prevent nesting of user notifications: if someone is already attached
      the tree in one place, nobody else can attach to the tree (Andy)
    * notify the listener of signals the tracee receives as well (Andy)
    * implement poll
v3: * lockdep fix (Oleg)
    * drop unnecessary WARN()s (Christian)
    * rearrange error returns to be more rpetty (Christian)
    * fix build in !CONFIG_SECCOMP_USER_NOTIFICATION case
v4: * fix implementation of poll to use poll_wait() (Jann)
    * change listener's fd flags to be 0 (Jann)
    * hoist filter initialization out of ifdefs to its own function
      init_user_notification()
    * add some more testing around poll() and closing the listener while a
      syscall is in action
    * s/GET_LISTENER/NEW_LISTENER, since you can't _get_ a listener, but it
      creates a new one (Matthew)
    * correctly handle pid namespaces, add some testcases (Matthew)
    * use EINPROGRESS instead of EINVAL when a notification response is
      written twice (Matthew)
    * fix comment typo from older version (SEND vs READ) (Matthew)
    * whitespace and logic simplification (Tobin)
    * add some Documentation/ bits on userspace trapping
v5: * fix documentation typos (Jann)
    * add signalled field to struct seccomp_notif (Jann)
    * switch to using ioctls instead of read()/write() for struct passing
      (Jann)
    * add an ioctl to ensure an id is still valid
v6: * docs typo fixes, update docs for ioctl() change (Christian)
v7: * switch struct seccomp_knotif's id member to a u64 (derp :)
    * use notify_lock in IS_ID_VALID query to avoid racing
    * s/signalled/signaled (Tyler)
    * fix docs to reflect that ids are not globally unique (Tyler)
    * add a test to check -ERESTARTSYS behavior (Tyler)
    * drop CONFIG_SECCOMP_USER_NOTIFICATION (Tyler)
    * reorder USER_NOTIF in seccomp return codes list (Tyler)
    * return size instead of sizeof(struct user_notif) (Tyler)
    * ENOENT instead of EINVAL when invalid id is passed (Tyler)
    * drop CONFIG_SECCOMP_USER_NOTIFICATION guards (Tyler)
    * s/IS_ID_VALID/ID_VALID and switch ioctl to be "well behaved" (Tyler)
    * add a new struct notification to minimize the additions to
      struct seccomp_filter, also pack the necessary additions a bit more
      cleverly (Tyler)
    * switch to keeping track of the task itself instead of the pid (we'll
      use this for implementing PUT_FD)
v8: * in recv, don't copy_to_user() while holding notify lock, in case
      userfaultfd blocks and causes all syscalls to block (Kees)
    * switch ioctl character to something more fun ! (Kees)
    * switch ioctl defines to use their own SECCOMP_IO* macros (Kees)
    * rename seccomp ioctls to be SECCOMP_IOCTL_* (Kees)
    * move comment of notify_lock to the right place (Jann)
    * drop comment abount reference count bounding in __get_seccomp_filter (Jann)
    * add lockdep_assert_held() in seccomp_next_notify_id() (Kees)
    * in seccomp_do_user_notification(), always increment semaphore before
      releasing lock, to prevent use after free of ->notif (Kees)
    * add another wake_up_poll() when a signal is received (Jann)
    * make all listener fds O_CLOEXEC (Jann/Kees)
    * use memset() instead of = {} initialization for structures (Kees)
    * move casting of buf pointer to ioctl, instead of in handler functions (Kees)
    * fix ENOENT testing in seccomp_notify_send() (Jann)
    * use ENOENT instead of -1 (EPERM) for ID_VALID ioctl (Jann)
    * use ()s around "nested" bit operations (Kees)
    * init struct notification members in the order they're declared (Jann)
    * rearrange things so no forward declaration of init_listener() is
      required (Kees)
    * switch to a flags based future-proofing mechanism for struct
      seccomp_notif and seccomp_notif_resp, thus avoiding version issues
      with structure length (Kees)
    * fix a memory leak in init_listener() in a failure case
    * fix a use-after-free of filter->notif in do_user_notification() when
      the listener fd is closed after a signal is sent
    * add a comment about semaphore state in the interrupt case in
      do_user_notification() + seccomp_notify_recv()
v9: * add SECCOMP_GET_NOTIF_SIZES to handle when struct seccomp_data
      changes in size
    * don't do locking all the way up the seccomp tree (Oleg)
    * rearrange the tests so that one test tests one thing
    * avoid an unkillable sleep by dropping the signaled flag (Oleg)
---
 Documentation/ioctl/ioctl-number.txt          |   1 +
 .../userspace-api/seccomp_filter.rst          |  84 ++++
 include/linux/seccomp.h                       |   7 +-
 include/uapi/linux/seccomp.h                  |  40 +-
 kernel/seccomp.c                              | 448 +++++++++++++++++-
 tools/testing/selftests/seccomp/seccomp_bpf.c | 447 ++++++++++++++++-
 6 files changed, 1017 insertions(+), 10 deletions(-)

Patch hide | download patch | download mbox

diff --git a/Documentation/ioctl/ioctl-number.txt b/Documentation/ioctl/ioctl-number.txt
index af6f6ba1fe80..c9558146ac58 100644
--- a/Documentation/ioctl/ioctl-number.txt
+++ b/Documentation/ioctl/ioctl-number.txt
@@ -79,6 +79,7 @@  Code  Seq#(hex)	Include File		Comments
 0x1b	all	InfiniBand Subsystem	<http://infiniband.sourceforge.net/>
 0x20	all	drivers/cdrom/cm206.h
 0x22	all	scsi/sg.h
+'!'	00-1F	uapi/linux/seccomp.h
 '#'	00-3F	IEEE 1394 Subsystem	Block for the entire subsystem
 '$'	00-0F	linux/perf_counter.h, linux/perf_event.h
 '%'	00-0F	include/uapi/linux/stm.h
diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst
index 82a468bc7560..b1b846d8a094 100644
--- a/Documentation/userspace-api/seccomp_filter.rst
+++ b/Documentation/userspace-api/seccomp_filter.rst
@@ -122,6 +122,11 @@  In precedence order, they are:
 	Results in the lower 16-bits of the return value being passed
 	to userland as the errno without executing the system call.
 
+``SECCOMP_RET_USER_NOTIF``:
+    Results in a ``struct seccomp_notif`` message sent on the userspace
+    notification fd, if it is attached, or ``-ENOSYS`` if it is not. See below
+    on discussion of how to handle user notifications.
+
 ``SECCOMP_RET_TRACE``:
 	When returned, this value will cause the kernel to attempt to
 	notify a ``ptrace()``-based tracer prior to executing the system
@@ -183,6 +188,85 @@  The ``samples/seccomp/`` directory contains both an x86-specific example
 and a more generic example of a higher level macro interface for BPF
 program generation.
 
+Userspace Notification
+======================
+
+The ``SECCOMP_RET_USER_NOTIF`` return code lets seccomp filters pass a
+particular syscall to userspace to be handled. This may be useful for
+applications like container managers, which wish to intercept particular
+syscalls (``mount()``, ``finit_module()``, etc.) and change their behavior.
+
+To acquire a notification FD, use the ``SECCOMP_FILTER_FLAG_NEW_LISTENER``
+argument to the ``seccomp()`` syscall:
+
+.. code-block:: c
+
+    fd = seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_NEW_LISTENER, &prog);
+
+which (on success) will return a listener fd for the filter, which can then be
+passed around via ``SCM_RIGHTS`` or similar. Note that filter fds correspond to
+a particular filter, and not a particular task. So if this task then forks,
+notifications from both tasks will appear on the same filter fd. Reads and
+writes to/from a filter fd are also synchronized, so a filter fd can safely
+have many readers.
+
+The interface for a seccomp notification fd consists of two structures:
+
+.. code-block:: c
+
+    struct seccomp_notif_sizes {
+        __u16 seccomp_notif;
+        __u16 seccomp_notif_resp;
+        __u16 seccomp_data;
+    };
+
+    struct seccomp_notif {
+        __u64 id;
+        __u32 pid;
+        __u32 flags;
+        struct seccomp_data data;
+    };
+
+    struct seccomp_notif_resp {
+        __u64 id;
+        __s64 val;
+        __s32 error;
+        __u32 flags;
+    };
+
+The ``struct seccomp_notif_sizes`` structure can be used to determine the size
+of the various structures used in seccomp notifications. The size of ``struct
+seccomp_data`` may change in the future, so code should use:
+
+.. code-block:: c
+
+    struct seccomp_notif_sizes sizes;
+    seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes);
+
+to determine the size of the various structures to allocate. See
+samples/seccomp/user-trap.c for an example.
+
+Users can read via ``ioctl(SECCOMP_IOCTL_NOTIF_RECV)``  (or ``poll()``) on a
+seccomp notification fd to receive a ``struct seccomp_notif``, which contains
+five members: the input length of the structure, a unique-per-filter ``id``,
+the ``pid`` of the task which triggered this request (which may be 0 if the
+task is in a pid ns not visible from the listener's pid namespace), a ``flags``
+member which for now only has ``SECCOMP_NOTIF_FLAG_SIGNALED``, representing
+whether or not the notification is a result of a non-fatal signal, and the
+``data`` passed to seccomp. Userspace can then make a decision based on this
+information about what to do, and ``ioctl(SECCOMP_IOCTL_NOTIF_SEND)`` a
+response, indicating what should be returned to userspace. The ``id`` member of
+``struct seccomp_notif_resp`` should be the same ``id`` as in ``struct
+seccomp_notif``.
+
+It is worth noting that ``struct seccomp_data`` contains the values of register
+arguments to the syscall, but does not contain pointers to memory. The task's
+memory is accessible to suitably privileged traces via ``ptrace()`` or
+``/proc/pid/mem``. However, care should be taken to avoid the TOCTOU mentioned
+above in this document: all arguments being read from the tracee's memory
+should be read into the tracer's memory before any policy decisions are made.
+This allows for an atomic decision on syscall arguments.
+
 Sysctls
 =======
 
diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index b5103c019cf4..84868d37b35d 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -4,9 +4,10 @@ 
 
 #include <uapi/linux/seccomp.h>
 
-#define SECCOMP_FILTER_FLAG_MASK	(SECCOMP_FILTER_FLAG_TSYNC	| \
-					 SECCOMP_FILTER_FLAG_LOG	| \
-					 SECCOMP_FILTER_FLAG_SPEC_ALLOW)
+#define SECCOMP_FILTER_FLAG_MASK	(SECCOMP_FILTER_FLAG_TSYNC | \
+					 SECCOMP_FILTER_FLAG_LOG | \
+					 SECCOMP_FILTER_FLAG_SPEC_ALLOW | \
+					 SECCOMP_FILTER_FLAG_NEW_LISTENER)
 
 #ifdef CONFIG_SECCOMP
 
diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
index 9efc0e73d50b..90734aa5aa36 100644
--- a/include/uapi/linux/seccomp.h
+++ b/include/uapi/linux/seccomp.h
@@ -15,11 +15,13 @@ 
 #define SECCOMP_SET_MODE_STRICT		0
 #define SECCOMP_SET_MODE_FILTER		1
 #define SECCOMP_GET_ACTION_AVAIL	2
+#define SECCOMP_GET_NOTIF_SIZES		3
 
 /* Valid flags for SECCOMP_SET_MODE_FILTER */
-#define SECCOMP_FILTER_FLAG_TSYNC	(1UL << 0)
-#define SECCOMP_FILTER_FLAG_LOG		(1UL << 1)
-#define SECCOMP_FILTER_FLAG_SPEC_ALLOW	(1UL << 2)
+#define SECCOMP_FILTER_FLAG_TSYNC		(1UL << 0)
+#define SECCOMP_FILTER_FLAG_LOG			(1UL << 1)
+#define SECCOMP_FILTER_FLAG_SPEC_ALLOW		(1UL << 2)
+#define SECCOMP_FILTER_FLAG_NEW_LISTENER	(1UL << 3)
 
 /*
  * All BPF programs must return a 32-bit value.
@@ -35,6 +37,7 @@ 
 #define SECCOMP_RET_KILL	 SECCOMP_RET_KILL_THREAD
 #define SECCOMP_RET_TRAP	 0x00030000U /* disallow and force a SIGSYS */
 #define SECCOMP_RET_ERRNO	 0x00050000U /* returns an errno */
+#define SECCOMP_RET_USER_NOTIF	 0x7fc00000U /* notifies userspace */
 #define SECCOMP_RET_TRACE	 0x7ff00000U /* pass to a tracer or disallow */
 #define SECCOMP_RET_LOG		 0x7ffc0000U /* allow after logging */
 #define SECCOMP_RET_ALLOW	 0x7fff0000U /* allow */
@@ -60,4 +63,35 @@  struct seccomp_data {
 	__u64 args[6];
 };
 
+struct seccomp_notif_sizes {
+	__u16 seccomp_notif;
+	__u16 seccomp_notif_resp;
+	__u16 seccomp_data;
+};
+
+struct seccomp_notif {
+	__u64 id;
+	__u32 pid;
+	__u32 flags;
+	struct seccomp_data data;
+};
+
+struct seccomp_notif_resp {
+	__u64 id;
+	__s64 val;
+	__s32 error;
+	__u32 flags;
+};
+
+#define SECCOMP_IOC_MAGIC		'!'
+#define SECCOMP_IO(nr)			_IO(SECCOMP_IOC_MAGIC, nr)
+#define SECCOMP_IOR(nr, type)		_IOR(SECCOMP_IOC_MAGIC, nr, type)
+#define SECCOMP_IOW(nr, type)		_IOW(SECCOMP_IOC_MAGIC, nr, type)
+#define SECCOMP_IOWR(nr, type)		_IOWR(SECCOMP_IOC_MAGIC, nr, type)
+
+/* Flags for seccomp notification fd ioctl. */
+#define SECCOMP_IOCTL_NOTIF_RECV	SECCOMP_IOWR(0, struct seccomp_notif)
+#define SECCOMP_IOCTL_NOTIF_SEND	SECCOMP_IOWR(1,	\
+						struct seccomp_notif_resp)
+#define SECCOMP_IOCTL_NOTIF_ID_VALID	SECCOMP_IOR(2, __u64)
 #endif /* _UAPI_LINUX_SECCOMP_H */
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 393e029f778a..15b6be97fc09 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -33,12 +33,74 @@ 
 #endif
 
