[v7,1/1] ns: add binfmt_misc to the user namespace

Submitted by Laurent Vivier on Nov. 7, 2019, 2:03 p.m.

Details

Message ID 20191107140304.8426-2-laurent@vivier.eu
State New
Series "ns: introduce binfmt_misc namespace"
Headers show

Commit Message

Laurent Vivier Nov. 7, 2019, 2:03 p.m.
This patch allows to have a different binfmt_misc configuration
for each new user namespace. By default, the binfmt_misc configuration
is the one of the previous level, but if the binfmt_misc filesystem is
mounted in the new namespace a new empty binfmt instance is created and
used in this namespace.

For instance, using "unshare" we can start a chroot of another
architecture and configure the binfmt_misc interpreter without being root
to run the binaries in this chroot.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Acked-by: Andrei Vagin <avagin@gmail.com>
---
 fs/binfmt_misc.c               | 115 +++++++++++++++++++++++++--------
 include/linux/user_namespace.h |  15 +++++
 kernel/user.c                  |  14 ++++
 kernel/user_namespace.c        |   3 +
 4 files changed, 119 insertions(+), 28 deletions(-)

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diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index cdb45829354d..ba5f0d2ade96 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -40,9 +40,6 @@  enum {
 	VERBOSE_STATUS = 1 /* make it zero to save 400 bytes kernel memory */
 };
 
-static LIST_HEAD(entries);
-static int enabled = 1;
-
 enum {Enabled, Magic};
 #define MISC_FMT_PRESERVE_ARGV0 (1 << 31)
 #define MISC_FMT_OPEN_BINARY (1 << 30)
@@ -62,10 +59,7 @@  typedef struct {
 	struct file *interp_file;
 } Node;
 
-static DEFINE_RWLOCK(entries_lock);
 static struct file_system_type bm_fs_type;
-static struct vfsmount *bm_mnt;
-static int entry_count;
 
 /*
  * Max length of the register string.  Determined by:
@@ -82,18 +76,37 @@  static int entry_count;
  */
 #define MAX_REGISTER_LENGTH 1920
 
+static struct binfmt_namespace *binfmt_ns(struct user_namespace *ns)
+{
+	struct binfmt_namespace *b_ns;
+
+	while (ns) {
+		b_ns = READ_ONCE(ns->binfmt_ns);
+		if (b_ns)
+			return b_ns;
+		ns = ns->parent;
+	}
+	/* as the first user namespace is initialized with
+	 * &init_binfmt_ns we should never come here
+	 * but we try to stay safe by logging a warning
+	 * and returning a sane value
+	 */
+	WARN_ON_ONCE(1);
+	return &init_binfmt_ns;
+}
+
 /*
  * Check if we support the binfmt
  * if we do, return the node, else NULL
  * locking is done in load_misc_binary
  */
-static Node *check_file(struct linux_binprm *bprm)
+static Node *check_file(struct binfmt_namespace *ns, struct linux_binprm *bprm)
 {
 	char *p = strrchr(bprm->interp, '.');
 	struct list_head *l;
 
 	/* Walk all the registered handlers. */
-	list_for_each(l, &entries) {
+	list_for_each(l, &ns->entries) {
 		Node *e = list_entry(l, Node, list);
 		char *s;
 		int j;
@@ -135,17 +148,18 @@  static int load_misc_binary(struct linux_binprm *bprm)
 	struct file *interp_file = NULL;
 	int retval;
 	int fd_binary = -1;
+	struct binfmt_namespace *ns = binfmt_ns(current_user_ns());
 
 	retval = -ENOEXEC;
-	if (!enabled)
+	if (!ns->enabled)
 		return retval;
 
 	/* to keep locking time low, we copy the interpreter string */
-	read_lock(&entries_lock);
-	fmt = check_file(bprm);
+	read_lock(&ns->entries_lock);
+	fmt = check_file(ns, bprm);
 	if (fmt)
 		dget(fmt->dentry);
-	read_unlock(&entries_lock);
+	read_unlock(&ns->entries_lock);
 	if (!fmt)
 		return retval;
 
@@ -611,19 +625,19 @@  static void bm_evict_inode(struct inode *inode)
 	kfree(e);
 }
 
-static void kill_node(Node *e)
+static void kill_node(struct binfmt_namespace *ns, Node *e)
 {
 	struct dentry *dentry;
 
-	write_lock(&entries_lock);
+	write_lock(&ns->entries_lock);
 	list_del_init(&e->list);
-	write_unlock(&entries_lock);
+	write_unlock(&ns->entries_lock);
 
 	dentry = e->dentry;
 	drop_nlink(d_inode(dentry));
 	d_drop(dentry);
 	dput(dentry);
-	simple_release_fs(&bm_mnt, &entry_count);
+	simple_release_fs(&ns->bm_mnt, &ns->entry_count);
 }
 
 /* /<entry> */
@@ -653,6 +667,9 @@  static ssize_t bm_entry_write(struct file *file, const char __user *buffer,
 	struct dentry *root;
 	Node *e = file_inode(file)->i_private;
 	int res = parse_command(buffer, count);
+	struct binfmt_namespace *ns;
+
+	ns = binfmt_ns(file->f_path.dentry->d_sb->s_user_ns);
 
 	switch (res) {
 	case 1:
@@ -669,7 +686,7 @@  static ssize_t bm_entry_write(struct file *file, const char __user *buffer,
 		inode_lock(d_inode(root));
 
 		if (!list_empty(&e->list))
-			kill_node(e);
+			kill_node(ns, e);
 
 		inode_unlock(d_inode(root));
 		break;
@@ -695,6 +712,7 @@  static ssize_t bm_register_write(struct file *file, const char __user *buffer,
 	struct inode *inode;
 	struct super_block *sb = file_inode(file)->i_sb;
 	struct dentry *root = sb->s_root, *dentry;
+	struct binfmt_namespace *ns;
 	int err = 0;
 
 	e = create_entry(buffer, count);
@@ -718,7 +736,9 @@  static ssize_t bm_register_write(struct file *file, const char __user *buffer,
 	if (!inode)
 		goto out2;
 
-	err = simple_pin_fs(&bm_fs_type, &bm_mnt, &entry_count);
+	ns = binfmt_ns(file->f_path.dentry->d_sb->s_user_ns);
+	err = simple_pin_fs(&bm_fs_type, &ns->bm_mnt,
+			    &ns->entry_count);
 	if (err) {
 		iput(inode);
 		inode = NULL;
@@ -727,12 +747,16 @@  static ssize_t bm_register_write(struct file *file, const char __user *buffer,
 
 	if (e->flags & MISC_FMT_OPEN_FILE) {
 		struct file *f;
+		const struct cred *old_cred;
 
