[RHEL7,COMMIT] ve/kthread: fix race when work can be added to stopped kthread worker

Submitted by Konstantin Khorenko on March 22, 2018, 10:55 a.m.

Details

Message ID 201803221055.w2MAt6Tg028726@finist_ce7.work
State New
Series "Series without cover letter"
Headers show

Commit Message

Konstantin Khorenko March 22, 2018, 10:55 a.m.
The commit is pushed to "branch-rh7-3.10.0-693.17.1.vz7.45.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-693.17.1.vz7.45.8
------>
commit 6d43ed10e40e842d0c522b24b2052947e1b52389
Author: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Date:   Thu Mar 22 13:55:06 2018 +0300

    ve/kthread: fix race when work can be added to stopped kthread worker
    
    Race can be reproduced with steps below:
    
    1) Add these test patch to increase the race probability:
    
    kernel/kmod.c
    @@ -977,6 +977,8 @@ int call_usermodehelper_fns_ve(struct ve_struct *ve,
            khelper = ve_is_super(ve) ? &khelper_worker : &ve->ve_umh_worker;
    
            if (ve_is_super(ve) || (get_exec_env() == ve)) {
    +               while(ve->is_running)
    +                       cond_resched();
                    err = call_usermodehelper_by(khelper, path, argv, envp, wait, init,
                                                 cleanup, data);
                    goto out_put;
    
    2) Set corepattern to type pipe in CT:
    
    echo "|/bin/dd of=/vz/coredumps/core-%e-%t.%p" > /proc/sys/kernel/core_pattern
    
    3) Produce a segfault inside a container and next try to stop the
    container killing init.
    
    Coredump creates "dd" work and ads it to ve_umh_worker, which is already
    stopped and will never handle these work, and our work will hang
    forever, and container will never stop:
    
    [root@kuchy ~]# cat /proc/6064/stack
    [<ffffffff810a86f9>] __call_usermodehelper_exec+0x179/0x1a0
    [<ffffffff810a8eae>] call_usermodehelper_by+0x5e/0xa0
    [<ffffffff810a901c>] call_usermodehelper_fns_ve+0xec/0x170
    [<ffffffff8128600c>] do_coredump+0x4dc/0xaf0
    [<ffffffff810a1cc5>] get_signal_to_deliver+0x1c5/0x5e0
    [<ffffffff8102a387>] do_signal+0x57/0x6b0
    [<ffffffff8102aa3f>] do_notify_resume+0x5f/0xb0
    [<ffffffff816df6ec>] retint_signal+0x48/0x8c
    [<ffffffffffffffff>] 0xffffffffffffffff
    
    Fix is:
    
    1) When run __call_usermodehelper_exec for ve, check that umh for these
    ve is not PF_EXITING, before adding work. (These is done between memory
    barriers of helper_lock and helper_unlock (implict).)
    
    2) In do_exit first set PF_EXITING flag, and then in ve_stop_ns wait for
    running_helpers to become 0 before stopping umh. (Between setting and
    checking helpers there is a full memory barrier.)
    
    Now we have 2 cases:
    
    If __call_usermodehelper_exec thread comes to helper_lock after do_exit
    thread finished wait_khelpers, then the former already see that umh task
    is PF_EXITING (due to 2) and will not queue the work.
    
    If __call_usermodehelper_exec comes to helper_lock before do_exit passes
    wait_khelpers, the former can see no PF_EXITING and can queue the work,
    and then do helper_unlock. After that if wait_khelpers see
    running_helpers == 0, it will also see the work queued (due to 1) and
    will run these work and complete it normaly.
    
    So ither way work won't hang.
    
    v3: using ve_struct::op_sem outside of ve code is bad so switch to a
    better solution and remove unneded lock.
    v4: actually remove lock in call_usermodehelper_fns_ve
    
    https://jira.sw.ru/browse/PSBM-82490
    
    Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
    Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
---
 include/linux/kmod.h |  1 +
 kernel/kmod.c        | 35 +++++++++++------------------------
 kernel/ve/ve.c       |  2 ++
 3 files changed, 14 insertions(+), 24 deletions(-)

Patch hide | download patch | download mbox

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 9bd5776b1302..cc9b9551a940 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -98,6 +98,7 @@  enum umh_disable_depth {
 extern void usermodehelper_init(void);
 
 extern int __usermodehelper_disable(enum umh_disable_depth depth);
+extern void wait_khelpers(void);
 extern void __usermodehelper_set_disable_depth(enum umh_disable_depth depth);
 
 static inline int usermodehelper_disable(void)
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 9c37e8b3ab01..1d2ea4fb17bb 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -803,6 +803,12 @@  int __usermodehelper_disable(enum umh_disable_depth depth)
 	return -EAGAIN;
 }
 
+void wait_khelpers(void)
+{
+	wait_event(running_helpers_waitq,
+		atomic_read(&running_helpers) == 0);
+}
+
 static void helper_lock(void)
 {
 	atomic_inc(&running_helpers);
@@ -891,7 +897,8 @@  static int __call_usermodehelper_exec(struct subprocess_info *sub_info,
 	if (sub_info->path[0] == '\0')
 		goto out;
 
-	if (usermodehelper_disabled) {
+	if (usermodehelper_disabled ||
+	    (ve && (ve->init_task->flags & PF_EXITING))) {
 		retval = -EBUSY;
 		goto out;
 	}
@@ -975,30 +982,10 @@  int call_usermodehelper_fns_ve(struct ve_struct *ve,
 	if (!ve)
 		return -EFAULT;
 
-	if (ve_is_super(ve) || (get_exec_env() == ve)) {
-		err = call_usermodehelper_by(path, argv, envp, wait, init, cleanup,
-					     data, ve_is_super(ve) ? NULL : ve);
-		goto out_put;
-	}
-
-	if (wait > UMH_WAIT_EXEC) {
-		printk(KERN_ERR "VE#%s: Sleeping call for containers UMH is "
-				"not supported\n", ve->ve_name);
-		err = -EINVAL;
-		goto out_put;
-	}
-
-	down_read(&ve->op_sem);
-	err = -EPIPE;
-	if (!ve->is_running)
-		goto out;
+	err = call_usermodehelper_by(path, argv, envp,
+			wait, init, cleanup, data,
+			ve_is_super(ve) ? NULL : ve);
 
-	err = call_usermodehelper_by(path, argv, envp, wait, init,
-				     cleanup, data, ve);
-
-out:
-	up_read(&ve->op_sem);
-out_put:
 	put_ve(ve);
 	return err;
 }
diff --git a/kernel/ve/ve.c b/kernel/ve/ve.c
index 68f84797a840..f562fb9e5cbf 100644
--- a/kernel/ve/ve.c
+++ b/kernel/ve/ve.c
@@ -556,6 +556,8 @@  void ve_stop_ns(struct pid_namespace *pid_ns)
 	if (!ve->ve_ns || ve->ve_ns->pid_ns != pid_ns)
 		return;
 
+	wait_khelpers();
+
 	down_write(&ve->op_sem);
 	/*
 	 * Here the VE changes its state into "not running".