 #ifdef CONFIG_SECCOMP_FILTER
+#include <linux/file.h>
 #include <linux/filter.h>
 #include <linux/pid.h>
 #include <linux/ptrace.h>
 #include <linux/security.h>
 #include <linux/tracehook.h>
 #include <linux/uaccess.h>
+#include <linux/anon_inodes.h>
+
+enum notify_state {
+	SECCOMP_NOTIFY_INIT,
+	SECCOMP_NOTIFY_SENT,
+	SECCOMP_NOTIFY_REPLIED,
+};
+
+struct seccomp_knotif {
+	/* The struct pid of the task whose filter triggered the notification */
+	struct task_struct *task;
+
+	/* The "cookie" for this request; this is unique for this filter. */
+	u64 id;
+
+	/*
+	 * The seccomp data. This pointer is valid the entire time this
+	 * notification is active, since it comes from __seccomp_filter which
+	 * eclipses the entire lifecycle here.
+	 */
+	const struct seccomp_data *data;
+
+	/*
+	 * Notification states. When SECCOMP_RET_USER_NOTIF is returned, a
+	 * struct seccomp_knotif is created and starts out in INIT. Once the
+	 * handler reads the notification off of an FD, it transitions to SENT.
+	 * If a signal is received the state transitions back to INIT and
+	 * another message is sent. When the userspace handler replies, state
+	 * transitions to REPLIED.
+	 */
+	enum notify_state state;
+
+	/* The return values, only valid when in SECCOMP_NOTIFY_REPLIED */
+	int error;
+	long val;
+
+	/* Signals when this has entered SECCOMP_NOTIFY_REPLIED */
+	struct completion ready;
+
+	struct list_head list;
+};
+
+/**
+ * struct notification - container for seccomp userspace notifications. Since
+ * most seccomp filters will not have notification listeners attached and this
+ * structure is fairly large, we store the notification-specific stuff in a
+ * separate structure.
+ *
+ * @request: A semaphore that users of this notification can wait on for
+ *           changes. Actual reads and writes are still controlled with
+ *           filter->notify_lock.
+ * @next_id: The id of the next request.
+ * @notifications: A list of struct seccomp_knotif elements.
+ * @wqh: A wait queue for poll.
+ */
+struct notification {
+	struct semaphore request;
+	u64 next_id;
+	struct list_head notifications;
+	wait_queue_head_t wqh;
+};
 
 /**
  * struct seccomp_filter - container for seccomp BPF programs
@@ -50,6 +112,8 @@ 
  * @log: true if all actions except for SECCOMP_RET_ALLOW should be logged
  * @prev: points to a previously installed, or inherited, filter
  * @prog: the BPF program to evaluate
+ * @notif: the struct that holds all notification related information
+ * @notify_lock: A lock for all notification-related accesses.
  *
  * seccomp_filter objects are organized in a tree linked via the @prev
  * pointer.  For any task, it appears to be a singly-linked list starting
@@ -66,6 +130,8 @@  struct seccomp_filter {
 	bool log;
 	struct seccomp_filter *prev;
 	struct bpf_prog *prog;
+	struct notification *notif;
+	struct mutex notify_lock;
 };
 
 /* Limit any path through the tree to 256KB worth of instructions. */
@@ -386,6 +452,7 @@  static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
 	if (!sfilter)
 		return ERR_PTR(-ENOMEM);
 
+	mutex_init(&sfilter->notify_lock);
 	ret = bpf_prog_create_from_user(&sfilter->prog, fprog,
 					seccomp_check_filter, save_orig);
 	if (ret < 0) {
@@ -479,7 +546,6 @@  static long seccomp_attach_filter(unsigned int flags,
 
 static void __get_seccomp_filter(struct seccomp_filter *filter)
 {
-	/* Reference count is bounded by the number of total processes. */
 	refcount_inc(&filter->usage);
 }
 
@@ -550,11 +616,13 @@  static void seccomp_send_sigsys(int syscall, int reason)
 #define SECCOMP_LOG_TRACE		(1 << 4)
 #define SECCOMP_LOG_LOG			(1 << 5)
 #define SECCOMP_LOG_ALLOW		(1 << 6)
+#define SECCOMP_LOG_USER_NOTIF		(1 << 7)
 
 static u32 seccomp_actions_logged = SECCOMP_LOG_KILL_PROCESS |
 				    SECCOMP_LOG_KILL_THREAD  |
 				    SECCOMP_LOG_TRAP  |
 				    SECCOMP_LOG_ERRNO |
+				    SECCOMP_LOG_USER_NOTIF |
 				    SECCOMP_LOG_TRACE |
 				    SECCOMP_LOG_LOG;
 
@@ -575,6 +643,9 @@  static inline void seccomp_log(unsigned long syscall, long signr, u32 action,
 	case SECCOMP_RET_TRACE:
 		log = requested && seccomp_actions_logged & SECCOMP_LOG_TRACE;
 		break;
+	case SECCOMP_RET_USER_NOTIF:
+		log = requested && seccomp_actions_logged & SECCOMP_LOG_USER_NOTIF;
+		break;
 	case SECCOMP_RET_LOG:
 		log = seccomp_actions_logged & SECCOMP_LOG_LOG;
 		break;
@@ -646,6 +717,68 @@  void secure_computing_strict(int this_syscall)
 #else
 
 #ifdef CONFIG_SECCOMP_FILTER
+static u64 seccomp_next_notify_id(struct seccomp_filter *filter)
+{
+	/*
+	 * Note: overflow is ok here, the id just needs to be unique per
+	 * filter.
+	 */
+	lockdep_assert_held(&filter->notify_lock);
+	return filter->notif->next_id++;
+}
+
+static void seccomp_do_user_notification(int this_syscall,
+					 struct seccomp_filter *match,
+					 const struct seccomp_data *sd)
+{
+	int err;
+	long ret = 0;
+	struct seccomp_knotif n = {};
+
+	mutex_lock(&match->notify_lock);
+	err = -ENOSYS;
+	if (!match->notif)
+		goto out;
+
+	n.task = current;
+	n.state = SECCOMP_NOTIFY_INIT;
+	n.data = sd;
+	n.id = seccomp_next_notify_id(match);
+	init_completion(&n.ready);
+	list_add(&n.list, &match->notif->notifications);
+
+	up(&match->notif->request);
+	wake_up_poll(&match->notif->wqh, EPOLLIN | EPOLLRDNORM);
+	mutex_unlock(&match->notify_lock);
+
+	/*
+	 * This is where we wait for a reply from userspace.
+	 */
+	err = wait_for_completion_interruptible(&n.ready);
+	mutex_lock(&match->notify_lock);
+	if (err == 0) {
+		ret = n.val;
+		err = n.error;
+	}
+
+	/*
+	 * Note that it's possible the listener died in between the time when
+	 * we were notified of a respons (or a signal) and when we were able to
+	 * re-acquire the lock, so only delete from the list if the
+	 * notification actually exists.
+	 *
+	 * Also note that this test is only valid because there's no way to
+	 * *reattach* to a notifier right now. If one is added, we'll need to
+	 * keep track of the notif itself and make sure they match here.
+	 */
+	if (match->notif)
+		list_del(&n.list);
+out:
+	mutex_unlock(&match->notify_lock);
+	syscall_set_return_value(current, task_pt_regs(current),
+				 err, ret);
+}
+
 static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 			    const bool recheck_after_trace)
 {
@@ -728,6 +861,10 @@  static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 
 		return 0;
 
+	case SECCOMP_RET_USER_NOTIF:
+		seccomp_do_user_notification(this_syscall, match, sd);
+		goto skip;
+
 	case SECCOMP_RET_LOG:
 		seccomp_log(this_syscall, 0, action, true);
 		return 0;
@@ -834,6 +971,263 @@  static long seccomp_set_mode_strict(void)
 }
 
 #ifdef CONFIG_SECCOMP_FILTER
+static int seccomp_notify_release(struct inode *inode, struct file *file)
+{
+	struct seccomp_filter *filter = file->private_data;
+	struct seccomp_knotif *knotif;
+
+	mutex_lock(&filter->notify_lock);
+
+	/*
+	 * If this file is being closed because e.g. the task who owned it
+	 * died, let's wake everyone up who was waiting on us.
+	 */
+	list_for_each_entry(knotif, &filter->notif->notifications, list) {
+		if (knotif->state == SECCOMP_NOTIFY_REPLIED)
+			continue;
+
+		knotif->state = SECCOMP_NOTIFY_REPLIED;
+		knotif->error = -ENOSYS;
+		knotif->val = 0;
+
+		complete(&knotif->ready);
+	}
+
+	kfree(filter->notif);
+	filter->notif = NULL;
+	mutex_unlock(&filter->notify_lock);
+	__put_seccomp_filter(filter);
+	return 0;
+}
+
+static long seccomp_notify_recv(struct seccomp_filter *filter,
+				void __user *buf)
+{
+	struct seccomp_knotif *knotif = NULL, *cur;
+	struct seccomp_notif unotif;
+	ssize_t ret;
+
+	memset(&unotif, 0, sizeof(unotif));
+
+	ret = down_interruptible(&filter->notif->request);
+	if (ret < 0)
+		return ret;
+
+	mutex_lock(&filter->notify_lock);
+	list_for_each_entry(cur, &filter->notif->notifications, list) {
+		if (cur->state == SECCOMP_NOTIFY_INIT) {
+			knotif = cur;
+			break;
+		}
+	}
+
+	/*
+	 * If we didn't find a notification, it could be that the task was
+	 * interrupted by a fatal signal between the time we were woken and
+	 * when we were able to acquire the rw lock.
+	 */
+	if (!knotif) {
+		ret = -ENOENT;
+		goto out;
+	}
+
+	unotif.id = knotif->id;
+	unotif.pid = task_pid_vnr(knotif->task);
+	unotif.data = *(knotif->data);
+
+	knotif->state = SECCOMP_NOTIFY_SENT;
+	wake_up_poll(&filter->notif->wqh, EPOLLOUT | EPOLLWRNORM);
+	ret = 0;
+out:
+	mutex_unlock(&filter->notify_lock);
+
+	if (ret == 0 && copy_to_user(buf, &unotif, sizeof(unotif))) {
+		ret = -EFAULT;
+
+		/*
+		 * Userspace screwed up. To make sure that we keep this
+		 * notification alive, let's reset it back to INIT. It
+		 * may have died when we released the lock, so we need to make
+		 * sure it's still around.
+		 */
+		knotif = NULL;
+		mutex_lock(&filter->notify_lock);
+		list_for_each_entry(cur, &filter->notif->notifications, list) {
+			if (cur->id == unotif.id) {
+				knotif = cur;
+				break;
+			}
+		}
+
+		if (knotif) {
+			knotif->state = SECCOMP_NOTIFY_INIT;
+			up(&filter->notif->request);
+		}
+		mutex_unlock(&filter->notify_lock);
+	}
+
+	return ret;
+}
+
+static long seccomp_notify_send(struct seccomp_filter *filter,
+				void __user *buf)
+{
+	struct seccomp_notif_resp resp = {};
+	struct seccomp_knotif *knotif = NULL, *cur;
+	long ret;
+
+	if (copy_from_user(&resp, buf, sizeof(resp)))
+		return -EFAULT;
+
+	if (resp.flags)
+		return -EINVAL;
+
+	ret = mutex_lock_interruptible(&filter->notify_lock);
+	if (ret < 0)
+		return ret;
+
+	list_for_each_entry(cur, &filter->notif->notifications, list) {
+		if (cur->id == resp.id) {
+			knotif = cur;
+			break;
+		}
+	}
+
+	if (!knotif) {
+		ret = -ENOENT;
+		goto out;
+	}
+
+	/* Allow exactly one reply. */
+	if (knotif->state != SECCOMP_NOTIFY_SENT) {
+		ret = -EINPROGRESS;
+		goto out;
+	}
+
+	ret = 0;
+	knotif->state = SECCOMP_NOTIFY_REPLIED;
+	knotif->error = resp.error;
+	knotif->val = resp.val;
+	complete(&knotif->ready);
+out:
+	mutex_unlock(&filter->notify_lock);
+	return ret;
+}
+
+static long seccomp_notify_id_valid(struct seccomp_filter *filter,
+				    void __user *buf)
+{
+	struct seccomp_knotif *knotif = NULL;
+	u64 id;
+	long ret;
+
+	if (copy_from_user(&id, buf, sizeof(id)))
+		return -EFAULT;
+
+	ret = mutex_lock_interruptible(&filter->notify_lock);
+	if (ret < 0)
+		return ret;
+
+	ret = -ENOENT;
+	list_for_each_entry(knotif, &filter->notif->notifications, list) {
+		if (knotif->id == id) {
+			if (knotif->state == SECCOMP_NOTIFY_SENT)
+				ret = 0;
+			goto out;
+		}
+	}
+
+out:
+	mutex_unlock(&filter->notify_lock);
+	return ret;
+}
+
+static long seccomp_notify_ioctl(struct file *file, unsigned int cmd,
+				 unsigned long arg)
+{
+	struct seccomp_filter *filter = file->private_data;
+	void __user *buf = (void __user *)arg;
+
+	switch (cmd) {
+	case SECCOMP_IOCTL_NOTIF_RECV:
+		return seccomp_notify_recv(filter, buf);
+	case SECCOMP_IOCTL_NOTIF_SEND:
+		return seccomp_notify_send(filter, buf);
+	case SECCOMP_IOCTL_NOTIF_ID_VALID:
+		return seccomp_notify_id_valid(filter, buf);
+	default:
+		return -EINVAL;
+	}
+}
+
+static __poll_t seccomp_notify_poll(struct file *file,
+				    struct poll_table_struct *poll_tab)
+{
+	struct seccomp_filter *filter = file->private_data;
+	__poll_t ret = 0;
+	struct seccomp_knotif *cur;
+
+	poll_wait(file, &filter->notif->wqh, poll_tab);
+
+	ret = mutex_lock_interruptible(&filter->notify_lock);
+	if (ret < 0)
+		return EPOLLERR;
+
+	list_for_each_entry(cur, &filter->notif->notifications, list) {
+		if (cur->state == SECCOMP_NOTIFY_INIT)
+			ret |= EPOLLIN | EPOLLRDNORM;
+		if (cur->state == SECCOMP_NOTIFY_SENT)
+			ret |= EPOLLOUT | EPOLLWRNORM;
+		if ((ret & EPOLLIN) && (ret & EPOLLOUT))
+			break;
+	}
+
+	mutex_unlock(&filter->notify_lock);
+
+	return ret;
+}
+
+static const struct file_operations seccomp_notify_ops = {
+	.poll = seccomp_notify_poll,
+	.release = seccomp_notify_release,
+	.unlocked_ioctl = seccomp_notify_ioctl,
+};
+
+static struct file *init_listener(struct seccomp_filter *filter)
+{
+	struct file *ret = ERR_PTR(-EBUSY);
+	struct seccomp_filter *cur;
+
+	for (cur = current->seccomp.filter; cur; cur = cur->prev) {
+		if (cur->notif)
+			goto out;
+	}
+
+	ret = ERR_PTR(-ENOMEM);
+	filter->notif = kzalloc(sizeof(*(filter->notif)), GFP_KERNEL);
+	if (!filter->notif)
+		goto out;
+
+	sema_init(&filter->notif->request, 0);
+	filter->notif->next_id = get_random_u64();
+	INIT_LIST_HEAD(&filter->notif->notifications);
+	init_waitqueue_head(&filter->notif->wqh);
+
+	ret = anon_inode_getfile("seccomp notify", &seccomp_notify_ops,
+				 filter, O_RDWR);
+	if (IS_ERR(ret))
+		goto out_notif;
+
+	/* The file has a reference to it now */
+	__get_seccomp_filter(filter);
+
+out_notif:
+	if (IS_ERR(ret))
+		kfree(filter->notif);
+out:
+	return ret;
+}
+
 /**
  * seccomp_set_mode_filter: internal function for setting seccomp filter
  * @flags:  flags to change filter behavior
@@ -853,6 +1247,8 @@  static long seccomp_set_mode_filter(unsigned int flags,
 	const unsigned long seccomp_mode = SECCOMP_MODE_FILTER;
 	struct seccomp_filter *prepared = NULL;
 	long ret = -EINVAL;
+	int listener = -1;
+	struct file *listener_f = NULL;
 