+		old_cred = override_creds(file->f_cred);
 		f = open_exec(e->interpreter);
+		revert_creds(old_cred);
 		if (IS_ERR(f)) {
 			err = PTR_ERR(f);
 			pr_notice("register: failed to install interpreter file %s\n", e->interpreter);
-			simple_release_fs(&bm_mnt, &entry_count);
+			simple_release_fs(&ns->bm_mnt,
+					  &ns->entry_count);
 			iput(inode);
 			inode = NULL;
 			goto out2;
@@ -745,9 +769,9 @@  static ssize_t bm_register_write(struct file *file, const char __user *buffer,
 	inode->i_fop = &bm_entry_operations;
 
 	d_instantiate(dentry, inode);
-	write_lock(&entries_lock);
-	list_add(&e->list, &entries);
-	write_unlock(&entries_lock);
+	write_lock(&ns->entries_lock);
+	list_add(&e->list, &ns->entries);
+	write_unlock(&ns->entries_lock);
 
 	err = 0;
 out2:
@@ -772,7 +796,9 @@  static const struct file_operations bm_register_operations = {
 static ssize_t
 bm_status_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
 {
-	char *s = enabled ? "enabled\n" : "disabled\n";
+	struct binfmt_namespace *ns =
+				binfmt_ns(file->f_path.dentry->d_sb->s_user_ns);
+	char *s = ns->enabled ? "enabled\n" : "disabled\n";
 
 	return simple_read_from_buffer(buf, nbytes, ppos, s, strlen(s));
 }
@@ -780,25 +806,28 @@  bm_status_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
 static ssize_t bm_status_write(struct file *file, const char __user *buffer,
 		size_t count, loff_t *ppos)
 {
+	struct binfmt_namespace *ns;
 	int res = parse_command(buffer, count);
 	struct dentry *root;
 
+	ns = binfmt_ns(file->f_path.dentry->d_sb->s_user_ns);
 	switch (res) {
 	case 1:
 		/* Disable all handlers. */
-		enabled = 0;
+		ns->enabled = 0;
 		break;
 	case 2:
 		/* Enable all handlers. */
-		enabled = 1;
+		ns->enabled = 1;
 		break;
 	case 3:
 		/* Delete all handlers. */
 		root = file_inode(file)->i_sb->s_root;
 		inode_lock(d_inode(root));
 
-		while (!list_empty(&entries))
-			kill_node(list_first_entry(&entries, Node, list));
+		while (!list_empty(&ns->entries))
+			kill_node(ns, list_first_entry(&ns->entries,
+						       Node, list));
 
 		inode_unlock(d_inode(root));
 		break;
@@ -825,24 +854,53 @@  static const struct super_operations s_ops = {
 static int bm_fill_super(struct super_block *sb, struct fs_context *fc)
 {
 	int err;
+	struct user_namespace *ns = sb->s_user_ns;
 	static const struct tree_descr bm_files[] = {
 		[2] = {"status", &bm_status_operations, S_IWUSR|S_IRUGO},
 		[3] = {"register", &bm_register_operations, S_IWUSR},
 		/* last one */ {""}
 	};
 
+	/* create a new binfmt namespace
+	 * if we are not in the first user namespace
+	 * but the binfmt namespace is the first one
+	 */
+	if (READ_ONCE(ns->binfmt_ns) == NULL) {
+		struct binfmt_namespace *new_ns;
+
+		new_ns = kmalloc(sizeof(struct binfmt_namespace),
+				 GFP_KERNEL);
+		if (new_ns == NULL)
+			return -ENOMEM;
+		INIT_LIST_HEAD(&new_ns->entries);
+		new_ns->enabled = 1;
+		rwlock_init(&new_ns->entries_lock);
+		new_ns->bm_mnt = NULL;
+		new_ns->entry_count = 0;
+		/* ensure new_ns is completely initialized before sharing it */
+		smp_wmb();
+		WRITE_ONCE(ns->binfmt_ns, new_ns);
+	}
+
 	err = simple_fill_super(sb, BINFMTFS_MAGIC, bm_files);
 	if (!err)
 		sb->s_op = &s_ops;
 	return err;
 }
 
+static void bm_free(struct fs_context *fc)
+{
+	if (fc->s_fs_info)
+		put_user_ns(fc->s_fs_info);
+}
+
 static int bm_get_tree(struct fs_context *fc)
 {
-	return get_tree_single(fc, bm_fill_super);
+	return get_tree_keyed(fc, bm_fill_super, get_user_ns(fc->user_ns));
 }
 
 static const struct fs_context_operations bm_context_ops = {
+	.free		= bm_free,
 	.get_tree	= bm_get_tree,
 };
 
@@ -861,6 +919,7 @@  static struct file_system_type bm_fs_type = {
 	.owner		= THIS_MODULE,
 	.name		= "binfmt_misc",
 	.init_fs_context = bm_init_fs_context,
+	.fs_flags	= FS_USERNS_MOUNT,
 	.kill_sb	= kill_litter_super,
 };
 MODULE_ALIAS_FS("binfmt_misc");
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index fb9f4f799554..16e6f3a97a01 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -52,6 +52,18 @@  enum ucount_type {
 	UCOUNT_COUNTS,
 };
 
+#if IS_ENABLED(CONFIG_BINFMT_MISC)
+struct binfmt_namespace {
+	struct list_head entries;
+	rwlock_t entries_lock;
+	int enabled;
+	struct vfsmount *bm_mnt;
+	int entry_count;
+} __randomize_layout;
+
+extern struct binfmt_namespace init_binfmt_ns;
+#endif
+
 struct user_namespace {
 	struct uid_gid_map	uid_map;
 	struct uid_gid_map	gid_map;
@@ -86,6 +98,9 @@  struct user_namespace {
 #endif
 	struct ucounts		*ucounts;
 	int ucount_max[UCOUNT_COUNTS];
+#if IS_ENABLED(CONFIG_BINFMT_MISC)
+	struct binfmt_namespace *binfmt_ns;
+#endif
 } __randomize_layout;
 
 struct ucounts {
diff --git a/kernel/user.c b/kernel/user.c
index 5235d7f49982..092b2b4d47a6 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -20,6 +20,17 @@ 
 #include <linux/user_namespace.h>
 #include <linux/proc_ns.h>
 
