Message-ID: <20211226133140.GA8064@mail.hallyn.com>
Date:   Sun, 26 Dec 2021 07:31:40 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Serge Hallyn <serge@...lyn.com>, Al Viro <viro@...iv.linux.org.uk>,
        Laurent Vivier <laurent@...ier.eu>,
        linux-fsdevel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Kees Cook <keescook@...omium.org>,
        Sargun Dhillon <sargun@...gun.me>,
        Jann Horn <jannh@...gle.com>,
        Henning Schild <henning.schild@...mens.com>,
        Andrei Vagin <avagin@...il.com>,
        Matthew Bobrowski <repnop@...gle.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        containers@...ts.linux.dev,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [PATCH v2 2/2] binfmt_misc: enable sandboxed mounts

On Thu, Dec 16, 2021 at 12:26:59PM +0100, Christian Brauner wrote:
> From: Christian Brauner <christian.brauner@...ntu.com>
> 
> Enable unprivileged sandboxes to create their own binfmt_misc mounts.
> This is based on Laurent's work in [1] but has been significantly
> reworked to fix various issues we identified in earlier versions.
> 
> While binfmt_misc can currently only be mounted in the initial user
> namespace, binary types registered in this binfmt_misc instance are
> available to all sandboxes (Either by having them installed in the
> sandbox or by registering the binary type with the F flag causing the
> interpreter to be opened right away). So binfmt_misc binary types are
> already delegated to sandboxes implicitly.
> 
> However, while a sandbox has access to all registered binary types in
> binfmt_misc a sandbox cannot currently register its own binary types
> in binfmt_misc. This has prevented various use-cases some of which were
> already outlined in [1] but we have a range of issues associated with
> this (cf. [3]-[5] below which are just a small sample).
> 
> Extend binfmt_misc to be mountable in non-initial user namespaces.
> Similar to other filesystem such as nfsd, mqueue, and sunrpc we use
> keyed superblock management. The key determines whether we need to
> create a new superblock or can reuse an already existing one. We use the
> user namespace of the mount as key. This means a new binfmt_misc
> superblock is created once per user namespace creation. Subsequent
> mounts of binfmt_misc in the same user namespace will mount the same
> binfmt_misc instance. We explicitly do not create a new binfmt_misc
> superblock on every binfmt_misc mount as the semantics for
> load_misc_binary() line up with the keying model. This also allows us to
> retrieve the relevant binfmt_misc instance based on the caller's user
> namespace which can be done in a simple (bounded to 32 levels) loop.
> 
> Similar to the current binfmt_misc semantics allowing access to the
> binary types in the initial binfmt_misc instance we do allow sandboxes
> access to their parent's binfmt_misc mounts if they do not have created
> a separate binfmt_misc instance.
> 
> Overall, this will unblock the use-cases mentioned below and in general
> will also allow to support and harden execution of another
> architecture's binaries in tight sandboxes. For instance, using the
> unshare binary it possible to start a chroot of another architecture and
> configure the binfmt_misc interpreter without being root to run the
> binaries in this chroot and without requiring the host to modify its
> binary type handlers.
> 
> Henning had already posted a few experiments in the cover letter at [1].