 	/* Validate flags. */
 	if (flags & ~SECCOMP_FILTER_FLAG_MASK)
@@ -863,13 +1259,28 @@  static long seccomp_set_mode_filter(unsigned int flags,
 	if (IS_ERR(prepared))
 		return PTR_ERR(prepared);
 
+	if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
+		listener = get_unused_fd_flags(O_CLOEXEC);
+		if (listener < 0) {
+			ret = listener;
+			goto out_free;
+		}
+
+		listener_f = init_listener(prepared);
+		if (IS_ERR(listener_f)) {
+			put_unused_fd(listener);
+			ret = PTR_ERR(listener_f);
+			goto out_free;
+		}
+	}
+
 	/*
 	 * Make sure we cannot change seccomp or nnp state via TSYNC
 	 * while another thread is in the middle of calling exec.
 	 */
 	if (flags & SECCOMP_FILTER_FLAG_TSYNC &&
 	    mutex_lock_killable(&current->signal->cred_guard_mutex))
-		goto out_free;
+		goto out_put_fd;
 
 	spin_lock_irq(&current->sighand->siglock);
 
@@ -887,6 +1298,16 @@  static long seccomp_set_mode_filter(unsigned int flags,
 	spin_unlock_irq(&current->sighand->siglock);
 	if (flags & SECCOMP_FILTER_FLAG_TSYNC)
 		mutex_unlock(&current->signal->cred_guard_mutex);
+out_put_fd:
+	if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
+		if (ret < 0) {
+			fput(listener_f);
+			put_unused_fd(listener);
+		} else {
+			fd_install(listener, listener_f);
+			ret = listener;
+		}
+	}
 out_free:
 	seccomp_filter_free(prepared);
 	return ret;
@@ -911,6 +1332,7 @@  static long seccomp_get_action_avail(const char __user *uaction)
 	case SECCOMP_RET_KILL_THREAD:
 	case SECCOMP_RET_TRAP:
 	case SECCOMP_RET_ERRNO:
+	case SECCOMP_RET_USER_NOTIF:
 	case SECCOMP_RET_TRACE:
 	case SECCOMP_RET_LOG:
 	case SECCOMP_RET_ALLOW:
@@ -922,6 +1344,20 @@  static long seccomp_get_action_avail(const char __user *uaction)
 	return 0;
 }
 
+static long seccomp_get_notif_sizes(void __user *usizes)
+{
+	struct seccomp_notif_sizes sizes = {
+		.seccomp_notif = sizeof(struct seccomp_notif),
+		.seccomp_notif_resp = sizeof(struct seccomp_notif_resp),
+		.seccomp_data = sizeof(struct seccomp_data),
+	};
+
+	if (copy_to_user(usizes, &sizes, sizeof(sizes)))
+		return -EFAULT;
+
+	return 0;
+}
+
 /* Common entry point for both prctl and syscall. */
 static long do_seccomp(unsigned int op, unsigned int flags,
 		       void __user *uargs)
@@ -938,6 +1374,11 @@  static long do_seccomp(unsigned int op, unsigned int flags,
 			return -EINVAL;
 
 		return seccomp_get_action_avail(uargs);
+	case SECCOMP_GET_NOTIF_SIZES:
+		if (flags != 0)
+			return -EINVAL;
+
+		return seccomp_get_notif_sizes(uargs);
 	default:
 		return -EINVAL;
 	}
@@ -1111,6 +1552,7 @@  long seccomp_get_metadata(struct task_struct *task,
 #define SECCOMP_RET_KILL_THREAD_NAME	"kill_thread"
 #define SECCOMP_RET_TRAP_NAME		"trap"
 #define SECCOMP_RET_ERRNO_NAME		"errno"
+#define SECCOMP_RET_USER_NOTIF_NAME	"user_notif"
 #define SECCOMP_RET_TRACE_NAME		"trace"
 #define SECCOMP_RET_LOG_NAME		"log"
 #define SECCOMP_RET_ALLOW_NAME		"allow"
@@ -1120,6 +1562,7 @@  static const char seccomp_actions_avail[] =
 				SECCOMP_RET_KILL_THREAD_NAME	" "
 				SECCOMP_RET_TRAP_NAME		" "
 				SECCOMP_RET_ERRNO_NAME		" "
+				SECCOMP_RET_USER_NOTIF_NAME     " "
 				SECCOMP_RET_TRACE_NAME		" "
 				SECCOMP_RET_LOG_NAME		" "
 				SECCOMP_RET_ALLOW_NAME;
@@ -1134,6 +1577,7 @@  static const struct seccomp_log_name seccomp_log_names[] = {
 	{ SECCOMP_LOG_KILL_THREAD, SECCOMP_RET_KILL_THREAD_NAME },
 	{ SECCOMP_LOG_TRAP, SECCOMP_RET_TRAP_NAME },
 	{ SECCOMP_LOG_ERRNO, SECCOMP_RET_ERRNO_NAME },
+	{ SECCOMP_LOG_USER_NOTIF, SECCOMP_RET_USER_NOTIF_NAME },
 	{ SECCOMP_LOG_TRACE, SECCOMP_RET_TRACE_NAME },
 	{ SECCOMP_LOG_LOG, SECCOMP_RET_LOG_NAME },
 	{ SECCOMP_LOG_ALLOW, SECCOMP_RET_ALLOW_NAME },
diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index e1473234968d..5c9768a1b8cd 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -5,6 +5,7 @@ 
  * Test code for seccomp bpf.
  */
 
+#define _GNU_SOURCE
 #include <sys/types.h>
 
 /*
@@ -40,10 +41,12 @@ 
 #include <sys/fcntl.h>
 #include <sys/mman.h>
 #include <sys/times.h>
+#include <sys/socket.h>
+#include <sys/ioctl.h>
 
-#define _GNU_SOURCE
 #include <unistd.h>
 #include <sys/syscall.h>
+#include <poll.h>
 
 #include "../kselftest_harness.h"
 
@@ -133,6 +136,10 @@  struct seccomp_data {
 #define SECCOMP_GET_ACTION_AVAIL 2
 #endif
 
+#ifndef SECCOMP_GET_NOTIF_SIZES
+#define SECCOMP_GET_NOTIF_SIZES 3
+#endif
+
 #ifndef SECCOMP_FILTER_FLAG_TSYNC
 #define SECCOMP_FILTER_FLAG_TSYNC (1UL << 0)
 #endif
@@ -154,6 +161,44 @@  struct seccomp_metadata {
 };
 #endif
 
+#ifndef SECCOMP_FILTER_FLAG_NEW_LISTENER
+#define SECCOMP_FILTER_FLAG_NEW_LISTENER	(1UL << 3)
+
+#define SECCOMP_RET_USER_NOTIF 0x7fc00000U
+
+#define SECCOMP_IOC_MAGIC		'!'
+#define SECCOMP_IO(nr)			_IO(SECCOMP_IOC_MAGIC, nr)
+#define SECCOMP_IOR(nr, type)		_IOR(SECCOMP_IOC_MAGIC, nr, type)
+#define SECCOMP_IOW(nr, type)		_IOW(SECCOMP_IOC_MAGIC, nr, type)
+#define SECCOMP_IOWR(nr, type)		_IOWR(SECCOMP_IOC_MAGIC, nr, type)
+
+/* Flags for seccomp notification fd ioctl. */
+#define SECCOMP_IOCTL_NOTIF_RECV	SECCOMP_IOWR(0, struct seccomp_notif)
+#define SECCOMP_IOCTL_NOTIF_SEND	SECCOMP_IOWR(1,	\
+						struct seccomp_notif_resp)
+#define SECCOMP_IOCTL_NOTIF_ID_VALID	SECCOMP_IOR(2, __u64)
+
+struct seccomp_notif {
+	__u64 id;
+	__u32 pid;
+	__u32 flags;
+	struct seccomp_data data;
+};
+
+struct seccomp_notif_resp {
+	__u64 id;
+	__s64 val;
+	__s32 error;
+	__u32 flags;
+};
+
+struct seccomp_notif_sizes {
+	__u16 seccomp_notif;
+	__u16 seccomp_notif_resp;
+	__u16 seccomp_data;
+};
+#endif
+
 #ifndef seccomp
 int seccomp(unsigned int op, unsigned int flags, void *args)
 {
@@ -2077,7 +2122,8 @@  TEST(detect_seccomp_filter_flags)
 {
 	unsigned int flags[] = { SECCOMP_FILTER_FLAG_TSYNC,
 				 SECCOMP_FILTER_FLAG_LOG,
-				 SECCOMP_FILTER_FLAG_SPEC_ALLOW };
+				 SECCOMP_FILTER_FLAG_SPEC_ALLOW,
+				 SECCOMP_FILTER_FLAG_NEW_LISTENER };
 	unsigned int flag, all_flags;
 	int i;
 	long ret;
@@ -2933,6 +2979,403 @@  TEST(get_metadata)
 	ASSERT_EQ(0, kill(pid, SIGKILL));
 }
 