+#if IS_ENABLED(CONFIG_BINFMT_MISC)
+struct binfmt_namespace init_binfmt_ns = {
+	.entries = LIST_HEAD_INIT(init_binfmt_ns.entries),
+	.enabled = 1,
+	.entries_lock = __RW_LOCK_UNLOCKED(init_binfmt_ns.entries_lock),
+	.bm_mnt = NULL,
+	.entry_count = 0,
+};
+EXPORT_SYMBOL_GPL(init_binfmt_ns);
+#endif
+
 /*
  * userns count is 1 for root user, 1 for init_uts_ns,
  * and 1 for... ?
@@ -67,6 +78,9 @@  struct user_namespace init_user_ns = {
 	.keyring_name_list = LIST_HEAD_INIT(init_user_ns.keyring_name_list),
 	.keyring_sem = __RWSEM_INITIALIZER(init_user_ns.keyring_sem),
 #endif
+#if IS_ENABLED(CONFIG_BINFMT_MISC)
+	.binfmt_ns = &init_binfmt_ns,
+#endif
 };
 EXPORT_SYMBOL_GPL(init_user_ns);
 
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 8eadadc478f9..f42c32269e20 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -191,6 +191,9 @@  static void free_user_ns(struct work_struct *work)
 			kfree(ns->projid_map.forward);
 			kfree(ns->projid_map.reverse);
 		}
+#if IS_ENABLED(CONFIG_BINFMT_MISC)
+		kfree(ns->binfmt_ns);
+#endif
 		retire_userns_sysctls(ns);
 		key_free_user_ns(ns);
 		ns_free_inum(&ns->ns);

