Merge tag 'idmapped-mounts-v5.12' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux

Pull idmapped mounts from Christian Brauner:
 "This introduces idmapped mounts which has been in the making for some
  time. Simply put, different mounts can expose the same file or
  directory with different ownership. This initial implementation comes
  with ports for fat, ext4 and with Christoph's port for xfs with more
  filesystems being actively worked on by independent people and
  maintainers.

  Idmapping mounts handle a wide range of long standing use-cases. Here
  are just a few:

   - Idmapped mounts make it possible to easily share files between
     multiple users or multiple machines especially in complex
     scenarios. For example, idmapped mounts will be used in the
     implementation of portable home directories in
     systemd-homed.service(8) where they allow users to move their home
     directory to an external storage device and use it on multiple
     computers where they are assigned different uids and gids. This
     effectively makes it possible to assign random uids and gids at
     login time.

   - It is possible to share files from the host with unprivileged
     containers without having to change ownership permanently through
     chown(2).

   - It is possible to idmap a container's rootfs and without having to
     mangle every file. For example, Chromebooks use it to share the
     user's Download folder with their unprivileged containers in their
     Linux subsystem.

   - It is possible to share files between containers with
     non-overlapping idmappings.

   - Filesystem that lack a proper concept of ownership such as fat can
     use idmapped mounts to implement discretionary access (DAC)
     permission checking.

   - They allow users to efficiently changing ownership on a per-mount
     basis without having to (recursively) chown(2) all files. In
     contrast to chown (2) changing ownership of large sets of files is
     instantenous with idmapped mounts. This is especially useful when
     ownership of a whole root filesystem of a virtual machine or
     container is changed. With idmapped mounts a single syscall
     mount_setattr syscall will be sufficient to change the ownership of
     all files.

   - Idmapped mounts always take the current ownership into account as
     idmappings specify what a given uid or gid is supposed to be mapped
     to. This contrasts with the chown(2) syscall which cannot by itself
     take the current ownership of the files it changes into account. It
     simply changes the ownership to the specified uid and gid. This is
     especially problematic when recursively chown(2)ing a large set of
     files which is commong with the aforementioned portable home
     directory and container and vm scenario.

   - Idmapped mounts allow to change ownership locally, restricting it
     to specific mounts, and temporarily as the ownership changes only
     apply as long as the mount exists.

  Several userspace projects have either already put up patches and
  pull-requests for this feature or will do so should you decide to pull
  this:

   - systemd: In a wide variety of scenarios but especially right away
     in their implementation of portable home directories.

         https://systemd.io/HOME_DIRECTORY/

   - container runtimes: containerd, runC, LXD:To share data between
     host and unprivileged containers, unprivileged and privileged
     containers, etc. The pull request for idmapped mounts support in
     containerd, the default Kubernetes runtime is already up for quite
     a while now: https://github.com/containerd/containerd/pull/4734

   - The virtio-fs developers and several users have expressed interest
     in using this feature with virtual machines once virtio-fs is
     ported.

   - ChromeOS: Sharing host-directories with unprivileged containers.

  I've tightly synced with all those projects and all of those listed
  here have also expressed their need/desire for this feature on the
  mailing list. For more info on how people use this there's a bunch of
  talks about this too. Here's just two recent ones:

      https://www.cncf.io/wp-content/uploads/2020/12/Rootless-Containers-in-Gitpod.pdf
      https://fosdem.org/2021/schedule/event/containers_idmap/

  This comes with an extensive xfstests suite covering both ext4 and
  xfs:

      https://git.kernel.org/brauner/xfstests-dev/h/idmapped_mounts

  It covers truncation, creation, opening, xattrs, vfscaps, setid
  execution, setgid inheritance and more both with idmapped and
  non-idmapped mounts. It already helped to discover an unrelated xfs
  setgid inheritance bug which has since been fixed in mainline. It will
  be sent for inclusion with the xfstests project should you decide to
  merge this.

  In order to support per-mount idmappings vfsmounts are marked with
  user namespaces. The idmapping of the user namespace will be used to
  map the ids of vfs objects when they are accessed through that mount.
  By default all vfsmounts are marked with the initial user namespace.
  The initial user namespace is used to indicate that a mount is not
  idmapped. All operations behave as before and this is verified in the
  testsuite.

  Based on prior discussions we want to attach the whole user namespace
  and not just a dedicated idmapping struct. This allows us to reuse all
  the helpers that already exist for dealing with idmappings instead of
  introducing a whole new range of helpers. In addition, if we decide in
  the future that we are confident enough to enable unprivileged users
  to setup idmapped mounts the permission checking can take into account
  whether the caller is privileged in the user namespace the mount is
  currently marked with.

  The user namespace the mount will be marked with can be specified by
  passing a file descriptor refering to the user namespace as an
  argument to the new mount_setattr() syscall together with the new
  MOUNT_ATTR_IDMAP flag. The system call follows the openat2() pattern
  of extensibility.