> But here's an additional example where an unprivileged container
> registers qemu-user-static binary handlers for various binary types in
> its separate binfmt_misc mount and is then seamlessly able to start
> containers with a different architecture without affecting the host:
> 
> root    [lxc monitor] /var/snap/lxd/common/lxd/containers f1
> 1000000  \_ /sbin/init
> 1000000      \_ /lib/systemd/systemd-journald
> 1000000      \_ /lib/systemd/systemd-udevd
> 1000100      \_ /lib/systemd/systemd-networkd
> 1000101      \_ /lib/systemd/systemd-resolved
> 1000000      \_ /usr/sbin/cron -f
> 1000103      \_ /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only
> 1000000      \_ /usr/bin/python3 /usr/bin/networkd-dispatcher --run-startup-triggers
> 1000104      \_ /usr/sbin/rsyslogd -n -iNONE
> 1000000      \_ /lib/systemd/systemd-logind
> 1000000      \_ /sbin/agetty -o -p -- \u --noclear --keep-baud console 115200,38400,9600 vt220
> 1000107      \_ dnsmasq --conf-file=/dev/null -u lxc-dnsmasq --strict-order --bind-interfaces --pid-file=/run/lxc/dnsmasq.pid --liste
> 1000000      \_ [lxc monitor] /var/lib/lxc f1-s390x
> 1100000          \_ /usr/bin/qemu-s390x-static /sbin/init
> 1100000              \_ /usr/bin/qemu-s390x-static /lib/systemd/systemd-journald
> 1100000              \_ /usr/bin/qemu-s390x-static /usr/sbin/cron -f
> 1100103              \_ /usr/bin/qemu-s390x-static /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-ac
> 1100000              \_ /usr/bin/qemu-s390x-static /usr/bin/python3 /usr/bin/networkd-dispatcher --run-startup-triggers
> 1100104              \_ /usr/bin/qemu-s390x-static /usr/sbin/rsyslogd -n -iNONE
> 1100000              \_ /usr/bin/qemu-s390x-static /lib/systemd/systemd-logind
> 1100000              \_ /usr/bin/qemu-s390x-static /sbin/agetty -o -p -- \u --noclear --keep-baud console 115200,38400,9600 vt220
> 1100000              \_ /usr/bin/qemu-s390x-static /sbin/agetty -o -p -- \u --noclear --keep-baud pts/0 115200,38400,9600 vt220
> 1100000              \_ /usr/bin/qemu-s390x-static /sbin/agetty -o -p -- \u --noclear --keep-baud pts/1 115200,38400,9600 vt220
> 1100000              \_ /usr/bin/qemu-s390x-static /sbin/agetty -o -p -- \u --noclear --keep-baud pts/2 115200,38400,9600 vt220
> 1100000              \_ /usr/bin/qemu-s390x-static /sbin/agetty -o -p -- \u --noclear --keep-baud pts/3 115200,38400,9600 vt220
> 1100000              \_ /usr/bin/qemu-s390x-static /lib/systemd/systemd-udevd
> 
> [1]: https://lore.kernel.org/all/20191216091220.465626-1-laurent@vivier.eu
> [2]: https://discuss.linuxcontainers.org/t/binfmt-misc-permission-denied
> [3]: https://discuss.linuxcontainers.org/t/lxd-binfmt-support-for-qemu-static-interpreters
> [4]: https://discuss.linuxcontainers.org/t/3-1-0-binfmt-support-service-in-unprivileged-guest-requires-write-access-on-hosts-proc-sys-fs-binfmt-misc
> [5]: https://discuss.linuxcontainers.org/t/qemu-user-static-not-working-4-11
> 
> Link: https://lore.kernel.org/r/20191216091220.465626-2-laurent@vivier.eu (origin)
> Link: https://lore.kernel.org/r/20211028103114.2849140-2-brauner@kernel.org (v1)
> Cc: Sargun Dhillon <sargun@...gun.me>
> Cc: Serge Hallyn <serge@...lyn.com>

(one typo below)

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

> Cc: Jann Horn <jannh@...gle.com>
> Cc: Henning Schild <henning.schild@...mens.com>
> Cc: Andrei Vagin <avagin@...il.com>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Cc: Laurent Vivier <laurent@...ier.eu>
> Cc: linux-fsdevel@...r.kernel.org
> Signed-off-by: Laurent Vivier <laurent@...ier.eu>
> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> ---
> /* v2 */
> - Serge Hallyn <serge@...lyn.com>:
>   - Use GFP_KERNEL_ACCOUNT for userspace triggered allocations when a
>     new binary type handler is registered.
> - Christian Brauner <christian.brauner@...ntu.com>:
>   - Switch authorship to me. I refused to do that earlier even though
>     Laurent said I should do so because I think it's genuinely bad form.
>     But by now I have changed so many things that it'd be unfair to
>     blame Laurent for any potential bugs in here.
>   - Add more comments that explain what's going on.
>   - Rename functions while changing them to better reflect what they are
>     doing to make the code easier to understand.
>   - In the first version when a specific binary type handler was removed
>     either through a write to the entry's file or all binary type
>     handlers were removed by a write to the binfmt_misc mount's status
>     file all cleanup work happened during inode eviction.
>     That includes removal of the relevant entries from entry list. While
>     that works fine I disliked that model after thinking about it for a
>     bit. Because it means that there was a window were someone has
>     already removed a or all binary handlers but they could still be
>     safely reached from load_misc_binary() when it has managed to take
>     the read_lock() on the entries list while inode eviction was already
>     happening. Again, that perfectly benign but it's cleaner to remove
>     the binary handler from the list immediately meaning that ones the
>     write to then entry's file or the binfmt_misc status file returns
>     the binary type cannot be executed anymore. That gives stronger
>     guarantees to the user.