Comments

Laurent Vivier Dec. 3, 2019, 9:44 a.m.
Ping

Thanks,
Laurent

Le 07/11/2019 à 15:03, Laurent Vivier a écrit :
> This patch allows to have a different binfmt_misc configuration
> for each new user namespace. By default, the binfmt_misc configuration
> is the one of the previous level, but if the binfmt_misc filesystem is
> mounted in the new namespace a new empty binfmt instance is created and
> used in this namespace.
> 
> For instance, using "unshare" we can start a chroot of another
> architecture and configure the binfmt_misc interpreter without being root
> to run the binaries in this chroot.
> 
> Signed-off-by: Laurent Vivier <laurent@vivier.eu>
> Acked-by: Andrei Vagin <avagin@gmail.com>
> ---
>  fs/binfmt_misc.c               | 115 +++++++++++++++++++++++++--------
>  include/linux/user_namespace.h |  15 +++++
>  kernel/user.c                  |  14 ++++
>  kernel/user_namespace.c        |   3 +
>  4 files changed, 119 insertions(+), 28 deletions(-)
> 
> diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
> index cdb45829354d..ba5f0d2ade96 100644
> --- a/fs/binfmt_misc.c
> +++ b/fs/binfmt_misc.c
> @@ -40,9 +40,6 @@ enum {
>  	VERBOSE_STATUS = 1 /* make it zero to save 400 bytes kernel memory */
>  };
>  
> -static LIST_HEAD(entries);
> -static int enabled = 1;
> -
>  enum {Enabled, Magic};
>  #define MISC_FMT_PRESERVE_ARGV0 (1 << 31)
>  #define MISC_FMT_OPEN_BINARY (1 << 30)
> @@ -62,10 +59,7 @@ typedef struct {
>  	struct file *interp_file;
>  } Node;
>  
> -static DEFINE_RWLOCK(entries_lock);
>  static struct file_system_type bm_fs_type;
> -static struct vfsmount *bm_mnt;
> -static int entry_count;
>  
>  /*
>   * Max length of the register string.  Determined by:
> @@ -82,18 +76,37 @@ static int entry_count;
>   */
>  #define MAX_REGISTER_LENGTH 1920
>  
> +static struct binfmt_namespace *binfmt_ns(struct user_namespace *ns)
> +{
> +	struct binfmt_namespace *b_ns;
> +
> +	while (ns) {
> +		b_ns = READ_ONCE(ns->binfmt_ns);
> +		if (b_ns)
> +			return b_ns;
> +		ns = ns->parent;
> +	}
> +	/* as the first user namespace is initialized with
> +	 * &init_binfmt_ns we should never come here
> +	 * but we try to stay safe by logging a warning
> +	 * and returning a sane value
> +	 */
> +	WARN_ON_ONCE(1);
> +	return &init_binfmt_ns;
> +}
> +
>  /*
>   * Check if we support the binfmt
>   * if we do, return the node, else NULL
>   * locking is done in load_misc_binary
>   */
> -static Node *check_file(struct linux_binprm *bprm)
> +static Node *check_file(struct binfmt_namespace *ns, struct linux_binprm *bprm)
>  {
>  	char *p = strrchr(bprm->interp, '.');
>  	struct list_head *l;
>  
>  	/* Walk all the registered handlers. */
> -	list_for_each(l, &entries) {
> +	list_for_each(l, &ns->entries) {
>  		Node *e = list_entry(l, Node, list);
>  		char *s;
>  		int j;
> @@ -135,17 +148,18 @@ static int load_misc_binary(struct linux_binprm *bprm)
>  	struct file *interp_file = NULL;
>  	int retval;
>  	int fd_binary = -1;
> +	struct binfmt_namespace *ns = binfmt_ns(current_user_ns());
>  
>  	retval = -ENOEXEC;
> -	if (!enabled)
> +	if (!ns->enabled)
>  		return retval;
>  
>  	/* to keep locking time low, we copy the interpreter string */
> -	read_lock(&entries_lock);
> -	fmt = check_file(bprm);
> +	read_lock(&ns->entries_lock);
> +	fmt = check_file(ns, bprm);
>  	if (fmt)
>  		dget(fmt->dentry);
> -	read_unlock(&entries_lock);
> +	read_unlock(&ns->entries_lock);
>  	if (!fmt)
>  		return retval;
>  
> @@ -611,19 +625,19 @@ static void bm_evict_inode(struct inode *inode)
>  	kfree(e);
>  }
>  
> -static void kill_node(Node *e)
> +static void kill_node(struct binfmt_namespace *ns, Node *e)
>  {
>  	struct dentry *dentry;
>  
> -	write_lock(&entries_lock);
> +	write_lock(&ns->entries_lock);
>  	list_del_init(&e->list);
> -	write_unlock(&entries_lock);
> +	write_unlock(&ns->entries_lock);
>  
>  	dentry = e->dentry;
>  	drop_nlink(d_inode(dentry));
>  	d_drop(dentry);
>  	dput(dentry);
> -	simple_release_fs(&bm_mnt, &entry_count);
> +	simple_release_fs(&ns->bm_mnt, &ns->entry_count);
>  }
>  
>  /* /<entry> */
> @@ -653,6 +667,9 @@ static ssize_t bm_entry_write(struct file *file, const char __user *buffer,
>  	struct dentry *root;
>  	Node *e = file_inode(file)->i_private;
>  	int res = parse_command(buffer, count);
> +	struct binfmt_namespace *ns;
> +
> +	ns = binfmt_ns(file->f_path.dentry->d_sb->s_user_ns);
>  
>  	switch (res) {
>  	case 1:
> @@ -669,7 +686,7 @@ static ssize_t bm_entry_write(struct file *file, const char __user *buffer,
>  		inode_lock(d_inode(root));
>  
>  		if (!list_empty(&e->list))
> -			kill_node(e);
> +			kill_node(ns, e);
>  
>  		inode_unlock(d_inode(root));
>  		break;
> @@ -695,6 +712,7 @@ static ssize_t bm_register_write(struct file *file, const char __user *buffer,
>  	struct inode *inode;
>  	struct super_block *sb = file_inode(file)->i_sb;
>  	struct dentry *root = sb->s_root, *dentry;
> +	struct binfmt_namespace *ns;
>  	int err = 0;
>  
>  	e = create_entry(buffer, count);
> @@ -718,7 +736,9 @@ static ssize_t bm_register_write(struct file *file, const char __user *buffer,
>  	if (!inode)
>  		goto out2;
>  
> -	err = simple_pin_fs(&bm_fs_type, &bm_mnt, &entry_count);
> +	ns = binfmt_ns(file->f_path.dentry->d_sb->s_user_ns);
> +	err = simple_pin_fs(&bm_fs_type, &ns->bm_mnt,
> +			    &ns->entry_count);
>  	if (err) {
>  		iput(inode);
>  		inode = NULL;
> @@ -727,12 +747,16 @@ static ssize_t bm_register_write(struct file *file, const char __user *buffer,