+static int user_trap_syscall(int nr, unsigned int flags)
+{
+	struct sock_filter filter[] = {
+		BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
+			offsetof(struct seccomp_data, nr)),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_USER_NOTIF),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+	};
+
+	struct sock_fprog prog = {
+		.len = (unsigned short)ARRAY_SIZE(filter),
+		.filter = filter,
+	};
+
+	return seccomp(SECCOMP_SET_MODE_FILTER, flags, &prog);
+}
+
+#define USER_NOTIF_MAGIC 116983961184613L
+TEST(user_notification_basic)
+{
+	pid_t pid;
+	long ret;
+	int status, listener;
+	struct seccomp_notif req = {};
+	struct seccomp_notif_resp resp = {};
+	struct pollfd pollfd;
+
+	struct sock_filter filter[] = {
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
+	};
+	struct sock_fprog prog = {
+		.len = (unsigned short)ARRAY_SIZE(filter),
+		.filter = filter,
+	};
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	/* Check that we get -ENOSYS with no listener attached */
+	if (pid == 0) {
+		if (user_trap_syscall(__NR_getpid, 0) < 0)
+			exit(1);
+		ret = syscall(__NR_getpid);
+		exit(ret >= 0 || errno != ENOSYS);
+	}
+
+	EXPECT_EQ(waitpid(pid, &status, 0), pid);
+	EXPECT_EQ(true, WIFEXITED(status));
+	EXPECT_EQ(0, WEXITSTATUS(status));
+
+	/* Add some no-op filters so for grins. */
+	EXPECT_EQ(seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog), 0);
+	EXPECT_EQ(seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog), 0);
+	EXPECT_EQ(seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog), 0);
+	EXPECT_EQ(seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog), 0);
+
+	/* Check that the basic notification machinery works */
+	listener = user_trap_syscall(__NR_getpid,
+				     SECCOMP_FILTER_FLAG_NEW_LISTENER);
+	EXPECT_GE(listener, 0);
+
+	/* Installing a second listener in the chain should EBUSY */
+	EXPECT_EQ(user_trap_syscall(__NR_getpid,
+				    SECCOMP_FILTER_FLAG_NEW_LISTENER),
+		  -1);
+	EXPECT_EQ(errno, EBUSY);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		ret = syscall(__NR_getpid);
+		exit(ret != USER_NOTIF_MAGIC);
+	}
+
+	pollfd.fd = listener;
+	pollfd.events = POLLIN | POLLOUT;
+
+	EXPECT_GT(poll(&pollfd, 1, -1), 0);
+	EXPECT_EQ(pollfd.revents, POLLIN);
+
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
+
+	pollfd.fd = listener;
+	pollfd.events = POLLIN | POLLOUT;
+
+	EXPECT_GT(poll(&pollfd, 1, -1), 0);
+	EXPECT_EQ(pollfd.revents, POLLOUT);
+
+	EXPECT_EQ(req.data.nr,  __NR_getpid);
+
+	resp.id = req.id;
+	resp.error = 0;
+	resp.val = USER_NOTIF_MAGIC;
+
+	/* check that we make sure flags == 0 */
+	resp.flags = 1;
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), -1);
+	EXPECT_EQ(errno, EINVAL);
+
+	resp.flags = 0;
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
+
+	EXPECT_EQ(waitpid(pid, &status, 0), pid);
+	EXPECT_EQ(true, WIFEXITED(status));
+	EXPECT_EQ(0, WEXITSTATUS(status));
+}
+
+TEST(user_notification_kill_in_middle)
+{
+	pid_t pid;
+	long ret;
+	int listener;
+	struct seccomp_notif req = {};
+	struct seccomp_notif_resp resp = {};
+
+	listener = user_trap_syscall(__NR_getpid,
+				     SECCOMP_FILTER_FLAG_NEW_LISTENER);
+	EXPECT_GE(listener, 0);
+
+	/*
+	 * Check that nothing bad happens when we kill the task in the middle
+	 * of a syscall.
+	 */
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		ret = syscall(__NR_getpid);
+		exit(ret != USER_NOTIF_MAGIC);
+	}
+
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_ID_VALID, &req.id), 0);
+
+	EXPECT_EQ(kill(pid, SIGKILL), 0);
+	EXPECT_EQ(waitpid(pid, NULL, 0), pid);
+
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_ID_VALID, &req.id), -1);
+
+	resp.id = req.id;
+	ret = ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp);
+	EXPECT_EQ(ret, -1);
+	EXPECT_EQ(errno, ENOENT);
+}
+
+static int handled = -1;
+
+static void signal_handler(int signal)
+{
+	if (write(handled, "c", 1) != 1)
+		perror("write from signal");
+}
+
+TEST(user_notification_signal)
+{
+	pid_t pid;
+	long ret;
+	int status, listener, sk_pair[2];
+	struct seccomp_notif req = {};
+	struct seccomp_notif_resp resp = {};
+	char c;
+
+	ASSERT_EQ(socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair), 0);
+
+	listener = user_trap_syscall(__NR_gettid,
+				     SECCOMP_FILTER_FLAG_NEW_LISTENER);
+	EXPECT_GE(listener, 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		close(sk_pair[0]);
+		handled = sk_pair[1];
+		if (signal(SIGUSR1, signal_handler) == SIG_ERR) {
+			perror("signal");
+			exit(1);
+		}
+		/*
+		 * ERESTARTSYS behavior is a bit hard to test, because we need
+		 * to rely on a signal that has not yet been handled. Let's at
+		 * least check that the error code gets propagated through, and
+		 * hope that it doesn't break when there is actually a signal :)
+		 */
+		ret = syscall(__NR_gettid);
+		exit(!(ret == -1 && errno == 512));
+	}
+
+	close(sk_pair[1]);
+
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
+
+	EXPECT_EQ(kill(pid, SIGUSR1), 0);
+
+	/*
+	 * Make sure the signal really is delivered, which means we're not
+	 * stuck in the user notification code any more and the notification
+	 * should be dead.
+	 */
+	EXPECT_EQ(read(sk_pair[0], &c, 1), 1);
+
+	resp.id = req.id;
+	resp.error = -EPERM;
+	resp.val = 0;
+
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), -1);
+	EXPECT_EQ(errno, ENOENT);
+
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
+
+	resp.id = req.id;
+	resp.error = -512; /* -ERESTARTSYS */
+	resp.val = 0;
+
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
+
+	EXPECT_EQ(waitpid(pid, &status, 0), pid);
+	EXPECT_EQ(true, WIFEXITED(status));
+	EXPECT_EQ(0, WEXITSTATUS(status));
+}
+
+TEST(user_notification_closed_listener)
+{
+	pid_t pid;
+	long ret;
+	int status, listener;
+
+	listener = user_trap_syscall(__NR_getpid,
+				     SECCOMP_FILTER_FLAG_NEW_LISTENER);
+	EXPECT_GE(listener, 0);
+
+	/*
+	 * Check that we get an ENOSYS when the listener is closed.
+	 */
+	pid = fork();
+	ASSERT_GE(pid, 0);
+	if (pid == 0) {
+		close(listener);
+		ret = syscall(__NR_getpid);
+		exit(ret != -1 && errno != ENOSYS);
+	}
+
+	close(listener);
+
+	EXPECT_EQ(waitpid(pid, &status, 0), pid);
+	EXPECT_EQ(true, WIFEXITED(status));
+	EXPECT_EQ(0, WEXITSTATUS(status));
+}
+
+/*
+ * Check that a pid in a child namespace still shows up as valid in ours.
+ */
+TEST(user_notification_child_pid_ns)
+{
+	pid_t pid;
+	int status, listener;
+	struct seccomp_notif req = {};
+	struct seccomp_notif_resp resp = {};
+
+	ASSERT_EQ(unshare(CLONE_NEWPID), 0);
+
+	listener = user_trap_syscall(__NR_getpid, SECCOMP_FILTER_FLAG_NEW_LISTENER);
+	ASSERT_GE(listener, 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0)
+		exit(syscall(__NR_getpid) != USER_NOTIF_MAGIC);
+
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
+	EXPECT_EQ(req.pid, pid);
+
+	resp.id = req.id;
+	resp.error = 0;
+	resp.val = USER_NOTIF_MAGIC;
+
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
+
+	EXPECT_EQ(waitpid(pid, &status, 0), pid);
+	EXPECT_EQ(true, WIFEXITED(status));
+	EXPECT_EQ(0, WEXITSTATUS(status));
+	close(listener);
+}
+
+/*
+ * Check that a pid in a sibling (i.e. unrelated) namespace shows up as 0, i.e.
+ * invalid.
+ */
+TEST(user_notification_sibling_pid_ns)
+{
+	pid_t pid, pid2;
+	int status, listener;
+	struct seccomp_notif req = {};
+	struct seccomp_notif_resp resp = {};
+
+	listener = user_trap_syscall(__NR_getpid, SECCOMP_FILTER_FLAG_NEW_LISTENER);
+	ASSERT_GE(listener, 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		ASSERT_EQ(unshare(CLONE_NEWPID), 0);
+
+		pid2 = fork();
+		ASSERT_GE(pid2, 0);
+
+		if (pid2 == 0)
+			exit(syscall(__NR_getpid) != USER_NOTIF_MAGIC);
+
+		EXPECT_EQ(waitpid(pid2, &status, 0), pid2);
+		EXPECT_EQ(true, WIFEXITED(status));
+		EXPECT_EQ(0, WEXITSTATUS(status));
+		exit(WEXITSTATUS(status));
+	}
+
+	/* Create the sibling ns, and sibling in it. */
+	EXPECT_EQ(unshare(CLONE_NEWPID), 0);
+	EXPECT_EQ(errno, 0);
+
+	pid2 = fork();
+	EXPECT_GE(pid2, 0);
+
+	if (pid2 == 0) {
+		ASSERT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
+		/*
+		 * The pid should be 0, i.e. the task is in some namespace that
+		 * we can't "see".
+		 */
+		ASSERT_EQ(req.pid, 0);
+
+		resp.id = req.id;
+		resp.error = 0;
+		resp.val = USER_NOTIF_MAGIC;
+
+		ASSERT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
+		exit(0);
+	}
+
+	close(listener);
+
+	EXPECT_EQ(waitpid(pid, &status, 0), pid);
+	EXPECT_EQ(true, WIFEXITED(status));
+	EXPECT_EQ(0, WEXITSTATUS(status));
+
+	EXPECT_EQ(waitpid(pid2, &status, 0), pid2);
+	EXPECT_EQ(true, WIFEXITED(status));
+	EXPECT_EQ(0, WEXITSTATUS(status));
+}
+
+TEST(user_notification_fault_recv)
+{
+	pid_t pid;
+	int status, listener;
+	struct seccomp_notif req = {};
+	struct seccomp_notif_resp resp = {};
+
+	listener = user_trap_syscall(__NR_getpid, SECCOMP_FILTER_FLAG_NEW_LISTENER);
+	ASSERT_GE(listener, 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0)
+		exit(syscall(__NR_getpid) != USER_NOTIF_MAGIC);
+
+	/* Do a bad recv() */
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, NULL), -1);
+	EXPECT_EQ(errno, EFAULT);
+
+	/* We should still be able to receive this notification, though. */
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
+	EXPECT_EQ(req.pid, pid);
+
+	resp.id = req.id;
+	resp.error = 0;
+	resp.val = USER_NOTIF_MAGIC;
+
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
+
+	EXPECT_EQ(waitpid(pid, &status, 0), pid);
+	EXPECT_EQ(true, WIFEXITED(status));
+	EXPECT_EQ(0, WEXITSTATUS(status));
+}
+
+TEST(seccomp_get_notif_sizes)
+{
+	struct seccomp_notif_sizes sizes;
+
+	EXPECT_EQ(seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes), 0);
+	EXPECT_EQ(sizes.seccomp_notif, sizeof(struct seccomp_notif));
+	EXPECT_EQ(sizes.seccomp_notif_resp, sizeof(struct seccomp_notif_resp));
+}
+
 /*
  * TODO:
  * - add microbenchmarks

Comments

Serge E. Hallyn Dec. 3, 2018, 5:26 a.m.
On Sun, Dec 02, 2018 at 08:28:26PM -0700, Tycho Andersen wrote:
> This patch introduces a means for syscalls matched in seccomp to notify
> some other task that a particular filter has been triggered.
> 
> The motivation for this is primarily for use with containers. For example,
> if a container does an init_module(), we obviously don't want to load this
> untrusted code, which may be compiled for the wrong version of the kernel
> anyway. Instead, we could parse the module image, figure out which module
> the container is trying to load and load it on the host.
> 
> As another example, containers cannot mount() in general since various
> filesystems assume a trusted image. However, if an orchestrator knows that
> e.g. a particular block device has not been exposed to a container for
> writing, it want to allow the container to mount that block device (that
> is, handle the mount for it).
> 
> This patch adds functionality that is already possible via at least two
> other means that I know about, both of which involve ptrace(): first, one
> could ptrace attach, and then iterate through syscalls via PTRACE_SYSCALL.
> Unfortunately this is slow, so a faster version would be to install a
> filter that does SECCOMP_RET_TRACE, which triggers a PTRACE_EVENT_SECCOMP.
> Since ptrace allows only one tracer, if the container runtime is that
> tracer, users inside the container (or outside) trying to debug it will not
> be able to use ptrace, which is annoying. It also means that older
> distributions based on Upstart cannot boot inside containers using ptrace,
> since upstart itself uses ptrace to monitor services while starting.
> 
> The actual implementation of this is fairly small, although getting the
> synchronization right was/is slightly complex.
> 
> Finally, it's worth noting that the classic seccomp TOCTOU of reading
> memory data from the task still applies here, but can be avoided with
> careful design of the userspace handler: if the userspace handler reads all
> of the task memory that is necessary before applying its security policy,
> the tracee's subsequent memory edits will not be read by the tracer.
> 
> Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> CC: Kees Cook <keescook@chromium.org>
> CC: Andy Lutomirski <luto@amacapital.net>
> CC: Oleg Nesterov <oleg@redhat.com>
> CC: Eric W. Biederman <ebiederm@xmission.com>
> CC: "Serge E. Hallyn" <serge@hallyn.com>

Acked-by: Serge Hallyn <serge@hallyn.com>

a few questions below, though, which I'm sure are just me reading too
late at night,