> ---
>  fs/binfmt_misc.c               | 202 +++++++++++++++++++++++++++------
>  include/linux/binfmts.h        |  10 ++
>  include/linux/user_namespace.h |   8 ++
>  kernel/user.c                  |  13 +++
>  kernel/user_namespace.c        |   3 +
>  5 files changed, 202 insertions(+), 34 deletions(-)
> 
> diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
> index 3fd99a20694b..40b74751ecfd 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)
> @@ -63,7 +60,6 @@ typedef struct {
>  	refcount_t users;		/* sync removal with load_misc_binary() */
>  } Node;
>  
> -static DEFINE_RWLOCK(entries_lock);
>  static struct file_system_type bm_fs_type;
>  
>  /*
> @@ -90,13 +86,14 @@ static struct file_system_type bm_fs_type;
>   *
>   * Return: binary type list entry on success, NULL on failure
>   */
> -static Node *search_binfmt_handler(struct linux_binprm *bprm)
> +static Node *search_binfmt_handler(struct binfmt_misc *misc,
> +				   struct linux_binprm *bprm)
>  {
>  	char *p = strrchr(bprm->interp, '.');
>  	Node *e;
>  
>  	/* Walk all the registered handlers. */
> -	list_for_each_entry(e, &entries, list) {
> +	list_for_each_entry(e, &misc->entries, list) {
>  		char *s;
>  		int j;
>  
> @@ -138,15 +135,16 @@ static Node *search_binfmt_handler(struct linux_binprm *bprm)
>   *
>   * Return: binary type list entry on success, NULL on failure
>   */
> -static Node *get_binfmt_handler(struct linux_binprm *bprm)
> +static Node *get_binfmt_handler(struct binfmt_misc *misc,
> +				struct linux_binprm *bprm)
>  {
>  	Node *e;
>  
> -	read_lock(&entries_lock);
> -	e = search_binfmt_handler(bprm);
> +	read_lock(&misc->entries_lock);
> +	e = search_binfmt_handler(misc, bprm);
>  	if (e)
>  		refcount_inc(&e->users);
> -	read_unlock(&entries_lock);
> +	read_unlock(&misc->entries_lock);
>  	return e;
>  }
>  
> @@ -167,6 +165,35 @@ static void put_binfmt_handler(Node *e)
>  	}
>  }
>  
> +/**
> + * load_binfmt_misc - load the binfmt_misc of the caller's user namespace
> + *
> + * To be called in load_misc_binary() to load the relevant struct binfmt_misc.
> + * If a user namespace doesn't have its own binfmt_misc mount it can make use
> + * of its ancestor's binfmt_misc handlers. This mimicks the behavior of
> + * pre-namespaced binfmt_misc where all registered binfmt_misc handlers where
> + * available to all user and user namespaces on the system.
> + *
> + * Return: the binfmt_misc instance of the caller's user namespace
> + */
> +static struct binfmt_misc *load_binfmt_misc(void)
> +{
> +	const struct user_namespace *user_ns;
> +	struct binfmt_misc *misc;
> +
> +	user_ns = current_user_ns();
> +	while (user_ns) {
> +		/* Pairs with smp_store_release() in bm_fill_super(). */
> +		misc = smp_load_acquire(&user_ns->binfmt_misc);
> +		if (misc)
> +			return misc;
> +
> +		user_ns = user_ns->parent;
> +	}
> +
> +	return &init_binfmt_misc;
> +}
> +
>  /*
>   * the loader itself
>   */
> @@ -174,13 +201,14 @@ static int load_misc_binary(struct linux_binprm *bprm)
>  {
>  	Node *fmt;
>  	struct file *interp_file = NULL;
> -	int retval;
> +	int retval = -ENOEXEC;
> +	struct binfmt_misc *misc;
>  
> -	retval = -ENOEXEC;
> -	if (!enabled)
> +	misc = load_binfmt_misc();
> +	if (!misc->enabled)
>  		return retval;
>  
> -	fmt = get_binfmt_handler(bprm);
> +	fmt = get_binfmt_handler(misc, bprm);
>  	if (!fmt)
>  		return retval;
>  
> @@ -238,9 +266,9 @@ static int load_misc_binary(struct linux_binprm *bprm)
>  	/*
>  	 * If we actually put the node here all concurrent calls to
>  	 * load_misc_binary() will have finished. We also know
> -	 * that for the refcount to be zero ->evict_inode() must have removed
> -	 * the node to be deleted from the list. All that is left for us is to
> -	 * close and free.