>  
>  	if (e->flags & MISC_FMT_OPEN_FILE) {
>  		struct file *f;
> +		const struct cred *old_cred;
>  
> +		old_cred = override_creds(file->f_cred);
>  		f = open_exec(e->interpreter);
> +		revert_creds(old_cred);
>  		if (IS_ERR(f)) {
>  			err = PTR_ERR(f);
>  			pr_notice("register: failed to install interpreter file %s\n", e->interpreter);
> -			simple_release_fs(&bm_mnt, &entry_count);
> +			simple_release_fs(&ns->bm_mnt,
> +					  &ns->entry_count);
>  			iput(inode);
>  			inode = NULL;
>  			goto out2;
> @@ -745,9 +769,9 @@ static ssize_t bm_register_write(struct file *file, const char __user *buffer,
>  	inode->i_fop = &bm_entry_operations;
>  
>  	d_instantiate(dentry, inode);
> -	write_lock(&entries_lock);
> -	list_add(&e->list, &entries);
> -	write_unlock(&entries_lock);
> +	write_lock(&ns->entries_lock);
> +	list_add(&e->list, &ns->entries);
> +	write_unlock(&ns->entries_lock);
>  
>  	err = 0;
>  out2:
> @@ -772,7 +796,9 @@ static const struct file_operations bm_register_operations = {
>  static ssize_t
>  bm_status_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
>  {
> -	char *s = enabled ? "enabled\n" : "disabled\n";
> +	struct binfmt_namespace *ns =
> +				binfmt_ns(file->f_path.dentry->d_sb->s_user_ns);
> +	char *s = ns->enabled ? "enabled\n" : "disabled\n";
>  
>  	return simple_read_from_buffer(buf, nbytes, ppos, s, strlen(s));
>  }
> @@ -780,25 +806,28 @@ bm_status_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
>  static ssize_t bm_status_write(struct file *file, const char __user *buffer,
>  		size_t count, loff_t *ppos)
>  {
> +	struct binfmt_namespace *ns;
>  	int res = parse_command(buffer, count);
>  	struct dentry *root;
>  
> +	ns = binfmt_ns(file->f_path.dentry->d_sb->s_user_ns);
>  	switch (res) {
>  	case 1:
>  		/* Disable all handlers. */
> -		enabled = 0;
> +		ns->enabled = 0;
>  		break;
>  	case 2:
>  		/* Enable all handlers. */
> -		enabled = 1;
> +		ns->enabled = 1;
>  		break;
>  	case 3:
>  		/* Delete all handlers. */
>  		root = file_inode(file)->i_sb->s_root;
>  		inode_lock(d_inode(root));
>  
> -		while (!list_empty(&entries))
> -			kill_node(list_first_entry(&entries, Node, list));
> +		while (!list_empty(&ns->entries))
> +			kill_node(ns, list_first_entry(&ns->entries,
> +						       Node, list));
>  
>  		inode_unlock(d_inode(root));
>  		break;
> @@ -825,24 +854,53 @@ static const struct super_operations s_ops = {
>  static int bm_fill_super(struct super_block *sb, struct fs_context *fc)
>  {
>  	int err;
> +	struct user_namespace *ns = sb->s_user_ns;
>  	static const struct tree_descr bm_files[] = {
>  		[2] = {"status", &bm_status_operations, S_IWUSR|S_IRUGO},
>  		[3] = {"register", &bm_register_operations, S_IWUSR},
>  		/* last one */ {""}
>  	};
>  
> +	/* create a new binfmt namespace
> +	 * if we are not in the first user namespace
> +	 * but the binfmt namespace is the first one
> +	 */
> +	if (READ_ONCE(ns->binfmt_ns) == NULL) {
> +		struct binfmt_namespace *new_ns;
> +
> +		new_ns = kmalloc(sizeof(struct binfmt_namespace),
> +				 GFP_KERNEL);
> +		if (new_ns == NULL)
> +			return -ENOMEM;
> +		INIT_LIST_HEAD(&new_ns->entries);
> +		new_ns->enabled = 1;
> +		rwlock_init(&new_ns->entries_lock);
> +		new_ns->bm_mnt = NULL;
> +		new_ns->entry_count = 0;
> +		/* ensure new_ns is completely initialized before sharing it */
> +		smp_wmb();
> +		WRITE_ONCE(ns->binfmt_ns, new_ns);
> +	}
> +
>  	err = simple_fill_super(sb, BINFMTFS_MAGIC, bm_files);
>  	if (!err)
>  		sb->s_op = &s_ops;
>  	return err;
>  }
>  
> +static void bm_free(struct fs_context *fc)
> +{
> +	if (fc->s_fs_info)
> +		put_user_ns(fc->s_fs_info);
> +}
> +
>  static int bm_get_tree(struct fs_context *fc)
>  {
> -	return get_tree_single(fc, bm_fill_super);
> +	return get_tree_keyed(fc, bm_fill_super, get_user_ns(fc->user_ns));
>  }
>  
>  static const struct fs_context_operations bm_context_ops = {
> +	.free		= bm_free,
>  	.get_tree	= bm_get_tree,
>  };
>  
> @@ -861,6 +919,7 @@ static struct file_system_type bm_fs_type = {
>  	.owner		= THIS_MODULE,
>  	.name		= "binfmt_misc",
>  	.init_fs_context = bm_init_fs_context,
> +	.fs_flags	= FS_USERNS_MOUNT,
>  	.kill_sb	= kill_litter_super,
>  };
>  MODULE_ALIAS_FS("binfmt_misc");
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index fb9f4f799554..16e6f3a97a01 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -52,6 +52,18 @@ enum ucount_type {
>  	UCOUNT_COUNTS,
>  };
>  
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +struct binfmt_namespace {
> +	struct list_head entries;
> +	rwlock_t entries_lock;
> +	int enabled;
> +	struct vfsmount *bm_mnt;
> +	int entry_count;
> +} __randomize_layout;
> +
> +extern struct binfmt_namespace init_binfmt_ns;
> +#endif
> +
>  struct user_namespace {
>  	struct uid_gid_map	uid_map;
>  	struct uid_gid_map	gid_map;
> @@ -86,6 +98,9 @@ struct user_namespace {
>  #endif
>  	struct ucounts		*ucounts;
>  	int ucount_max[UCOUNT_COUNTS];
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +	struct binfmt_namespace *binfmt_ns;
> +#endif
>  } __randomize_layout;
>  
>  struct ucounts {
> diff --git a/kernel/user.c b/kernel/user.c
> index 5235d7f49982..092b2b4d47a6 100644
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -20,6 +20,17 @@
>  #include <linux/user_namespace.h>
>  #include <linux/proc_ns.h>
>  
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +struct binfmt_namespace init_binfmt_ns = {
> +	.entries = LIST_HEAD_INIT(init_binfmt_ns.entries),