> CC: Christian Brauner <christian@brauner.io>
> CC: Tyler Hicks <tyhicks@canonical.com>
> CC: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
> ---
> v2: * make id a u64; the idea here being that it will never overflow,
>       because 64 is huge (one syscall every nanosecond => wrap every 584
>       years) (Andy)
>     * prevent nesting of user notifications: if someone is already attached
>       the tree in one place, nobody else can attach to the tree (Andy)
>     * notify the listener of signals the tracee receives as well (Andy)
>     * implement poll
> v3: * lockdep fix (Oleg)
>     * drop unnecessary WARN()s (Christian)
>     * rearrange error returns to be more rpetty (Christian)
>     * fix build in !CONFIG_SECCOMP_USER_NOTIFICATION case
> v4: * fix implementation of poll to use poll_wait() (Jann)
>     * change listener's fd flags to be 0 (Jann)
>     * hoist filter initialization out of ifdefs to its own function
>       init_user_notification()
>     * add some more testing around poll() and closing the listener while a
>       syscall is in action
>     * s/GET_LISTENER/NEW_LISTENER, since you can't _get_ a listener, but it
>       creates a new one (Matthew)
>     * correctly handle pid namespaces, add some testcases (Matthew)
>     * use EINPROGRESS instead of EINVAL when a notification response is
>       written twice (Matthew)
>     * fix comment typo from older version (SEND vs READ) (Matthew)
>     * whitespace and logic simplification (Tobin)
>     * add some Documentation/ bits on userspace trapping
> v5: * fix documentation typos (Jann)
>     * add signalled field to struct seccomp_notif (Jann)
>     * switch to using ioctls instead of read()/write() for struct passing
>       (Jann)
>     * add an ioctl to ensure an id is still valid
> v6: * docs typo fixes, update docs for ioctl() change (Christian)
> v7: * switch struct seccomp_knotif's id member to a u64 (derp :)
>     * use notify_lock in IS_ID_VALID query to avoid racing
>     * s/signalled/signaled (Tyler)
>     * fix docs to reflect that ids are not globally unique (Tyler)
>     * add a test to check -ERESTARTSYS behavior (Tyler)
>     * drop CONFIG_SECCOMP_USER_NOTIFICATION (Tyler)
>     * reorder USER_NOTIF in seccomp return codes list (Tyler)
>     * return size instead of sizeof(struct user_notif) (Tyler)
>     * ENOENT instead of EINVAL when invalid id is passed (Tyler)
>     * drop CONFIG_SECCOMP_USER_NOTIFICATION guards (Tyler)
>     * s/IS_ID_VALID/ID_VALID and switch ioctl to be "well behaved" (Tyler)
>     * add a new struct notification to minimize the additions to
>       struct seccomp_filter, also pack the necessary additions a bit more
>       cleverly (Tyler)
>     * switch to keeping track of the task itself instead of the pid (we'll
>       use this for implementing PUT_FD)
> v8: * in recv, don't copy_to_user() while holding notify lock, in case
>       userfaultfd blocks and causes all syscalls to block (Kees)
>     * switch ioctl character to something more fun ! (Kees)
>     * switch ioctl defines to use their own SECCOMP_IO* macros (Kees)
>     * rename seccomp ioctls to be SECCOMP_IOCTL_* (Kees)
>     * move comment of notify_lock to the right place (Jann)
>     * drop comment abount reference count bounding in __get_seccomp_filter (Jann)
>     * add lockdep_assert_held() in seccomp_next_notify_id() (Kees)
>     * in seccomp_do_user_notification(), always increment semaphore before
>       releasing lock, to prevent use after free of ->notif (Kees)
>     * add another wake_up_poll() when a signal is received (Jann)
>     * make all listener fds O_CLOEXEC (Jann/Kees)
>     * use memset() instead of = {} initialization for structures (Kees)
>     * move casting of buf pointer to ioctl, instead of in handler functions (Kees)
>     * fix ENOENT testing in seccomp_notify_send() (Jann)
>     * use ENOENT instead of -1 (EPERM) for ID_VALID ioctl (Jann)
>     * use ()s around "nested" bit operations (Kees)
>     * init struct notification members in the order they're declared (Jann)
>     * rearrange things so no forward declaration of init_listener() is
>       required (Kees)
>     * switch to a flags based future-proofing mechanism for struct
>       seccomp_notif and seccomp_notif_resp, thus avoiding version issues
>       with structure length (Kees)
>     * fix a memory leak in init_listener() in a failure case
>     * fix a use-after-free of filter->notif in do_user_notification() when
>       the listener fd is closed after a signal is sent
>     * add a comment about semaphore state in the interrupt case in
>       do_user_notification() + seccomp_notify_recv()
> v9: * add SECCOMP_GET_NOTIF_SIZES to handle when struct seccomp_data
>       changes in size
>     * don't do locking all the way up the seccomp tree (Oleg)
>     * rearrange the tests so that one test tests one thing
>     * avoid an unkillable sleep by dropping the signaled flag (Oleg)
> ---
>  Documentation/ioctl/ioctl-number.txt          |   1 +
>  .../userspace-api/seccomp_filter.rst          |  84 ++++
>  include/linux/seccomp.h                       |   7 +-
>  include/uapi/linux/seccomp.h                  |  40 +-
>  kernel/seccomp.c                              | 448 +++++++++++++++++-
>  tools/testing/selftests/seccomp/seccomp_bpf.c | 447 ++++++++++++++++-
>  6 files changed, 1017 insertions(+), 10 deletions(-)
> 
> diff --git a/Documentation/ioctl/ioctl-number.txt b/Documentation/ioctl/ioctl-number.txt
> index af6f6ba1fe80..c9558146ac58 100644
> --- a/Documentation/ioctl/ioctl-number.txt
> +++ b/Documentation/ioctl/ioctl-number.txt
> @@ -79,6 +79,7 @@ Code  Seq#(hex)	Include File		Comments
>  0x1b	all	InfiniBand Subsystem	<http://infiniband.sourceforge.net/>
>  0x20	all	drivers/cdrom/cm206.h
>  0x22	all	scsi/sg.h
> +'!'	00-1F	uapi/linux/seccomp.h
>  '#'	00-3F	IEEE 1394 Subsystem	Block for the entire subsystem
>  '$'	00-0F	linux/perf_counter.h, linux/perf_event.h
>  '%'	00-0F	include/uapi/linux/stm.h
> diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst
> index 82a468bc7560..b1b846d8a094 100644
> --- a/Documentation/userspace-api/seccomp_filter.rst
> +++ b/Documentation/userspace-api/seccomp_filter.rst
> @@ -122,6 +122,11 @@ In precedence order, they are:
>  	Results in the lower 16-bits of the return value being passed
>  	to userland as the errno without executing the system call.
>  
> +``SECCOMP_RET_USER_NOTIF``:
> +    Results in a ``struct seccomp_notif`` message sent on the userspace
> +    notification fd, if it is attached, or ``-ENOSYS`` if it is not. See below
> +    on discussion of how to handle user notifications.
> +
>  ``SECCOMP_RET_TRACE``:
>  	When returned, this value will cause the kernel to attempt to
>  	notify a ``ptrace()``-based tracer prior to executing the system
> @@ -183,6 +188,85 @@ The ``samples/seccomp/`` directory contains both an x86-specific example
>  and a more generic example of a higher level macro interface for BPF
>  program generation.
>  
> +Userspace Notification
> +======================
> +
> +The ``SECCOMP_RET_USER_NOTIF`` return code lets seccomp filters pass a
> +particular syscall to userspace to be handled. This may be useful for
> +applications like container managers, which wish to intercept particular
> +syscalls (``mount()``, ``finit_module()``, etc.) and change their behavior.
> +
> +To acquire a notification FD, use the ``SECCOMP_FILTER_FLAG_NEW_LISTENER``
> +argument to the ``seccomp()`` syscall:
> +
> +.. code-block:: c
> +
> +    fd = seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_NEW_LISTENER, &prog);
> +
> +which (on success) will return a listener fd for the filter, which can then be
> +passed around via ``SCM_RIGHTS`` or similar. Note that filter fds correspond to
> +a particular filter, and not a particular task. So if this task then forks,
> +notifications from both tasks will appear on the same filter fd. Reads and
> +writes to/from a filter fd are also synchronized, so a filter fd can safely
> +have many readers.
> +
> +The interface for a seccomp notification fd consists of two structures:
> +
> +.. code-block:: c
> +
> +    struct seccomp_notif_sizes {
> +        __u16 seccomp_notif;
> +        __u16 seccomp_notif_resp;
> +        __u16 seccomp_data;
> +    };
> +
> +    struct seccomp_notif {
> +        __u64 id;
> +        __u32 pid;
> +        __u32 flags;
> +        struct seccomp_data data;
> +    };
> +
> +    struct seccomp_notif_resp {
> +        __u64 id;
> +        __s64 val;
> +        __s32 error;
> +        __u32 flags;
> +    };
> +
> +The ``struct seccomp_notif_sizes`` structure can be used to determine the size
> +of the various structures used in seccomp notifications. The size of ``struct
> +seccomp_data`` may change in the future, so code should use:
> +
> +.. code-block:: c
> +
> +    struct seccomp_notif_sizes sizes;
> +    seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes);
> +
> +to determine the size of the various structures to allocate. See
> +samples/seccomp/user-trap.c for an example.
> +
> +Users can read via ``ioctl(SECCOMP_IOCTL_NOTIF_RECV)``  (or ``poll()``) on a
> +seccomp notification fd to receive a ``struct seccomp_notif``, which contains
> +five members: the input length of the structure, a unique-per-filter ``id``,
> +the ``pid`` of the task which triggered this request (which may be 0 if the
> +task is in a pid ns not visible from the listener's pid namespace), a ``flags``
> +member which for now only has ``SECCOMP_NOTIF_FLAG_SIGNALED``, representing
> +whether or not the notification is a result of a non-fatal signal, and the
> +``data`` passed to seccomp. Userspace can then make a decision based on this
> +information about what to do, and ``ioctl(SECCOMP_IOCTL_NOTIF_SEND)`` a
> +response, indicating what should be returned to userspace. The ``id`` member of
> +``struct seccomp_notif_resp`` should be the same ``id`` as in ``struct
> +seccomp_notif``.
> +
> +It is worth noting that ``struct seccomp_data`` contains the values of register
> +arguments to the syscall, but does not contain pointers to memory. The task's
> +memory is accessible to suitably privileged traces via ``ptrace()`` or
> +``/proc/pid/mem``. However, care should be taken to avoid the TOCTOU mentioned
> +above in this document: all arguments being read from the tracee's memory
> +should be read into the tracer's memory before any policy decisions are made.
> +This allows for an atomic decision on syscall arguments.
> +
>  Sysctls
>  =======
>  
> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
> index b5103c019cf4..84868d37b35d 100644
> --- a/include/linux/seccomp.h
> +++ b/include/linux/seccomp.h
> @@ -4,9 +4,10 @@
>  
>  #include <uapi/linux/seccomp.h>
>  
> -#define SECCOMP_FILTER_FLAG_MASK	(SECCOMP_FILTER_FLAG_TSYNC	| \
> -					 SECCOMP_FILTER_FLAG_LOG	| \
> -					 SECCOMP_FILTER_FLAG_SPEC_ALLOW)
> +#define SECCOMP_FILTER_FLAG_MASK	(SECCOMP_FILTER_FLAG_TSYNC | \
> +					 SECCOMP_FILTER_FLAG_LOG | \
> +					 SECCOMP_FILTER_FLAG_SPEC_ALLOW | \
> +					 SECCOMP_FILTER_FLAG_NEW_LISTENER)
>  
>  #ifdef CONFIG_SECCOMP
>  
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index 9efc0e73d50b..90734aa5aa36 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -15,11 +15,13 @@
>  #define SECCOMP_SET_MODE_STRICT		0
>  #define SECCOMP_SET_MODE_FILTER		1
>  #define SECCOMP_GET_ACTION_AVAIL	2
> +#define SECCOMP_GET_NOTIF_SIZES		3
>  
>  /* Valid flags for SECCOMP_SET_MODE_FILTER */
> -#define SECCOMP_FILTER_FLAG_TSYNC	(1UL << 0)
> -#define SECCOMP_FILTER_FLAG_LOG		(1UL << 1)
> -#define SECCOMP_FILTER_FLAG_SPEC_ALLOW	(1UL << 2)
> +#define SECCOMP_FILTER_FLAG_TSYNC		(1UL << 0)
> +#define SECCOMP_FILTER_FLAG_LOG			(1UL << 1)
> +#define SECCOMP_FILTER_FLAG_SPEC_ALLOW		(1UL << 2)
> +#define SECCOMP_FILTER_FLAG_NEW_LISTENER	(1UL << 3)
>  
>  /*
>   * All BPF programs must return a 32-bit value.
> @@ -35,6 +37,7 @@
>  #define SECCOMP_RET_KILL	 SECCOMP_RET_KILL_THREAD
>  #define SECCOMP_RET_TRAP	 0x00030000U /* disallow and force a SIGSYS */
>  #define SECCOMP_RET_ERRNO	 0x00050000U /* returns an errno */
> +#define SECCOMP_RET_USER_NOTIF	 0x7fc00000U /* notifies userspace */
>  #define SECCOMP_RET_TRACE	 0x7ff00000U /* pass to a tracer or disallow */
>  #define SECCOMP_RET_LOG		 0x7ffc0000U /* allow after logging */
>  #define SECCOMP_RET_ALLOW	 0x7fff0000U /* allow */
> @@ -60,4 +63,35 @@ struct seccomp_data {
>  	__u64 args[6];
>  };
>  
> +struct seccomp_notif_sizes {
> +	__u16 seccomp_notif;
> +	__u16 seccomp_notif_resp;
> +	__u16 seccomp_data;
> +};
> +
> +struct seccomp_notif {
> +	__u64 id;
> +	__u32 pid;
> +	__u32 flags;
> +	struct seccomp_data data;
> +};
> +
> +struct seccomp_notif_resp {
> +	__u64 id;
> +	__s64 val;
> +	__s32 error;
> +	__u32 flags;
> +};
> +
> +#define SECCOMP_IOC_MAGIC		'!'
> +#define SECCOMP_IO(nr)			_IO(SECCOMP_IOC_MAGIC, nr)
> +#define SECCOMP_IOR(nr, type)		_IOR(SECCOMP_IOC_MAGIC, nr, type)
> +#define SECCOMP_IOW(nr, type)		_IOW(SECCOMP_IOC_MAGIC, nr, type)
> +#define SECCOMP_IOWR(nr, type)		_IOWR(SECCOMP_IOC_MAGIC, nr, type)
> +
> +/* Flags for seccomp notification fd ioctl. */
> +#define SECCOMP_IOCTL_NOTIF_RECV	SECCOMP_IOWR(0, struct seccomp_notif)
> +#define SECCOMP_IOCTL_NOTIF_SEND	SECCOMP_IOWR(1,	\
> +						struct seccomp_notif_resp)
> +#define SECCOMP_IOCTL_NOTIF_ID_VALID	SECCOMP_IOR(2, __u64)
>  #endif /* _UAPI_LINUX_SECCOMP_H */
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 393e029f778a..15b6be97fc09 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -33,12 +33,74 @@
>  #endif
>  
>  #ifdef CONFIG_SECCOMP_FILTER
> +#include <linux/file.h>
>  #include <linux/filter.h>
>  #include <linux/pid.h>
>  #include <linux/ptrace.h>
>  #include <linux/security.h>
>  #include <linux/tracehook.h>
>  #include <linux/uaccess.h>
> +#include <linux/anon_inodes.h>
> +
> +enum notify_state {
> +	SECCOMP_NOTIFY_INIT,
> +	SECCOMP_NOTIFY_SENT,
> +	SECCOMP_NOTIFY_REPLIED,
> +};
> +
> +struct seccomp_knotif {
> +	/* The struct pid of the task whose filter triggered the notification */
> +	struct task_struct *task;
> +
> +	/* The "cookie" for this request; this is unique for this filter. */
> +	u64 id;
> +
> +	/*
> +	 * The seccomp data. This pointer is valid the entire time this
> +	 * notification is active, since it comes from __seccomp_filter which

define 'active' - is a notification in state REPLIED still active?

Actually while looking at that, I came to wondering - when are knotifs
freed?  Seems like only during seccomp_notify_release(), i.e. when the
tracing task stops polling for events?  Is that going to be a problem?
Or am I misreading this?