> +	 * that for the refcount to be zero someone must have concurently

(typo) concurrently

> +	 * removed the binary type handler from the list and it's our job to
> +	 * free it.
>  	 */
>  	put_binfmt_handler(fmt);
>  
> @@ -332,7 +360,7 @@ static Node *create_entry(const char __user *buffer, size_t count)
>  
>  	err = -ENOMEM;
>  	memsize = sizeof(Node) + count + 8;
> -	e = kmalloc(memsize, GFP_KERNEL);
> +	e = kmalloc(memsize, GFP_KERNEL_ACCOUNT);
>  	if (!e)
>  		goto out;
>  
> @@ -444,7 +472,7 @@ static Node *create_entry(const char __user *buffer, size_t count)
>  
>  			if (e->mask) {
>  				int i;
> -				char *masked = kmalloc(e->size, GFP_KERNEL);
> +				char *masked = kmalloc(e->size, GFP_KERNEL_ACCOUNT);
>  
>  				print_hex_dump_bytes(
>  					KBUILD_MODNAME ": register:  mask[decoded]: ",
> @@ -598,6 +626,22 @@ static struct inode *bm_get_inode(struct super_block *sb, int mode)
>  	return inode;
>  }
>  
> +/**
> + * i_binfmt_misc - retrieve struct binfmt_misc from a binfmt_misc inode
> + * @inode: inode of the relevant binfmt_misc instance
> + *
> + * This helper retrieves struct binfmt_misc from a binfmt_misc inode. This can
> + * be done without any memory barriers because we are guaranteed that
> + * user_ns->binfmt_misc is fully initialized. It was fully initialized when the
> + * binfmt_misc mount was first created.
> + *
> + * Return: struct binfmt_misc of the relevant binfmt_misc instance
> + */
> +static struct binfmt_misc *i_binfmt_misc(struct inode *inode)
> +{
> +	return inode->i_sb->s_user_ns->binfmt_misc;
> +}
> +
>  /**
>   * bm_evict_inode - cleanup data associated with @inode
>   * @inode: inode to which the data is attached
> @@ -618,10 +662,13 @@ static void bm_evict_inode(struct inode *inode)
>  	clear_inode(inode);
>  
>  	if (e) {
> -		write_lock(&entries_lock);
> +		struct binfmt_misc *misc;
> +
> +		misc = i_binfmt_misc(inode);
> +		write_lock(&misc->entries_lock);
>  		if (!list_empty(&e->list))
>  			list_del_init(&e->list);
> -		write_unlock(&entries_lock);
> +		write_unlock(&misc->entries_lock);
>  		put_binfmt_handler(e);
>  	}
>  }
> @@ -675,11 +722,11 @@ static void unlink_binfmt_dentry(struct dentry *dentry)
>   * to use writes to files in order to delete binary type handlers. But it has
>   * worked for so long that it's not a pressing issue.
>   */
> -static void remove_binfmt_handler(Node *e)
> +static void remove_binfmt_handler(struct binfmt_misc *misc, Node *e)
>  {
> -	write_lock(&entries_lock);
> +	write_lock(&misc->entries_lock);
>  	list_del_init(&e->list);
> -	write_unlock(&entries_lock);
> +	write_unlock(&misc->entries_lock);
>  	unlink_binfmt_dentry(e->dentry);
>  }
>  
> @@ -735,7 +782,7 @@ static ssize_t bm_entry_write(struct file *file, const char __user *buffer,
>  		 * actually remove the entry from the list.