> +	.enabled = 1,
> +	.entries_lock = __RW_LOCK_UNLOCKED(init_binfmt_ns.entries_lock),
> +	.bm_mnt = NULL,
> +	.entry_count = 0,
> +};
> +EXPORT_SYMBOL_GPL(init_binfmt_ns);
> +#endif
> +
>  /*
>   * userns count is 1 for root user, 1 for init_uts_ns,
>   * and 1 for... ?
> @@ -67,6 +78,9 @@ struct user_namespace init_user_ns = {
>  	.keyring_name_list = LIST_HEAD_INIT(init_user_ns.keyring_name_list),
>  	.keyring_sem = __RWSEM_INITIALIZER(init_user_ns.keyring_sem),
>  #endif
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +	.binfmt_ns = &init_binfmt_ns,
> +#endif
>  };
>  EXPORT_SYMBOL_GPL(init_user_ns);
>  
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 8eadadc478f9..f42c32269e20 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -191,6 +191,9 @@ static void free_user_ns(struct work_struct *work)
>  			kfree(ns->projid_map.forward);
>  			kfree(ns->projid_map.reverse);
>  		}
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +		kfree(ns->binfmt_ns);
> +#endif
>  		retire_userns_sysctls(ns);
>  		key_free_user_ns(ns);
>  		ns_free_inum(&ns->ns);
>
Christian Brauner Dec. 3, 2019, 3:40 p.m.
On Thu, Nov 07, 2019 at 03:03:04PM +0100, Laurent Vivier wrote:
> This patch allows to have a different binfmt_misc configuration
> for each new user namespace. By default, the binfmt_misc configuration
> is the one of the previous level, but if the binfmt_misc filesystem is
> mounted in the new namespace a new empty binfmt instance is created and
> used in this namespace.
> 
> For instance, using "unshare" we can start a chroot of another
> architecture and configure the binfmt_misc interpreter without being root
> to run the binaries in this chroot.
> 
> Signed-off-by: Laurent Vivier <laurent@vivier.eu>
> Acked-by: Andrei Vagin <avagin@gmail.com>
> ---
>  fs/binfmt_misc.c               | 115 +++++++++++++++++++++++++--------
>  include/linux/user_namespace.h |  15 +++++
>  kernel/user.c                  |  14 ++++
>  kernel/user_namespace.c        |   3 +
>  4 files changed, 119 insertions(+), 28 deletions(-)
> 
> diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
> index cdb45829354d..ba5f0d2ade96 100644
> --- a/fs/binfmt_misc.c
> +++ b/fs/binfmt_misc.c
> @@ -40,9 +40,6 @@ enum {
>  	VERBOSE_STATUS = 1 /* make it zero to save 400 bytes kernel memory */
>  };
>  
> -static LIST_HEAD(entries);
> -static int enabled = 1;
> -
>  enum {Enabled, Magic};
>  #define MISC_FMT_PRESERVE_ARGV0 (1 << 31)
>  #define MISC_FMT_OPEN_BINARY (1 << 30)
> @@ -62,10 +59,7 @@ typedef struct {
>  	struct file *interp_file;
>  } Node;
>  
> -static DEFINE_RWLOCK(entries_lock);
>  static struct file_system_type bm_fs_type;
> -static struct vfsmount *bm_mnt;
> -static int entry_count;
>  
>  /*
>   * Max length of the register string.  Determined by:
> @@ -82,18 +76,37 @@ static int entry_count;
>   */
>  #define MAX_REGISTER_LENGTH 1920
>  
> +static struct binfmt_namespace *binfmt_ns(struct user_namespace *ns)
> +{
> +	struct binfmt_namespace *b_ns;
> +
> +	while (ns) {
> +		b_ns = READ_ONCE(ns->binfmt_ns);
> +		if (b_ns)
> +			return b_ns;
> +		ns = ns->parent;
> +	}
> +	/* as the first user namespace is initialized with
> +	 * &init_binfmt_ns we should never come here
> +	 * but we try to stay safe by logging a warning
> +	 * and returning a sane value
> +	 */
> +	WARN_ON_ONCE(1);
> +	return &init_binfmt_ns;
> +}
> +
>  /*
>   * Check if we support the binfmt
>   * if we do, return the node, else NULL
>   * locking is done in load_misc_binary
>   */
> -static Node *check_file(struct linux_binprm *bprm)
> +static Node *check_file(struct binfmt_namespace *ns, struct linux_binprm *bprm)
>  {
>  	char *p = strrchr(bprm->interp, '.');
>  	struct list_head *l;
>  
>  	/* Walk all the registered handlers. */
> -	list_for_each(l, &entries) {
> +	list_for_each(l, &ns->entries) {
>  		Node *e = list_entry(l, Node, list);
>  		char *s;
>  		int j;
> @@ -135,17 +148,18 @@ static int load_misc_binary(struct linux_binprm *bprm)
>  	struct file *interp_file = NULL;
>  	int retval;
>  	int fd_binary = -1;
> +	struct binfmt_namespace *ns = binfmt_ns(current_user_ns());
>  
>  	retval = -ENOEXEC;
> -	if (!enabled)
> +	if (!ns->enabled)
>  		return retval;
>  
>  	/* to keep locking time low, we copy the interpreter string */
> -	read_lock(&entries_lock);
> -	fmt = check_file(bprm);
> +	read_lock(&ns->entries_lock);
> +	fmt = check_file(ns, bprm);
>  	if (fmt)
>  		dget(fmt->dentry);
> -	read_unlock(&entries_lock);
> +	read_unlock(&ns->entries_lock);
>  	if (!fmt)
>  		return retval;
>  
> @@ -611,19 +625,19 @@ static void bm_evict_inode(struct inode *inode)
>  	kfree(e);
>  }
>  
> -static void kill_node(Node *e)
> +static void kill_node(struct binfmt_namespace *ns, Node *e)
>  {
>  	struct dentry *dentry;
>  
> -	write_lock(&entries_lock);
> +	write_lock(&ns->entries_lock);
>  	list_del_init(&e->list);
> -	write_unlock(&entries_lock);
> +	write_unlock(&ns->entries_lock);
>  
>  	dentry = e->dentry;
>  	drop_nlink(d_inode(dentry));
>  	d_drop(dentry);
>  	dput(dentry);
> -	simple_release_fs(&bm_mnt, &entry_count);
> +	simple_release_fs(&ns->bm_mnt, &ns->entry_count);
>  }
>  
>  /* /<entry> */
> @@ -653,6 +667,9 @@ static ssize_t bm_entry_write(struct file *file, const char __user *buffer,
>  	struct dentry *root;
>  	Node *e = file_inode(file)->i_private;
>  	int res = parse_command(buffer, count);
> +	struct binfmt_namespace *ns;
> +
> +	ns = binfmt_ns(file->f_path.dentry->d_sb->s_user_ns);