  The following conditions must be met in order to create an idmapped
  mount:

   - The caller must currently have the CAP_SYS_ADMIN capability in the
     user namespace the underlying filesystem has been mounted in.

   - The underlying filesystem must support idmapped mounts.

   - The mount must not already be idmapped. This also implies that the
     idmapping of a mount cannot be altered once it has been idmapped.

   - The mount must be a detached/anonymous mount, i.e. it must have
     been created by calling open_tree() with the OPEN_TREE_CLONE flag
     and it must not already have been visible in the filesystem.

  The last two points guarantee easier semantics for userspace and the
  kernel and make the implementation significantly simpler.

  By default vfsmounts are marked with the initial user namespace and no
  behavioral or performance changes are observed.

  The manpage with a detailed description can be found here:

      https://git.kernel.org/brauner/man-pages/c/1d7b902e2875a1ff342e036a9f866a995640aea8

  In order to support idmapped mounts, filesystems need to be changed
  and mark themselves with the FS_ALLOW_IDMAP flag in fs_flags. The
  patches to convert individual filesystem are not very large or
  complicated overall as can be seen from the included fat, ext4, and
  xfs ports. Patches for other filesystems are actively worked on and
  will be sent out separately. The xfstestsuite can be used to verify
  that port has been done correctly.

  The mount_setattr() syscall is motivated independent of the idmapped
  mounts patches and it's been around since July 2019. One of the most
  valuable features of the new mount api is the ability to perform
  mounts based on file descriptors only.

  Together with the lookup restrictions available in the openat2()
  RESOLVE_* flag namespace which we added in v5.6 this is the first time
  we are close to hardened and race-free (e.g. symlinks) mounting and
  path resolution.

  While userspace has started porting to the new mount api to mount
  proper filesystems and create new bind-mounts it is currently not
  possible to change mount options of an already existing bind mount in
  the new mount api since the mount_setattr() syscall is missing.

  With the addition of the mount_setattr() syscall we remove this last
  restriction and userspace can now fully port to the new mount api,
  covering every use-case the old mount api could. We also add the
  crucial ability to recursively change mount options for a whole mount
  tree, both removing and adding mount options at the same time. This
  syscall has been requested multiple times by various people and
  projects.

  There is a simple tool available at

      https://github.com/brauner/mount-idmapped

  that allows to create idmapped mounts so people can play with this
  patch series. I'll add support for the regular mount binary should you
  decide to pull this in the following weeks:

  Here's an example to a simple idmapped mount of another user's home
  directory:

	u1001@f2-vm:/$ sudo ./mount --idmap both:1000:1001:1 /home/ubuntu/ /mnt

	u1001@f2-vm:/$ ls -al /home/ubuntu/
	total 28
	drwxr-xr-x 2 ubuntu ubuntu 4096 Oct 28 22:07 .
	drwxr-xr-x 4 root   root   4096 Oct 28 04:00 ..
	-rw------- 1 ubuntu ubuntu 3154 Oct 28 22:12 .bash_history
	-rw-r--r-- 1 ubuntu ubuntu  220 Feb 25  2020 .bash_logout
	-rw-r--r-- 1 ubuntu ubuntu 3771 Feb 25  2020 .bashrc
	-rw-r--r-- 1 ubuntu ubuntu  807 Feb 25  2020 .profile
	-rw-r--r-- 1 ubuntu ubuntu    0 Oct 16 16:11 .sudo_as_admin_successful
	-rw------- 1 ubuntu ubuntu 1144 Oct 28 00:43 .viminfo

	u1001@f2-vm:/$ ls -al /mnt/
	total 28
	drwxr-xr-x  2 u1001 u1001 4096 Oct 28 22:07 .
	drwxr-xr-x 29 root  root  4096 Oct 28 22:01 ..
	-rw-------  1 u1001 u1001 3154 Oct 28 22:12 .bash_history
	-rw-r--r--  1 u1001 u1001  220 Feb 25  2020 .bash_logout
	-rw-r--r--  1 u1001 u1001 3771 Feb 25  2020 .bashrc
	-rw-r--r--  1 u1001 u1001  807 Feb 25  2020 .profile
	-rw-r--r--  1 u1001 u1001    0 Oct 16 16:11 .sudo_as_admin_successful
	-rw-------  1 u1001 u1001 1144 Oct 28 00:43 .viminfo

	u1001@f2-vm:/$ touch /mnt/my-file

	u1001@f2-vm:/$ setfacl -m u:1001:rwx /mnt/my-file

	u1001@f2-vm:/$ sudo setcap -n 1001 cap_net_raw+ep /mnt/my-file

	u1001@f2-vm:/$ ls -al /mnt/my-file
	-rw-rwxr--+ 1 u1001 u1001 0 Oct 28 22:14 /mnt/my-file

	u1001@f2-vm:/$ ls -al /home/ubuntu/my-file
	-rw-rwxr--+ 1 ubuntu ubuntu 0 Oct 28 22:14 /home/ubuntu/my-file

	u1001@f2-vm:/$ getfacl /mnt/my-file
	getfacl: Removing leading '/' from absolute path names
	# file: mnt/my-file
	# owner: u1001
	# group: u1001
	user::rw-
	user:u1001:rwx
	group::rw-
	mask::rwx
	other::r--

	u1001@f2-vm:/$ getfacl /home/ubuntu/my-file
	getfacl: Removing leading '/' from absolute path names
	# file: home/ubuntu/my-file
	# owner: ubuntu
	# group: ubuntu
	user::rw-
	user:ubuntu:rwx
	group::rw-
	mask::rwx
	other::r--"