>  		 */
>  		if (!list_empty(&e->list))
> -			remove_binfmt_handler(e);
> +			remove_binfmt_handler(i_binfmt_misc(inode), e);
>  
>  		inode_unlock(inode);
>  		break;
> @@ -761,6 +808,7 @@ 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_misc *misc;
>  	int err = 0;
>  	struct file *f = NULL;
>  
> @@ -770,7 +818,18 @@ static ssize_t bm_register_write(struct file *file, const char __user *buffer,
>  		return PTR_ERR(e);
>  
>  	if (e->flags & MISC_FMT_OPEN_FILE) {
> +		const struct cred *old_cred;
> +
> +		/*
> +		 * Now that we support unprivileged binfmt_misc mounts make
> +		 * sure we use the credentials that the register @file was
> +		 * opened with to also open the interpreter. Before that this
> +		 * didn't matter much as only a privileged process could open
> +		 * the register file.
> +		 */
> +		old_cred = override_creds(file->f_cred);
>  		f = open_exec(e->interpreter);
> +		revert_creds(old_cred);
>  		if (IS_ERR(f)) {
>  			pr_notice("register: failed to install interpreter file %s\n",
>  				 e->interpreter);
> @@ -802,9 +861,10 @@ 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);
> +	misc = i_binfmt_misc(inode);
> +	write_lock(&misc->entries_lock);
> +	list_add(&e->list, &misc->entries);
> +	write_unlock(&misc->entries_lock);
>  
>  	err = 0;
>  out2:
> @@ -831,26 +891,31 @@ 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_misc *misc;
> +	char *s;
>  
> +	misc = i_binfmt_misc(file_inode(file));
> +	s = misc->enabled ? "enabled\n" : "disabled\n";
>  	return simple_read_from_buffer(buf, nbytes, ppos, s, strlen(s));
>  }
>  
>  static ssize_t bm_status_write(struct file *file, const char __user *buffer,
>  		size_t count, loff_t *ppos)
>  {
> +	struct binfmt_misc *misc;
>  	int res = parse_command(buffer, count);
>  	Node *e, *next;
>  	struct inode *inode;
>  
> +	misc = i_binfmt_misc(file_inode(file));
>  	switch (res) {
>  	case 1:
>  		/* Disable all handlers. */
> -		enabled = 0;
> +		misc->enabled = false;
>  		break;
>  	case 2:
>  		/* Enable all handlers. */
> -		enabled = 1;
> +		misc->enabled = true;
>  		break;
>  	case 3:
>  		/* Delete all handlers. */
> @@ -866,8 +931,8 @@ static ssize_t bm_status_write(struct file *file, const char __user *buffer,
>  		 * read-only. So we only need to take the write lock when we
>  		 * actually remove the entry from the list.
>  		 */
> -		list_for_each_entry_safe(e, next, &entries, list)
> -			remove_binfmt_handler(e);
> +		list_for_each_entry_safe(e, next, &misc->entries, list)
> +			remove_binfmt_handler(misc, e);
>  
>  		inode_unlock(inode);
>  		break;
> @@ -886,32 +951,100 @@ static const struct file_operations bm_status_operations = {
>  
>  /* Superblock handling */
>  
> +static void bm_put_super(struct super_block *sb)
> +{
> +	struct user_namespace *user_ns = sb->s_fs_info;
> +
> +	sb->s_fs_info = NULL;
> +	put_user_ns(user_ns);
> +}
> +
>  static const struct super_operations s_ops = {
>  	.statfs		= simple_statfs,
>  	.evict_inode	= bm_evict_inode,
> +	.put_super	= bm_put_super,
>  };
>  
>  static int bm_fill_super(struct super_block *sb, struct fs_context *fc)
>  {
>  	int err;
> +	struct user_namespace *user_ns = sb->s_user_ns;
> +	struct binfmt_misc *misc;
>  	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 */ {""}
>  	};
>  
> +	if (WARN_ON(user_ns != current_user_ns()))
> +		return -EINVAL;
> +
> +	/*
> +	 * Lazily allocate a new binfmt_misc instance for this namespace, i.e.
> +	 * do it here during the first mount of binfmt_misc. We don't need to
> +	 * waste memory for every user namespace allocation. It's likely much
> +	 * more common to not mount a separate binfmt_misc instance than it is
> +	 * to mount one.
> +	 *
> +	 * While multiple superblocks can exist they are keyed by userns in
> +	 * s_fs_info for binfmt_misc. Hence, the vfs guarantees that
> +	 * bm_fill_super() is called exactly once whenever a binfmt_misc
> +	 * superblock for a userns is created. This in turn lets us conclude
> +	 * that when a binfmt_misc superblock is created for the first time for
> +	 * a userns there's no one racing us. Therefore we don't need any
> +	 * barriers when we dereference binfmt_misc.