Sorry for being so late to the party.

The naked dereferences here are not very pretty and also likely mean you
access the wrong dentry when you do (weird but possible) things like:

mount -t overlay overlay -o lowerdir=/proc/sys/fs/binfmt_misc,workdir=/work,upperdir=/upper /merged

(which might already cause trouble in other parts of this code)

so I think it's better if you replace 
file->f_path.dentry->d_sb->s_user_ns
with
file_dentry(file)->d_sb->s_user_ns
in places where you do naked dereferences now.
Henning Schild Dec. 13, 2019, 5:51 p.m.
Hi all,

that is a very useful contribution, which will hopefully be considered.

Tested-by: Henning Schild <henning.schild@siemens.com>

Having a private binfmt_misc allows having different interpreters
(paths to them) for the same format in each namespace. Imagine chroots
of different architectures where the static qemu is in a different
place i.e. /usr/bin/qemu-arm-static, vs
/sbin/qemu-arm-static, or /home/dev/qemu/qemu-arm-static

And it also eventually allows setting up binfmt_misc for a chroot
without requiring root.

Henning

Am Tue, 3 Dec 2019 10:44:44 +0100
schrieb Laurent Vivier <laurent@vivier.eu>:

> Ping
> 
> Thanks,
> Laurent
> 
> Le 07/11/2019 à 15:03, Laurent Vivier a écrit :
> > This patch allows to have a different binfmt_misc configuration
> > for each new user namespace. By default, the binfmt_misc
> > configuration is the one of the previous level, but if the
> > binfmt_misc filesystem is mounted in the new namespace a new empty
> > binfmt instance is created and used in this namespace.
> > 
> > For instance, using "unshare" we can start a chroot of another
> > architecture and configure the binfmt_misc interpreter without
> > being root to run the binaries in this chroot.
> > 
> > Signed-off-by: Laurent Vivier <laurent@vivier.eu>
> > Acked-by: Andrei Vagin <avagin@gmail.com>
> > ---
> >  fs/binfmt_misc.c               | 115
> > +++++++++++++++++++++++++-------- include/linux/user_namespace.h |
> > 15 +++++ kernel/user.c                  |  14 ++++
> >  kernel/user_namespace.c        |   3 +
> >  4 files changed, 119 insertions(+), 28 deletions(-)
> > 
> > diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
> > index cdb45829354d..ba5f0d2ade96 100644
> > --- a/fs/binfmt_misc.c
> > +++ b/fs/binfmt_misc.c
> > @@ -40,9 +40,6 @@ enum {
> >  	VERBOSE_STATUS = 1 /* make it zero to save 400 bytes
> > kernel memory */ };
> >  
> > -static LIST_HEAD(entries);
> > -static int enabled = 1;
> > -
> >  enum {Enabled, Magic};
> >  #define MISC_FMT_PRESERVE_ARGV0 (1 << 31)
> >  #define MISC_FMT_OPEN_BINARY (1 << 30)
> > @@ -62,10 +59,7 @@ typedef struct {
> >  	struct file *interp_file;
> >  } Node;
> >  
> > -static DEFINE_RWLOCK(entries_lock);
> >  static struct file_system_type bm_fs_type;
> > -static struct vfsmount *bm_mnt;
> > -static int entry_count;
> >  
> >  /*
> >   * Max length of the register string.  Determined by:
> > @@ -82,18 +76,37 @@ static int entry_count;
> >   */
> >  #define MAX_REGISTER_LENGTH 1920
> >  
> > +static struct binfmt_namespace *binfmt_ns(struct user_namespace
> > *ns) +{
> > +	struct binfmt_namespace *b_ns;
> > +
> > +	while (ns) {
> > +		b_ns = READ_ONCE(ns->binfmt_ns);
> > +		if (b_ns)
> > +			return b_ns;
> > +		ns = ns->parent;
> > +	}
> > +	/* as the first user namespace is initialized with
> > +	 * &init_binfmt_ns we should never come here
> > +	 * but we try to stay safe by logging a warning
> > +	 * and returning a sane value
> > +	 */
> > +	WARN_ON_ONCE(1);
> > +	return &init_binfmt_ns;
> > +}
> > +
> >  /*
> >   * Check if we support the binfmt
> >   * if we do, return the node, else NULL
> >   * locking is done in load_misc_binary
> >   */
> > -static Node *check_file(struct linux_binprm *bprm)
> > +static Node *check_file(struct binfmt_namespace *ns, struct
> > linux_binprm *bprm) {
> >  	char *p = strrchr(bprm->interp, '.');
> >  	struct list_head *l;
> >  
> >  	/* Walk all the registered handlers. */
> > -	list_for_each(l, &entries) {
> > +	list_for_each(l, &ns->entries) {
> >  		Node *e = list_entry(l, Node, list);
> >  		char *s;
> >  		int j;
> > @@ -135,17 +148,18 @@ static int load_misc_binary(struct
> > linux_binprm *bprm) struct file *interp_file = NULL;
> >  	int retval;
> >  	int fd_binary = -1;
> > +	struct binfmt_namespace *ns = binfmt_ns(current_user_ns());
> >  
> >  	retval = -ENOEXEC;
> > -	if (!enabled)
> > +	if (!ns->enabled)
> >  		return retval;
> >  
> >  	/* to keep locking time low, we copy the interpreter
> > string */
> > -	read_lock(&entries_lock);
> > -	fmt = check_file(bprm);
> > +	read_lock(&ns->entries_lock);
> > +	fmt = check_file(ns, bprm);
> >  	if (fmt)
> >  		dget(fmt->dentry);
> > -	read_unlock(&entries_lock);
> > +	read_unlock(&ns->entries_lock);
> >  	if (!fmt)
> >  		return retval;
> >  
> > @@ -611,19 +625,19 @@ static void bm_evict_inode(struct inode
> > *inode) kfree(e);
> >  }
> >  
> > -static void kill_node(Node *e)
> > +static void kill_node(struct binfmt_namespace *ns, Node *e)
> >  {
> >  	struct dentry *dentry;
> >  
> > -	write_lock(&entries_lock);
> > +	write_lock(&ns->entries_lock);
> >  	list_del_init(&e->list);
> > -	write_unlock(&entries_lock);
> > +	write_unlock(&ns->entries_lock);
> >  
> >  	dentry = e->dentry;
> >  	drop_nlink(d_inode(dentry));
> >  	d_drop(dentry);
> >  	dput(dentry);
> > -	simple_release_fs(&bm_mnt, &entry_count);
> > +	simple_release_fs(&ns->bm_mnt, &ns->entry_count);
> >  }
> >  
> >  /* /<entry> */
> > @@ -653,6 +667,9 @@ static ssize_t bm_entry_write(struct file
> > *file, const char __user *buffer, struct dentry *root;
> >  	Node *e = file_inode(file)->i_private;
> >  	int res = parse_command(buffer, count);
> > +	struct binfmt_namespace *ns;
> > +
> > +	ns = binfmt_ns(file->f_path.dentry->d_sb->s_user_ns);
> >  
> >  	switch (res) {
> >  	case 1:
> > @@ -669,7 +686,7 @@ static ssize_t bm_entry_write(struct file
> > *file, const char __user *buffer, inode_lock(d_inode(root));
> >  
> >  		if (!list_empty(&e->list))
> > -			kill_node(e);
> > +			kill_node(ns, e);
> >  
> >  		inode_unlock(d_inode(root));
> >  		break;
> > @@ -695,6 +712,7 @@ static ssize_t bm_register_write(struct file
> > *file, const char __user *buffer, struct inode *inode;
> >  	struct super_block *sb = file_inode(file)->i_sb;
> >  	struct dentry *root = sb->s_root, *dentry;
> > +	struct binfmt_namespace *ns;
> >  	int err = 0;
> >  
> >  	e = create_entry(buffer, count);
> > @@ -718,7 +736,9 @@ static ssize_t bm_register_write(struct file
> > *file, const char __user *buffer, if (!inode)
> >  		goto out2;
> >  
> > -	err = simple_pin_fs(&bm_fs_type, &bm_mnt, &entry_count);
> > +	ns = binfmt_ns(file->f_path.dentry->d_sb->s_user_ns);
> > +	err = simple_pin_fs(&bm_fs_type, &ns->bm_mnt,
> > +			    &ns->entry_count);
> >  	if (err) {
> >  		iput(inode);
> >  		inode = NULL;
> > @@ -727,12 +747,16 @@ static ssize_t bm_register_write(struct file
> > *file, const char __user *buffer, 
> >  	if (e->flags & MISC_FMT_OPEN_FILE) {
> >  		struct file *f;
> > +		const struct cred *old_cred;
> >  
> > +		old_cred = override_creds(file->f_cred);
> >  		f = open_exec(e->interpreter);
> > +		revert_creds(old_cred);
> >  		if (IS_ERR(f)) {