> +	 * eclipses the entire lifecycle here.
> +	 */
> +	const struct seccomp_data *data;
> +
> +	/*
> +	 * Notification states. When SECCOMP_RET_USER_NOTIF is returned, a
> +	 * struct seccomp_knotif is created and starts out in INIT. Once the
> +	 * handler reads the notification off of an FD, it transitions to SENT.
> +	 * If a signal is received the state transitions back to INIT and
> +	 * another message is sent. When the userspace handler replies, state
> +	 * transitions to REPLIED.
> +	 */
> +	enum notify_state state;
> +
> +	/* The return values, only valid when in SECCOMP_NOTIFY_REPLIED */
> +	int error;
> +	long val;
> +
> +	/* Signals when this has entered SECCOMP_NOTIFY_REPLIED */
> +	struct completion ready;
> +
> +	struct list_head list;
> +};
> +
> +/**
> + * struct notification - container for seccomp userspace notifications. Since
> + * most seccomp filters will not have notification listeners attached and this
> + * structure is fairly large, we store the notification-specific stuff in a
> + * separate structure.
> + *
> + * @request: A semaphore that users of this notification can wait on for
> + *           changes. Actual reads and writes are still controlled with
> + *           filter->notify_lock.
> + * @next_id: The id of the next request.
> + * @notifications: A list of struct seccomp_knotif elements.
> + * @wqh: A wait queue for poll.
> + */
> +struct notification {
> +	struct semaphore request;
> +	u64 next_id;
> +	struct list_head notifications;
> +	wait_queue_head_t wqh;
> +};
>  
>  /**
>   * struct seccomp_filter - container for seccomp BPF programs
> @@ -50,6 +112,8 @@
>   * @log: true if all actions except for SECCOMP_RET_ALLOW should be logged
>   * @prev: points to a previously installed, or inherited, filter
>   * @prog: the BPF program to evaluate
> + * @notif: the struct that holds all notification related information
> + * @notify_lock: A lock for all notification-related accesses.
>   *
>   * seccomp_filter objects are organized in a tree linked via the @prev
>   * pointer.  For any task, it appears to be a singly-linked list starting
> @@ -66,6 +130,8 @@ struct seccomp_filter {
>  	bool log;
>  	struct seccomp_filter *prev;
>  	struct bpf_prog *prog;
> +	struct notification *notif;
> +	struct mutex notify_lock;
>  };
>  
>  /* Limit any path through the tree to 256KB worth of instructions. */
> @@ -386,6 +452,7 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
>  	if (!sfilter)
>  		return ERR_PTR(-ENOMEM);
>  
> +	mutex_init(&sfilter->notify_lock);
>  	ret = bpf_prog_create_from_user(&sfilter->prog, fprog,
>  					seccomp_check_filter, save_orig);
>  	if (ret < 0) {
> @@ -479,7 +546,6 @@ static long seccomp_attach_filter(unsigned int flags,
>  
>  static void __get_seccomp_filter(struct seccomp_filter *filter)
>  {
> -	/* Reference count is bounded by the number of total processes. */
>  	refcount_inc(&filter->usage);
>  }
>  
> @@ -550,11 +616,13 @@ static void seccomp_send_sigsys(int syscall, int reason)
>  #define SECCOMP_LOG_TRACE		(1 << 4)
>  #define SECCOMP_LOG_LOG			(1 << 5)
>  #define SECCOMP_LOG_ALLOW		(1 << 6)
> +#define SECCOMP_LOG_USER_NOTIF		(1 << 7)
>  
>  static u32 seccomp_actions_logged = SECCOMP_LOG_KILL_PROCESS |
>  				    SECCOMP_LOG_KILL_THREAD  |
>  				    SECCOMP_LOG_TRAP  |
>  				    SECCOMP_LOG_ERRNO |
> +				    SECCOMP_LOG_USER_NOTIF |
>  				    SECCOMP_LOG_TRACE |
>  				    SECCOMP_LOG_LOG;
>  
> @@ -575,6 +643,9 @@ static inline void seccomp_log(unsigned long syscall, long signr, u32 action,
>  	case SECCOMP_RET_TRACE:
>  		log = requested && seccomp_actions_logged & SECCOMP_LOG_TRACE;
>  		break;
> +	case SECCOMP_RET_USER_NOTIF:
> +		log = requested && seccomp_actions_logged & SECCOMP_LOG_USER_NOTIF;
> +		break;
>  	case SECCOMP_RET_LOG:
>  		log = seccomp_actions_logged & SECCOMP_LOG_LOG;
>  		break;
> @@ -646,6 +717,68 @@ void secure_computing_strict(int this_syscall)
>  #else
>  
>  #ifdef CONFIG_SECCOMP_FILTER
> +static u64 seccomp_next_notify_id(struct seccomp_filter *filter)
> +{
> +	/*
> +	 * Note: overflow is ok here, the id just needs to be unique per
> +	 * filter.
> +	 */
> +	lockdep_assert_held(&filter->notify_lock);
> +	return filter->notif->next_id++;
> +}
> +
> +static void seccomp_do_user_notification(int this_syscall,
> +					 struct seccomp_filter *match,
> +					 const struct seccomp_data *sd)
> +{
> +	int err;
> +	long ret = 0;
> +	struct seccomp_knotif n = {};
> +
> +	mutex_lock(&match->notify_lock);
> +	err = -ENOSYS;
> +	if (!match->notif)
> +		goto out;
> +
> +	n.task = current;
> +	n.state = SECCOMP_NOTIFY_INIT;
> +	n.data = sd;
> +	n.id = seccomp_next_notify_id(match);
> +	init_completion(&n.ready);
> +	list_add(&n.list, &match->notif->notifications);
> +
> +	up(&match->notif->request);
> +	wake_up_poll(&match->notif->wqh, EPOLLIN | EPOLLRDNORM);
> +	mutex_unlock(&match->notify_lock);
> +
> +	/*
> +	 * This is where we wait for a reply from userspace.
> +	 */
> +	err = wait_for_completion_interruptible(&n.ready);
> +	mutex_lock(&match->notify_lock);
> +	if (err == 0) {
> +		ret = n.val;
> +		err = n.error;
> +	}
> +
> +	/*
> +	 * Note that it's possible the listener died in between the time when
> +	 * we were notified of a respons (or a signal) and when we were able to
> +	 * re-acquire the lock, so only delete from the list if the
> +	 * notification actually exists.
> +	 *
> +	 * Also note that this test is only valid because there's no way to
> +	 * *reattach* to a notifier right now. If one is added, we'll need to
> +	 * keep track of the notif itself and make sure they match here.
> +	 */
> +	if (match->notif)
> +		list_del(&n.list);
> +out:
> +	mutex_unlock(&match->notify_lock);
> +	syscall_set_return_value(current, task_pt_regs(current),
> +				 err, ret);
> +}
> +
>  static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>  			    const bool recheck_after_trace)
>  {
> @@ -728,6 +861,10 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>  
>  		return 0;
>  
> +	case SECCOMP_RET_USER_NOTIF:
> +		seccomp_do_user_notification(this_syscall, match, sd);
> +		goto skip;
> +
>  	case SECCOMP_RET_LOG:
>  		seccomp_log(this_syscall, 0, action, true);
>  		return 0;
> @@ -834,6 +971,263 @@ static long seccomp_set_mode_strict(void)
>  }
>  
>  #ifdef CONFIG_SECCOMP_FILTER
> +static int seccomp_notify_release(struct inode *inode, struct file *file)
> +{
> +	struct seccomp_filter *filter = file->private_data;
> +	struct seccomp_knotif *knotif;
> +
> +	mutex_lock(&filter->notify_lock);
> +
> +	/*
> +	 * If this file is being closed because e.g. the task who owned it
> +	 * died, let's wake everyone up who was waiting on us.
> +	 */
> +	list_for_each_entry(knotif, &filter->notif->notifications, list) {
> +		if (knotif->state == SECCOMP_NOTIFY_REPLIED)
> +			continue;
> +
> +		knotif->state = SECCOMP_NOTIFY_REPLIED;
> +		knotif->error = -ENOSYS;
> +		knotif->val = 0;
> +
> +		complete(&knotif->ready);
> +	}
> +
> +	kfree(filter->notif);
> +	filter->notif = NULL;
> +	mutex_unlock(&filter->notify_lock);
> +	__put_seccomp_filter(filter);
> +	return 0;
> +}
> +
> +static long seccomp_notify_recv(struct seccomp_filter *filter,
> +				void __user *buf)
> +{
> +	struct seccomp_knotif *knotif = NULL, *cur;
> +	struct seccomp_notif unotif;
> +	ssize_t ret;
> +
> +	memset(&unotif, 0, sizeof(unotif));
> +
> +	ret = down_interruptible(&filter->notif->request);
> +	if (ret < 0)
> +		return ret;
> +
> +	mutex_lock(&filter->notify_lock);
> +	list_for_each_entry(cur, &filter->notif->notifications, list) {
> +		if (cur->state == SECCOMP_NOTIFY_INIT) {
> +			knotif = cur;
> +			break;
> +		}
> +	}
> +
> +	/*
> +	 * If we didn't find a notification, it could be that the task was
> +	 * interrupted by a fatal signal between the time we were woken and
> +	 * when we were able to acquire the rw lock.
> +	 */
> +	if (!knotif) {
> +		ret = -ENOENT;
> +		goto out;
> +	}
> +
> +	unotif.id = knotif->id;
> +	unotif.pid = task_pid_vnr(knotif->task);
> +	unotif.data = *(knotif->data);
> +
> +	knotif->state = SECCOMP_NOTIFY_SENT;
> +	wake_up_poll(&filter->notif->wqh, EPOLLOUT | EPOLLWRNORM);
> +	ret = 0;
> +out:
> +	mutex_unlock(&filter->notify_lock);
> +
> +	if (ret == 0 && copy_to_user(buf, &unotif, sizeof(unotif))) {
> +		ret = -EFAULT;
> +
> +		/*
> +		 * Userspace screwed up. To make sure that we keep this
> +		 * notification alive, let's reset it back to INIT. It

Is keeping the notification alive the right thing to do?

If userspace has messed up in something this touchy, it seems unlikely
that it'll to better if we give it a do-over...  I'm not sure whether
killing the whole thing (victim and tracer) is the right thing or not.