> +	 */
> +	misc = user_ns->binfmt_misc;
> +	if (!misc) {
> +		/*
> +		 * If it turns out that most user namespaces actually want to
> +		 * register their own binary type handler and therefore all
> +		 * create their own separate binfm_misc mounts we should
> +		 * consider turning this into a kmem cache.
> +		 */
> +		misc = kzalloc(sizeof(struct binfmt_misc), GFP_KERNEL);
> +		if (!misc)
> +			return -ENOMEM;
> +
> +		INIT_LIST_HEAD(&misc->entries);
> +		rwlock_init(&misc->entries_lock);
> +
> +		/* Pairs with smp_load_acquire() in load_binfmt_misc(). */
> +		smp_store_release(&user_ns->binfmt_misc, misc);
> +	}
> +
> +	/*
> +	 * When the binfmt_misc superblock for this userns is shutdown
> +	 * ->enabled might have been set to false and we don't reinitialize
> +	 * ->enabled again in put_super() as someone might already be mounting
> +	 * binfmt_misc again. It also would be pointless since by the time
> +	 * ->put_super() is called we know that the binary type list for this
> +	 * bintfmt_misc mount is empty making load_misc_binary() return
> +	 * -ENOEXEC independent of whether ->enabled is true. Instead, if
> +	 * someone mounts binfmt_misc for the first time or again we simply
> +	 * reset ->enabled to true.
> +	 */
> +	misc->enabled = true;
> +
>  	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,
>  };
>  
> @@ -930,6 +1063,7 @@ 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/binfmts.h b/include/linux/binfmts.h
> index 049cf9421d83..42efcefc56c7 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -102,6 +102,16 @@ struct linux_binfmt {
>  	unsigned long min_coredump;	/* minimal dump size */
>  } __randomize_layout;
>  
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +struct binfmt_misc {
> +	struct list_head entries;
> +	rwlock_t entries_lock;
> +	bool enabled;
> +} __randomize_layout;
> +
> +extern struct binfmt_misc init_binfmt_misc;
> +#endif
> +
>  extern void __register_binfmt(struct linux_binfmt *fmt, int insert);
>  
>  /* Registration of default binfmt handlers */
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index 33a4240e6a6f..a49f8f121fc4 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -63,6 +63,10 @@ enum ucount_type {
>  
>  #define MAX_PER_NAMESPACE_UCOUNTS UCOUNT_RLIMIT_NPROC
>  
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +struct binfmt_misc;
> +#endif
> +
>  struct user_namespace {
>  	struct uid_gid_map	uid_map;
>  	struct uid_gid_map	gid_map;
> @@ -99,6 +103,10 @@ struct user_namespace {
>  #endif
>  	struct ucounts		*ucounts;
>  	long ucount_max[UCOUNT_COUNTS];
> +
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +	struct binfmt_misc *binfmt_misc;
> +#endif
>  } __randomize_layout;
>  
>  struct ucounts {
> diff --git a/kernel/user.c b/kernel/user.c
> index e2cf8c22b539..d2e7575dbfa2 100644
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -18,8 +18,18 @@
>  #include <linux/interrupt.h>
>  #include <linux/export.h>
>  #include <linux/user_namespace.h>
> +#include <linux/binfmts.h>
>  #include <linux/proc_ns.h>
>  
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +struct binfmt_misc init_binfmt_misc = {
> +	.entries = LIST_HEAD_INIT(init_binfmt_misc.entries),
> +	.enabled = true,
> +	.entries_lock = __RW_LOCK_UNLOCKED(init_binfmt_misc.entries_lock),
> +};
> +EXPORT_SYMBOL_GPL(init_binfmt_misc);
> +#endif
> +
>  /*
>   * userns count is 1 for root user, 1 for init_uts_ns,
>   * and 1 for... ?
> @@ -67,6 +77,9 @@ 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_misc = &init_binfmt_misc,
> +#endif
>  };
>  EXPORT_SYMBOL_GPL(init_user_ns);
>  
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 6b2e3ca7ee99..2bdf2ff69148 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -196,6 +196,9 @@ 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_misc);
> +#endif
>  		retire_userns_sysctls(ns);
>  		key_free_user_ns(ns);
>  		ns_free_inum(&ns->ns);
> -- 
> 2.30.2

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