> >  			err = PTR_ERR(f);
> >  			pr_notice("register: failed to install
> > interpreter file %s\n", e->interpreter);
> > -			simple_release_fs(&bm_mnt, &entry_count);
> > +			simple_release_fs(&ns->bm_mnt,
> > +					  &ns->entry_count);
> >  			iput(inode);
> >  			inode = NULL;
> >  			goto out2;
> > @@ -745,9 +769,9 @@ static ssize_t bm_register_write(struct file
> > *file, const char __user *buffer, inode->i_fop =
> > &bm_entry_operations; 
> >  	d_instantiate(dentry, inode);
> > -	write_lock(&entries_lock);
> > -	list_add(&e->list, &entries);
> > -	write_unlock(&entries_lock);
> > +	write_lock(&ns->entries_lock);
> > +	list_add(&e->list, &ns->entries);
> > +	write_unlock(&ns->entries_lock);
> >  
> >  	err = 0;
> >  out2:
> > @@ -772,7 +796,9 @@ static const struct file_operations
> > bm_register_operations = { static ssize_t
> >  bm_status_read(struct file *file, char __user *buf, size_t nbytes,
> > loff_t *ppos) {
> > -	char *s = enabled ? "enabled\n" : "disabled\n";
> > +	struct binfmt_namespace *ns =
> > +
> > binfmt_ns(file->f_path.dentry->d_sb->s_user_ns);
> > +	char *s = ns->enabled ? "enabled\n" : "disabled\n";
> >  
> >  	return simple_read_from_buffer(buf, nbytes, ppos, s,
> > strlen(s)); }
> > @@ -780,25 +806,28 @@ bm_status_read(struct file *file, char __user
> > *buf, size_t nbytes, loff_t *ppos) static ssize_t
> > bm_status_write(struct file *file, const char __user *buffer,
> > size_t count, loff_t *ppos) {
> > +	struct binfmt_namespace *ns;
> >  	int res = parse_command(buffer, count);
> >  	struct dentry *root;
> >  
> > +	ns = binfmt_ns(file->f_path.dentry->d_sb->s_user_ns);
> >  	switch (res) {
> >  	case 1:
> >  		/* Disable all handlers. */
> > -		enabled = 0;
> > +		ns->enabled = 0;
> >  		break;
> >  	case 2:
> >  		/* Enable all handlers. */
> > -		enabled = 1;
> > +		ns->enabled = 1;
> >  		break;
> >  	case 3:
> >  		/* Delete all handlers. */
> >  		root = file_inode(file)->i_sb->s_root;
> >  		inode_lock(d_inode(root));
> >  
> > -		while (!list_empty(&entries))
> > -			kill_node(list_first_entry(&entries, Node,
> > list));
> > +		while (!list_empty(&ns->entries))
> > +			kill_node(ns,
> > list_first_entry(&ns->entries,
> > +						       Node,
> > list)); 
> >  		inode_unlock(d_inode(root));
> >  		break;
> > @@ -825,24 +854,53 @@ static const struct super_operations s_ops = {
> >  static int bm_fill_super(struct super_block *sb, struct fs_context
> > *fc) {
> >  	int err;
> > +	struct user_namespace *ns = sb->s_user_ns;
> >  	static const struct tree_descr bm_files[] = {
> >  		[2] = {"status", &bm_status_operations,
> > S_IWUSR|S_IRUGO}, [3] = {"register", &bm_register_operations,
> > S_IWUSR}, /* last one */ {""}
> >  	};
> >  
> > +	/* create a new binfmt namespace
> > +	 * if we are not in the first user namespace
> > +	 * but the binfmt namespace is the first one
> > +	 */
> > +	if (READ_ONCE(ns->binfmt_ns) == NULL) {
> > +		struct binfmt_namespace *new_ns;
> > +
> > +		new_ns = kmalloc(sizeof(struct binfmt_namespace),
> > +				 GFP_KERNEL);
> > +		if (new_ns == NULL)
> > +			return -ENOMEM;
> > +		INIT_LIST_HEAD(&new_ns->entries);
> > +		new_ns->enabled = 1;
> > +		rwlock_init(&new_ns->entries_lock);
> > +		new_ns->bm_mnt = NULL;
> > +		new_ns->entry_count = 0;
> > +		/* ensure new_ns is completely initialized before
> > sharing it */
> > +		smp_wmb();
> > +		WRITE_ONCE(ns->binfmt_ns, new_ns);
> > +	}
> > +
> >  	err = simple_fill_super(sb, BINFMTFS_MAGIC, bm_files);
> >  	if (!err)
> >  		sb->s_op = &s_ops;
> >  	return err;
> >  }
> >  
> > +static void bm_free(struct fs_context *fc)
> > +{
> > +	if (fc->s_fs_info)
> > +		put_user_ns(fc->s_fs_info);
> > +}
> > +
> >  static int bm_get_tree(struct fs_context *fc)
> >  {
> > -	return get_tree_single(fc, bm_fill_super);
> > +	return get_tree_keyed(fc, bm_fill_super,
> > get_user_ns(fc->user_ns)); }
> >  
> >  static const struct fs_context_operations bm_context_ops = {
> > +	.free		= bm_free,
> >  	.get_tree	= bm_get_tree,
> >  };
> >  
> > @@ -861,6 +919,7 @@ static struct file_system_type bm_fs_type = {
> >  	.owner		= THIS_MODULE,
> >  	.name		= "binfmt_misc",
> >  	.init_fs_context = bm_init_fs_context,
> > +	.fs_flags	= FS_USERNS_MOUNT,
> >  	.kill_sb	= kill_litter_super,
> >  };
> >  MODULE_ALIAS_FS("binfmt_misc");
> > diff --git a/include/linux/user_namespace.h
> > b/include/linux/user_namespace.h index fb9f4f799554..16e6f3a97a01
> > 100644 --- a/include/linux/user_namespace.h
> > +++ b/include/linux/user_namespace.h
> > @@ -52,6 +52,18 @@ enum ucount_type {
> >  	UCOUNT_COUNTS,
> >  };
> >  
> > +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> > +struct binfmt_namespace {
> > +	struct list_head entries;
> > +	rwlock_t entries_lock;
> > +	int enabled;
> > +	struct vfsmount *bm_mnt;
> > +	int entry_count;
> > +} __randomize_layout;
> > +
> > +extern struct binfmt_namespace init_binfmt_ns;
> > +#endif
> > +
> >  struct user_namespace {
> >  	struct uid_gid_map	uid_map;
> >  	struct uid_gid_map	gid_map;
> > @@ -86,6 +98,9 @@ struct user_namespace {
> >  #endif
> >  	struct ucounts		*ucounts;
> >  	int ucount_max[UCOUNT_COUNTS];
> > +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> > +	struct binfmt_namespace *binfmt_ns;
> > +#endif
> >  } __randomize_layout;
> >  
> >  struct ucounts {
> > diff --git a/kernel/user.c b/kernel/user.c
> > index 5235d7f49982..092b2b4d47a6 100644
> > --- a/kernel/user.c
> > +++ b/kernel/user.c
> > @@ -20,6 +20,17 @@
> >  #include <linux/user_namespace.h>
> >  #include <linux/proc_ns.h>
> >  
> > +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> > +struct binfmt_namespace init_binfmt_ns = {
> > +	.entries = LIST_HEAD_INIT(init_binfmt_ns.entries),
> > +	.enabled = 1,
> > +	.entries_lock =
> > __RW_LOCK_UNLOCKED(init_binfmt_ns.entries_lock),
> > +	.bm_mnt = NULL,
> > +	.entry_count = 0,
> > +};
> > +EXPORT_SYMBOL_GPL(init_binfmt_ns);
> > +#endif
> > +
> >  /*
> >   * userns count is 1 for root user, 1 for init_uts_ns,
> >   * and 1 for... ?
> > @@ -67,6 +78,9 @@ struct user_namespace init_user_ns = {
> >  	.keyring_name_list =
> > LIST_HEAD_INIT(init_user_ns.keyring_name_list), .keyring_sem =
> > __RWSEM_INITIALIZER(init_user_ns.keyring_sem), #endif
> > +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> > +	.binfmt_ns = &init_binfmt_ns,
> > +#endif
> >  };
> >  EXPORT_SYMBOL_GPL(init_user_ns);
> >  
> > diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> > index 8eadadc478f9..f42c32269e20 100644
> > --- a/kernel/user_namespace.c
> > +++ b/kernel/user_namespace.c
> > @@ -191,6 +191,9 @@ static void free_user_ns(struct work_struct
> > *work) kfree(ns->projid_map.forward);
> >  			kfree(ns->projid_map.reverse);
> >  		}
> > +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> > +		kfree(ns->binfmt_ns);
> > +#endif
> >  		retire_userns_sysctls(ns);
> >  		key_free_user_ns(ns);
> >  		ns_free_inum(&ns->ns);
> >   
>
James Bottomley Dec. 13, 2019, 7:59 p.m.
On Fri, 2019-12-13 at 18:51 +0100, Henning Schild wrote:
> Hi all,
> 
> that is a very useful contribution, which will hopefully be
> considered.

I'm technically the maintainer on the you touched it last you own it
basis, so if Christian's concerns get addressed I'll shepherd it
upstream.

James