> +		 * may have died when we released the lock, so we need to make
> +		 * sure it's still around.
> +		 */
> +		knotif = NULL;
> +		mutex_lock(&filter->notify_lock);
> +		list_for_each_entry(cur, &filter->notif->notifications, list) {
> +			if (cur->id == unotif.id) {
> +				knotif = cur;
> +				break;
> +			}
> +		}
> +
> +		if (knotif) {
> +			knotif->state = SECCOMP_NOTIFY_INIT;
> +			up(&filter->notif->request);
> +		}
> +		mutex_unlock(&filter->notify_lock);
> +	}
> +
> +	return ret;
> +}
> +
> +static long seccomp_notify_send(struct seccomp_filter *filter,
> +				void __user *buf)
> +{
> +	struct seccomp_notif_resp resp = {};
> +	struct seccomp_knotif *knotif = NULL, *cur;
> +	long ret;
> +
> +	if (copy_from_user(&resp, buf, sizeof(resp)))
> +		return -EFAULT;
> +
> +	if (resp.flags)
> +		return -EINVAL;
> +
> +	ret = mutex_lock_interruptible(&filter->notify_lock);
> +	if (ret < 0)
> +		return ret;
> +
> +	list_for_each_entry(cur, &filter->notif->notifications, list) {
> +		if (cur->id == resp.id) {
> +			knotif = cur;
> +			break;
> +		}
> +	}
> +
> +	if (!knotif) {
> +		ret = -ENOENT;
> +		goto out;
> +	}
> +
> +	/* Allow exactly one reply. */
> +	if (knotif->state != SECCOMP_NOTIFY_SENT) {
> +		ret = -EINPROGRESS;
> +		goto out;
> +	}
> +
> +	ret = 0;
> +	knotif->state = SECCOMP_NOTIFY_REPLIED;
> +	knotif->error = resp.error;
> +	knotif->val = resp.val;
> +	complete(&knotif->ready);
> +out:
> +	mutex_unlock(&filter->notify_lock);
> +	return ret;
> +}
> +
> +static long seccomp_notify_id_valid(struct seccomp_filter *filter,
> +				    void __user *buf)
> +{
> +	struct seccomp_knotif *knotif = NULL;
> +	u64 id;
> +	long ret;
> +
> +	if (copy_from_user(&id, buf, sizeof(id)))
> +		return -EFAULT;
> +
> +	ret = mutex_lock_interruptible(&filter->notify_lock);
> +	if (ret < 0)
> +		return ret;
> +
> +	ret = -ENOENT;
> +	list_for_each_entry(knotif, &filter->notif->notifications, list) {
> +		if (knotif->id == id) {
> +			if (knotif->state == SECCOMP_NOTIFY_SENT)
> +				ret = 0;
> +			goto out;
> +		}
> +	}
> +
> +out:
> +	mutex_unlock(&filter->notify_lock);
> +	return ret;
> +}
> +
> +static long seccomp_notify_ioctl(struct file *file, unsigned int cmd,
> +				 unsigned long arg)
> +{
> +	struct seccomp_filter *filter = file->private_data;
> +	void __user *buf = (void __user *)arg;
> +
> +	switch (cmd) {
> +	case SECCOMP_IOCTL_NOTIF_RECV:
> +		return seccomp_notify_recv(filter, buf);
> +	case SECCOMP_IOCTL_NOTIF_SEND:
> +		return seccomp_notify_send(filter, buf);
> +	case SECCOMP_IOCTL_NOTIF_ID_VALID:
> +		return seccomp_notify_id_valid(filter, buf);
> +	default:
> +		return -EINVAL;
> +	}
> +}
> +
> +static __poll_t seccomp_notify_poll(struct file *file,
> +				    struct poll_table_struct *poll_tab)
> +{
> +	struct seccomp_filter *filter = file->private_data;
> +	__poll_t ret = 0;
> +	struct seccomp_knotif *cur;
> +
> +	poll_wait(file, &filter->notif->wqh, poll_tab);
> +
> +	ret = mutex_lock_interruptible(&filter->notify_lock);
> +	if (ret < 0)
> +		return EPOLLERR;
> +
> +	list_for_each_entry(cur, &filter->notif->notifications, list) {
> +		if (cur->state == SECCOMP_NOTIFY_INIT)
> +			ret |= EPOLLIN | EPOLLRDNORM;
> +		if (cur->state == SECCOMP_NOTIFY_SENT)
> +			ret |= EPOLLOUT | EPOLLWRNORM;
> +		if ((ret & EPOLLIN) && (ret & EPOLLOUT))
> +			break;
> +	}
> +
> +	mutex_unlock(&filter->notify_lock);
> +
> +	return ret;
> +}
> +
> +static const struct file_operations seccomp_notify_ops = {
> +	.poll = seccomp_notify_poll,
> +	.release = seccomp_notify_release,
> +	.unlocked_ioctl = seccomp_notify_ioctl,
> +};
> +
> +static struct file *init_listener(struct seccomp_filter *filter)
> +{
> +	struct file *ret = ERR_PTR(-EBUSY);
> +	struct seccomp_filter *cur;
> +
> +	for (cur = current->seccomp.filter; cur; cur = cur->prev) {
> +		if (cur->notif)
> +			goto out;
> +	}
> +
> +	ret = ERR_PTR(-ENOMEM);
> +	filter->notif = kzalloc(sizeof(*(filter->notif)), GFP_KERNEL);
> +	if (!filter->notif)
> +		goto out;
> +
> +	sema_init(&filter->notif->request, 0);
> +	filter->notif->next_id = get_random_u64();
> +	INIT_LIST_HEAD(&filter->notif->notifications);
> +	init_waitqueue_head(&filter->notif->wqh);
> +
> +	ret = anon_inode_getfile("seccomp notify", &seccomp_notify_ops,
> +				 filter, O_RDWR);
> +	if (IS_ERR(ret))
> +		goto out_notif;
> +
> +	/* The file has a reference to it now */
> +	__get_seccomp_filter(filter);
> +
> +out_notif:
> +	if (IS_ERR(ret))
> +		kfree(filter->notif);
> +out:
> +	return ret;
> +}
> +
>  /**
>   * seccomp_set_mode_filter: internal function for setting seccomp filter
>   * @flags:  flags to change filter behavior
> @@ -853,6 +1247,8 @@ static long seccomp_set_mode_filter(unsigned int flags,
>  	const unsigned long seccomp_mode = SECCOMP_MODE_FILTER;
>  	struct seccomp_filter *prepared = NULL;
>  	long ret = -EINVAL;
> +	int listener = -1;
> +	struct file *listener_f = NULL;
>  
>  	/* Validate flags. */
>  	if (flags & ~SECCOMP_FILTER_FLAG_MASK)
> @@ -863,13 +1259,28 @@ static long seccomp_set_mode_filter(unsigned int flags,
>  	if (IS_ERR(prepared))
>  		return PTR_ERR(prepared);
>  
> +	if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
> +		listener = get_unused_fd_flags(O_CLOEXEC);
> +		if (listener < 0) {
> +			ret = listener;
> +			goto out_free;
> +		}
> +
> +		listener_f = init_listener(prepared);
> +		if (IS_ERR(listener_f)) {
> +			put_unused_fd(listener);
> +			ret = PTR_ERR(listener_f);
> +			goto out_free;
> +		}
> +	}
> +
>  	/*
>  	 * Make sure we cannot change seccomp or nnp state via TSYNC
>  	 * while another thread is in the middle of calling exec.
>  	 */
>  	if (flags & SECCOMP_FILTER_FLAG_TSYNC &&
>  	    mutex_lock_killable(&current->signal->cred_guard_mutex))
> -		goto out_free;
> +		goto out_put_fd;
>  
>  	spin_lock_irq(&current->sighand->siglock);
>  
> @@ -887,6 +1298,16 @@ static long seccomp_set_mode_filter(unsigned int flags,
>  	spin_unlock_irq(&current->sighand->siglock);
>  	if (flags & SECCOMP_FILTER_FLAG_TSYNC)
>  		mutex_unlock(&current->signal->cred_guard_mutex);
> +out_put_fd:
> +	if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
> +		if (ret < 0) {
> +			fput(listener_f);
> +			put_unused_fd(listener);
> +		} else {
> +			fd_install(listener, listener_f);
> +			ret = listener;
> +		}
> +	}
>  out_free:
>  	seccomp_filter_free(prepared);
>  	return ret;
> @@ -911,6 +1332,7 @@ static long seccomp_get_action_avail(const char __user *uaction)
>  	case SECCOMP_RET_KILL_THREAD:
>  	case SECCOMP_RET_TRAP:
>  	case SECCOMP_RET_ERRNO:
> +	case SECCOMP_RET_USER_NOTIF:
>  	case SECCOMP_RET_TRACE:
>  	case SECCOMP_RET_LOG:
>  	case SECCOMP_RET_ALLOW:
> @@ -922,6 +1344,20 @@ static long seccomp_get_action_avail(const char __user *uaction)
>  	return 0;
>  }
>  
> +static long seccomp_get_notif_sizes(void __user *usizes)
> +{
> +	struct seccomp_notif_sizes sizes = {
> +		.seccomp_notif = sizeof(struct seccomp_notif),
> +		.seccomp_notif_resp = sizeof(struct seccomp_notif_resp),
> +		.seccomp_data = sizeof(struct seccomp_data),
> +	};
> +
> +	if (copy_to_user(usizes, &sizes, sizeof(sizes)))
> +		return -EFAULT;
> +
> +	return 0;
> +}
> +
>  /* Common entry point for both prctl and syscall. */
>  static long do_seccomp(unsigned int op, unsigned int flags,
>  		       void __user *uargs)
> @@ -938,6 +1374,11 @@ static long do_seccomp(unsigned int op, unsigned int flags,
>  			return -EINVAL;
>  
>  		return seccomp_get_action_avail(uargs);
> +	case SECCOMP_GET_NOTIF_SIZES:
> +		if (flags != 0)
> +			return -EINVAL;
> +
> +		return seccomp_get_notif_sizes(uargs);
>  	default:
>  		return -EINVAL;
>  	}
> @@ -1111,6 +1552,7 @@ long seccomp_get_metadata(struct task_struct *task,
>  #define SECCOMP_RET_KILL_THREAD_NAME	"kill_thread"
>  #define SECCOMP_RET_TRAP_NAME		"trap"
>  #define SECCOMP_RET_ERRNO_NAME		"errno"
> +#define SECCOMP_RET_USER_NOTIF_NAME	"user_notif"
>  #define SECCOMP_RET_TRACE_NAME		"trace"
>  #define SECCOMP_RET_LOG_NAME		"log"
>  #define SECCOMP_RET_ALLOW_NAME		"allow"
> @@ -1120,6 +1562,7 @@ static const char seccomp_actions_avail[] =
>  				SECCOMP_RET_KILL_THREAD_NAME	" "
>  				SECCOMP_RET_TRAP_NAME		" "
>  				SECCOMP_RET_ERRNO_NAME		" "
> +				SECCOMP_RET_USER_NOTIF_NAME     " "
>  				SECCOMP_RET_TRACE_NAME		" "
>  				SECCOMP_RET_LOG_NAME		" "
>  				SECCOMP_RET_ALLOW_NAME;
> @@ -1134,6 +1577,7 @@ static const struct seccomp_log_name seccomp_log_names[] = {
>  	{ SECCOMP_LOG_KILL_THREAD, SECCOMP_RET_KILL_THREAD_NAME },
>  	{ SECCOMP_LOG_TRAP, SECCOMP_RET_TRAP_NAME },
>  	{ SECCOMP_LOG_ERRNO, SECCOMP_RET_ERRNO_NAME },
> +	{ SECCOMP_LOG_USER_NOTIF, SECCOMP_RET_USER_NOTIF_NAME },
>  	{ SECCOMP_LOG_TRACE, SECCOMP_RET_TRACE_NAME },
>  	{ SECCOMP_LOG_LOG, SECCOMP_RET_LOG_NAME },
>  	{ SECCOMP_LOG_ALLOW, SECCOMP_RET_ALLOW_NAME },
> diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> index e1473234968d..5c9768a1b8cd 100644
> --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> @@ -5,6 +5,7 @@
>   * Test code for seccomp bpf.
>   */
>  
> +#define _GNU_SOURCE
>  #include <sys/types.h>
>  
>  /*
> @@ -40,10 +41,12 @@
>  #include <sys/fcntl.h>
>  #include <sys/mman.h>
>  #include <sys/times.h>
> +#include <sys/socket.h>
> +#include <sys/ioctl.h>
>  
> -#define _GNU_SOURCE
>  #include <unistd.h>
>  #include <sys/syscall.h>
> +#include <poll.h>
>  
>  #include "../kselftest_harness.h"
>  
> @@ -133,6 +136,10 @@ struct seccomp_data {
>  #define SECCOMP_GET_ACTION_AVAIL 2
>  #endif
>  
> +#ifndef SECCOMP_GET_NOTIF_SIZES
> +#define SECCOMP_GET_NOTIF_SIZES 3
> +#endif
> +
>  #ifndef SECCOMP_FILTER_FLAG_TSYNC
>  #define SECCOMP_FILTER_FLAG_TSYNC (1UL << 0)
>  #endif
> @@ -154,6 +161,44 @@ struct seccomp_metadata {
>  };
>  #endif
>  
> +#ifndef SECCOMP_FILTER_FLAG_NEW_LISTENER
> +#define SECCOMP_FILTER_FLAG_NEW_LISTENER	(1UL << 3)
> +
> +#define SECCOMP_RET_USER_NOTIF 0x7fc00000U
> +
> +#define SECCOMP_IOC_MAGIC		'!'
> +#define SECCOMP_IO(nr)			_IO(SECCOMP_IOC_MAGIC, nr)
> +#define SECCOMP_IOR(nr, type)		_IOR(SECCOMP_IOC_MAGIC, nr, type)
> +#define SECCOMP_IOW(nr, type)		_IOW(SECCOMP_IOC_MAGIC, nr, type)
> +#define SECCOMP_IOWR(nr, type)		_IOWR(SECCOMP_IOC_MAGIC, nr, type)
> +
> +/* Flags for seccomp notification fd ioctl. */
> +#define SECCOMP_IOCTL_NOTIF_RECV	SECCOMP_IOWR(0, struct seccomp_notif)
> +#define SECCOMP_IOCTL_NOTIF_SEND	SECCOMP_IOWR(1,	\
> +						struct seccomp_notif_resp)
> +#define SECCOMP_IOCTL_NOTIF_ID_VALID	SECCOMP_IOR(2, __u64)
> +
> +struct seccomp_notif {
> +	__u64 id;
> +	__u32 pid;
> +	__u32 flags;
> +	struct seccomp_data data;
> +};
> +
> +struct seccomp_notif_resp {
> +	__u64 id;
> +	__s64 val;
> +	__s32 error;
> +	__u32 flags;
> +};
> +
> +struct seccomp_notif_sizes {
> +	__u16 seccomp_notif;
> +	__u16 seccomp_notif_resp;
> +	__u16 seccomp_data;
> +};
> +#endif
> +
>  #ifndef seccomp
>  int seccomp(unsigned int op, unsigned int flags, void *args)
>  {
> @@ -2077,7 +2122,8 @@ TEST(detect_seccomp_filter_flags)
>  {
>  	unsigned int flags[] = { SECCOMP_FILTER_FLAG_TSYNC,
>  				 SECCOMP_FILTER_FLAG_LOG,
> -				 SECCOMP_FILTER_FLAG_SPEC_ALLOW };
> +				 SECCOMP_FILTER_FLAG_SPEC_ALLOW,
> +				 SECCOMP_FILTER_FLAG_NEW_LISTENER };
>  	unsigned int flag, all_flags;
>  	int i;
>  	long ret;
> @@ -2933,6 +2979,403 @@ TEST(get_metadata)
>  	ASSERT_EQ(0, kill(pid, SIGKILL));
>  }
>  
> +static int user_trap_syscall(int nr, unsigned int flags)
> +{
> +	struct sock_filter filter[] = {
> +		BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
> +			offsetof(struct seccomp_data, nr)),
> +		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1),
> +		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_USER_NOTIF),
> +		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
> +	};
> +
> +	struct sock_fprog prog = {
> +		.len = (unsigned short)ARRAY_SIZE(filter),
> +		.filter = filter,
> +	};
> +
> +	return seccomp(SECCOMP_SET_MODE_FILTER, flags, &prog);
> +}
> +
> +#define USER_NOTIF_MAGIC 116983961184613L
> +TEST(user_notification_basic)
> +{
> +	pid_t pid;
> +	long ret;
> +	int status, listener;
> +	struct seccomp_notif req = {};
> +	struct seccomp_notif_resp resp = {};
> +	struct pollfd pollfd;
> +
> +	struct sock_filter filter[] = {
> +		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
> +	};
> +	struct sock_fprog prog = {
> +		.len = (unsigned short)ARRAY_SIZE(filter),
> +		.filter = filter,
> +	};
> +
> +	pid = fork();
> +	ASSERT_GE(pid, 0);
> +
> +	/* Check that we get -ENOSYS with no listener attached */
> +	if (pid == 0) {
> +		if (user_trap_syscall(__NR_getpid, 0) < 0)
> +			exit(1);
> +		ret = syscall(__NR_getpid);
> +		exit(ret >= 0 || errno != ENOSYS);
> +	}
> +
> +	EXPECT_EQ(waitpid(pid, &status, 0), pid);
> +	EXPECT_EQ(true, WIFEXITED(status));
> +	EXPECT_EQ(0, WEXITSTATUS(status));
> +
> +	/* Add some no-op filters so for grins. */
> +	EXPECT_EQ(seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog), 0);
> +	EXPECT_EQ(seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog), 0);
> +	EXPECT_EQ(seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog), 0);
> +	EXPECT_EQ(seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog), 0);
> +
> +	/* Check that the basic notification machinery works */
> +	listener = user_trap_syscall(__NR_getpid,
> +				     SECCOMP_FILTER_FLAG_NEW_LISTENER);
> +	EXPECT_GE(listener, 0);
> +
> +	/* Installing a second listener in the chain should EBUSY */
> +	EXPECT_EQ(user_trap_syscall(__NR_getpid,
> +				    SECCOMP_FILTER_FLAG_NEW_LISTENER),
> +		  -1);
> +	EXPECT_EQ(errno, EBUSY);
> +
> +	pid = fork();
> +	ASSERT_GE(pid, 0);
> +
> +	if (pid == 0) {
> +		ret = syscall(__NR_getpid);
> +		exit(ret != USER_NOTIF_MAGIC);
> +	}
> +
> +	pollfd.fd = listener;
> +	pollfd.events = POLLIN | POLLOUT;
> +
> +	EXPECT_GT(poll(&pollfd, 1, -1), 0);
> +	EXPECT_EQ(pollfd.revents, POLLIN);
> +
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
> +
> +	pollfd.fd = listener;
> +	pollfd.events = POLLIN | POLLOUT;
> +
> +	EXPECT_GT(poll(&pollfd, 1, -1), 0);
> +	EXPECT_EQ(pollfd.revents, POLLOUT);
> +
> +	EXPECT_EQ(req.data.nr,  __NR_getpid);
> +
> +	resp.id = req.id;
> +	resp.error = 0;
> +	resp.val = USER_NOTIF_MAGIC;
> +
> +	/* check that we make sure flags == 0 */
> +	resp.flags = 1;
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), -1);
> +	EXPECT_EQ(errno, EINVAL);
> +
> +	resp.flags = 0;
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
> +
> +	EXPECT_EQ(waitpid(pid, &status, 0), pid);
> +	EXPECT_EQ(true, WIFEXITED(status));
> +	EXPECT_EQ(0, WEXITSTATUS(status));
> +}
> +
> +TEST(user_notification_kill_in_middle)
> +{
> +	pid_t pid;
> +	long ret;
> +	int listener;
> +	struct seccomp_notif req = {};
> +	struct seccomp_notif_resp resp = {};
> +
> +	listener = user_trap_syscall(__NR_getpid,
> +				     SECCOMP_FILTER_FLAG_NEW_LISTENER);
> +	EXPECT_GE(listener, 0);
> +
> +	/*
> +	 * Check that nothing bad happens when we kill the task in the middle
> +	 * of a syscall.
> +	 */
> +	pid = fork();
> +	ASSERT_GE(pid, 0);
> +
> +	if (pid == 0) {
> +		ret = syscall(__NR_getpid);
> +		exit(ret != USER_NOTIF_MAGIC);
> +	}
> +
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_ID_VALID, &req.id), 0);
> +
> +	EXPECT_EQ(kill(pid, SIGKILL), 0);
> +	EXPECT_EQ(waitpid(pid, NULL, 0), pid);
> +
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_ID_VALID, &req.id), -1);
> +
> +	resp.id = req.id;
> +	ret = ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp);
> +	EXPECT_EQ(ret, -1);
> +	EXPECT_EQ(errno, ENOENT);
> +}
> +
> +static int handled = -1;
> +
> +static void signal_handler(int signal)
> +{
> +	if (write(handled, "c", 1) != 1)
> +		perror("write from signal");
> +}
> +
> +TEST(user_notification_signal)
> +{
> +	pid_t pid;
> +	long ret;
> +	int status, listener, sk_pair[2];
> +	struct seccomp_notif req = {};
> +	struct seccomp_notif_resp resp = {};
> +	char c;
> +
> +	ASSERT_EQ(socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair), 0);
> +
> +	listener = user_trap_syscall(__NR_gettid,
> +				     SECCOMP_FILTER_FLAG_NEW_LISTENER);
> +	EXPECT_GE(listener, 0);
> +
> +	pid = fork();
> +	ASSERT_GE(pid, 0);
> +
> +	if (pid == 0) {
> +		close(sk_pair[0]);
> +		handled = sk_pair[1];
> +		if (signal(SIGUSR1, signal_handler) == SIG_ERR) {
> +			perror("signal");
> +			exit(1);
> +		}
> +		/*
> +		 * ERESTARTSYS behavior is a bit hard to test, because we need
> +		 * to rely on a signal that has not yet been handled. Let's at
> +		 * least check that the error code gets propagated through, and
> +		 * hope that it doesn't break when there is actually a signal :)
> +		 */
> +		ret = syscall(__NR_gettid);
> +		exit(!(ret == -1 && errno == 512));
> +	}
> +
> +	close(sk_pair[1]);
> +
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
> +
> +	EXPECT_EQ(kill(pid, SIGUSR1), 0);
> +
> +	/*
> +	 * Make sure the signal really is delivered, which means we're not
> +	 * stuck in the user notification code any more and the notification
> +	 * should be dead.
> +	 */
> +	EXPECT_EQ(read(sk_pair[0], &c, 1), 1);
> +
> +	resp.id = req.id;
> +	resp.error = -EPERM;
> +	resp.val = 0;
> +
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), -1);
> +	EXPECT_EQ(errno, ENOENT);
> +
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
> +
> +	resp.id = req.id;
> +	resp.error = -512; /* -ERESTARTSYS */
> +	resp.val = 0;
> +
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
> +
> +	EXPECT_EQ(waitpid(pid, &status, 0), pid);
> +	EXPECT_EQ(true, WIFEXITED(status));
> +	EXPECT_EQ(0, WEXITSTATUS(status));
> +}
> +
> +TEST(user_notification_closed_listener)
> +{
> +	pid_t pid;
> +	long ret;
> +	int status, listener;
> +
> +	listener = user_trap_syscall(__NR_getpid,
> +				     SECCOMP_FILTER_FLAG_NEW_LISTENER);
> +	EXPECT_GE(listener, 0);
> +
> +	/*
> +	 * Check that we get an ENOSYS when the listener is closed.
> +	 */
> +	pid = fork();
> +	ASSERT_GE(pid, 0);
> +	if (pid == 0) {
> +		close(listener);
> +		ret = syscall(__NR_getpid);
> +		exit(ret != -1 && errno != ENOSYS);
> +	}
> +
> +	close(listener);
> +
> +	EXPECT_EQ(waitpid(pid, &status, 0), pid);
> +	EXPECT_EQ(true, WIFEXITED(status));
> +	EXPECT_EQ(0, WEXITSTATUS(status));
> +}
> +
> +/*
> + * Check that a pid in a child namespace still shows up as valid in ours.
> + */
> +TEST(user_notification_child_pid_ns)
> +{
> +	pid_t pid;
> +	int status, listener;
> +	struct seccomp_notif req = {};
> +	struct seccomp_notif_resp resp = {};
> +
> +	ASSERT_EQ(unshare(CLONE_NEWPID), 0);
> +
> +	listener = user_trap_syscall(__NR_getpid, SECCOMP_FILTER_FLAG_NEW_LISTENER);
> +	ASSERT_GE(listener, 0);
> +
> +	pid = fork();
> +	ASSERT_GE(pid, 0);
> +
> +	if (pid == 0)
> +		exit(syscall(__NR_getpid) != USER_NOTIF_MAGIC);
> +
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
> +	EXPECT_EQ(req.pid, pid);
> +
> +	resp.id = req.id;
> +	resp.error = 0;
> +	resp.val = USER_NOTIF_MAGIC;
> +
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
> +
> +	EXPECT_EQ(waitpid(pid, &status, 0), pid);
> +	EXPECT_EQ(true, WIFEXITED(status));
> +	EXPECT_EQ(0, WEXITSTATUS(status));
> +	close(listener);
> +}
> +
> +/*
> + * Check that a pid in a sibling (i.e. unrelated) namespace shows up as 0, i.e.
> + * invalid.
> + */
> +TEST(user_notification_sibling_pid_ns)
> +{
> +	pid_t pid, pid2;
> +	int status, listener;
> +	struct seccomp_notif req = {};
> +	struct seccomp_notif_resp resp = {};
> +
> +	listener = user_trap_syscall(__NR_getpid, SECCOMP_FILTER_FLAG_NEW_LISTENER);
> +	ASSERT_GE(listener, 0);
> +
> +	pid = fork();
> +	ASSERT_GE(pid, 0);
> +
> +	if (pid == 0) {
> +		ASSERT_EQ(unshare(CLONE_NEWPID), 0);
> +
> +		pid2 = fork();
> +		ASSERT_GE(pid2, 0);
> +
> +		if (pid2 == 0)
> +			exit(syscall(__NR_getpid) != USER_NOTIF_MAGIC);
> +
> +		EXPECT_EQ(waitpid(pid2, &status, 0), pid2);
> +		EXPECT_EQ(true, WIFEXITED(status));
> +		EXPECT_EQ(0, WEXITSTATUS(status));
> +		exit(WEXITSTATUS(status));
> +	}
> +
> +	/* Create the sibling ns, and sibling in it. */
> +	EXPECT_EQ(unshare(CLONE_NEWPID), 0);
> +	EXPECT_EQ(errno, 0);
> +
> +	pid2 = fork();
> +	EXPECT_GE(pid2, 0);
> +
> +	if (pid2 == 0) {
> +		ASSERT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
> +		/*
> +		 * The pid should be 0, i.e. the task is in some namespace that
> +		 * we can't "see".
> +		 */
> +		ASSERT_EQ(req.pid, 0);
> +
> +		resp.id = req.id;
> +		resp.error = 0;
> +		resp.val = USER_NOTIF_MAGIC;
> +
> +		ASSERT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
> +		exit(0);
> +	}
> +
> +	close(listener);
> +
> +	EXPECT_EQ(waitpid(pid, &status, 0), pid);
> +	EXPECT_EQ(true, WIFEXITED(status));
> +	EXPECT_EQ(0, WEXITSTATUS(status));
> +
> +	EXPECT_EQ(waitpid(pid2, &status, 0), pid2);
> +	EXPECT_EQ(true, WIFEXITED(status));
> +	EXPECT_EQ(0, WEXITSTATUS(status));
> +}
> +
> +TEST(user_notification_fault_recv)
> +{
> +	pid_t pid;
> +	int status, listener;
> +	struct seccomp_notif req = {};
> +	struct seccomp_notif_resp resp = {};
> +
> +	listener = user_trap_syscall(__NR_getpid, SECCOMP_FILTER_FLAG_NEW_LISTENER);
> +	ASSERT_GE(listener, 0);
> +
> +	pid = fork();
> +	ASSERT_GE(pid, 0);
> +
> +	if (pid == 0)
> +		exit(syscall(__NR_getpid) != USER_NOTIF_MAGIC);
> +
> +	/* Do a bad recv() */
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, NULL), -1);
> +	EXPECT_EQ(errno, EFAULT);
> +
> +	/* We should still be able to receive this notification, though. */
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
> +	EXPECT_EQ(req.pid, pid);
> +
> +	resp.id = req.id;
> +	resp.error = 0;
> +	resp.val = USER_NOTIF_MAGIC;
> +
> +	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
> +
> +	EXPECT_EQ(waitpid(pid, &status, 0), pid);
> +	EXPECT_EQ(true, WIFEXITED(status));
> +	EXPECT_EQ(0, WEXITSTATUS(status));
> +}
> +
> +TEST(seccomp_get_notif_sizes)
> +{
> +	struct seccomp_notif_sizes sizes;
> +
> +	EXPECT_EQ(seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes), 0);
> +	EXPECT_EQ(sizes.seccomp_notif, sizeof(struct seccomp_notif));
> +	EXPECT_EQ(sizes.seccomp_notif_resp, sizeof(struct seccomp_notif_resp));
> +}
> +
>  /*
>   * TODO:
>   * - add microbenchmarks
> -- 
> 2.19.1
Tycho Andersen Dec. 3, 2018, 3:52 p.m.
On Sun, Dec 02, 2018 at 11:26:50PM -0600, Serge E. Hallyn wrote:
> On Sun, Dec 02, 2018 at 08:28:26PM -0700, Tycho Andersen wrote:
> > +struct seccomp_knotif {
> > +	/* The struct pid of the task whose filter triggered the notification */
> > +	struct task_struct *task;
> > +
> > +	/* The "cookie" for this request; this is unique for this filter. */
> > +	u64 id;
> > +
> > +	/*
> > +	 * The seccomp data. This pointer is valid the entire time this
> > +	 * notification is active, since it comes from __seccomp_filter which
> 
> define 'active' - is a notification in state REPLIED still active?

Yes,

> Actually while looking at that, I came to wondering - when are knotifs
> freed?  Seems like only during seccomp_notify_release(), i.e. when the
> tracing task stops polling for events?  Is that going to be a problem?
> Or am I misreading this?

they're stack allocated in do_user_notification(). So "active" in this
sense really means "somewhere in do_user_notification()".

> > +	if (ret == 0 && copy_to_user(buf, &unotif, sizeof(unotif))) {
> > +		ret = -EFAULT;
> > +
> > +		/*
> > +		 * Userspace screwed up. To make sure that we keep this
> > +		 * notification alive, let's reset it back to INIT. It
> 
> Is keeping the notification alive the right thing to do?
> 
> If userspace has messed up in something this touchy, it seems unlikely
> that it'll to better if we give it a do-over...  I'm not sure whether
> killing the whole thing (victim and tracer) is the right thing or not.

I suppose we could do that too. I just didn't want to get into a
situation where the notification is lost and the task is stuck because
userspace screwed up here.

Tycho
Serge E. Hallyn Dec. 4, 2018, 12:10 a.m.
On Mon, Dec 03, 2018 at 08:52:11AM -0700, Tycho Andersen wrote:
> On Sun, Dec 02, 2018 at 11:26:50PM -0600, Serge E. Hallyn wrote:
> > On Sun, Dec 02, 2018 at 08:28:26PM -0700, Tycho Andersen wrote:
> > > +struct seccomp_knotif {
> > > +	/* The struct pid of the task whose filter triggered the notification */
> > > +	struct task_struct *task;
> > > +
> > > +	/* The "cookie" for this request; this is unique for this filter. */
> > > +	u64 id;
> > > +
> > > +	/*
> > > +	 * The seccomp data. This pointer is valid the entire time this
> > > +	 * notification is active, since it comes from __seccomp_filter which
> > 
> > define 'active' - is a notification in state REPLIED still active?
> 
> Yes,
> 
> > Actually while looking at that, I came to wondering - when are knotifs
> > freed?  Seems like only during seccomp_notify_release(), i.e. when the
> > tracing task stops polling for events?  Is that going to be a problem?
> > Or am I misreading this?
> 
> they're stack allocated in do_user_notification(). So "active" in this
> sense really means "somewhere in do_user_notification()".

D'oh!  I see, thanks :)

> > > +	if (ret == 0 && copy_to_user(buf, &unotif, sizeof(unotif))) {
> > > +		ret = -EFAULT;
> > > +
> > > +		/*
> > > +		 * Userspace screwed up. To make sure that we keep this
> > > +		 * notification alive, let's reset it back to INIT. It
> > 
> > Is keeping the notification alive the right thing to do?
> > 
> > If userspace has messed up in something this touchy, it seems unlikely
> > that it'll to better if we give it a do-over...  I'm not sure whether
> > killing the whole thing (victim and tracer) is the right thing or not.
> 
> I suppose we could do that too. I just didn't want to get into a
> situation where the notification is lost and the task is stuck because
> userspace screwed up here.

Yeah, that's probably best - I'm just trying to see whether there is
any way that this could be abused.  My underactive imagination hasn't
come up with anything yet.

-serge