* tag 'idmapped-mounts-v5.12' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux: (41 commits)
  xfs: remove the possibly unused mp variable in xfs_file_compat_ioctl
  xfs: support idmapped mounts
  ext4: support idmapped mounts
  fat: handle idmapped mounts
  tests: add mount_setattr() selftests
  fs: introduce MOUNT_ATTR_IDMAP
  fs: add mount_setattr()
  fs: add attr_flags_to_mnt_flags helper
  fs: split out functions to hold writers
  namespace: only take read lock in do_reconfigure_mnt()
  mount: make {lock,unlock}_mount_hash() static
  namespace: take lock_mount_hash() directly when changing flags
  nfs: do not export idmapped mounts
  overlayfs: do not mount on top of idmapped mounts
  ecryptfs: do not mount on top of idmapped mounts
  ima: handle idmapped mounts
  apparmor: handle idmapped mounts
  fs: make helpers idmap mount aware
  exec: handle idmapped mounts
  would_dump: handle idmapped mounts
  ...

1 files changed, 444 insertions, 40 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 9d33909d0f9e..062fe984a697 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -25,6 +25,7 @@
 #include <linux/proc_ns.h>
 #include <linux/magic.h>
 #include <linux/memblock.h>
+#include <linux/proc_fs.h>
 #include <linux/task_work.h>
 #include <linux/sched/task.h>
 #include <uapi/linux/mount.h>
@@ -73,6 +74,15 @@ static DECLARE_RWSEM(namespace_sem);
 static HLIST_HEAD(unmounted);	/* protected by namespace_sem */
 static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */
 
+struct mount_kattr {
+	unsigned int attr_set;
+	unsigned int attr_clr;
+	unsigned int propagation;
+	unsigned int lookup_flags;
+	bool recurse;
+	struct user_namespace *mnt_userns;
+};
+
 /* /sys/fs */
 struct kobject *fs_kobj;
 EXPORT_SYMBOL_GPL(fs_kobj);
@@ -87,6 +97,16 @@ EXPORT_SYMBOL_GPL(fs_kobj);
  */
 __cacheline_aligned_in_smp DEFINE_SEQLOCK(mount_lock);
 
+static inline void lock_mount_hash(void)
+{
+	write_seqlock(&mount_lock);
+}
+
+static inline void unlock_mount_hash(void)
+{
+	write_sequnlock(&mount_lock);
+}
+
 static inline struct hlist_head *m_hash(struct vfsmount *mnt, struct dentry *dentry)
 {
 	unsigned long tmp = ((unsigned long)mnt / L1_CACHE_BYTES);
@@ -210,6 +230,7 @@ static struct mount *alloc_vfsmnt(const char *name)
 		INIT_HLIST_NODE(&mnt->mnt_mp_list);
 		INIT_LIST_HEAD(&mnt->mnt_umounting);
 		INIT_HLIST_HEAD(&mnt->mnt_stuck_children);
+		mnt->mnt.mnt_userns = &init_user_ns;
 	}
 	return mnt;
 
@@ -459,11 +480,8 @@ void mnt_drop_write_file(struct file *file)
 }
 EXPORT_SYMBOL(mnt_drop_write_file);
 
-static int mnt_make_readonly(struct mount *mnt)
+static inline int mnt_hold_writers(struct mount *mnt)
 {
-	int ret = 0;
-
-	lock_mount_hash();
 	mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
 	/*
 	 * After storing MNT_WRITE_HOLD, we'll read the counters. This store
@@ -488,25 +506,30 @@ static int mnt_make_readonly(struct mount *mnt)
 	 * we're counting up here.
 	 */
 	if (mnt_get_writers(mnt) > 0)
-		ret = -EBUSY;
-	else
-		mnt->mnt.mnt_flags |= MNT_READONLY;
+		return -EBUSY;
+
+	return 0;
+}
+
+static inline void mnt_unhold_writers(struct mount *mnt)
+{
 	/*
 	 * MNT_READONLY must become visible before ~MNT_WRITE_HOLD, so writers
 	 * that become unheld will see MNT_READONLY.
 	 */
 	smp_wmb();
 	mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
-	unlock_mount_hash();
-	return ret;
 }
 
-static int __mnt_unmake_readonly(struct mount *mnt)
+static int mnt_make_readonly(struct mount *mnt)
 {
-	lock_mount_hash();
-	mnt->mnt.mnt_flags &= ~MNT_READONLY;
-	unlock_mount_hash();
-	return 0;
+	int ret;
+
+	ret = mnt_hold_writers(mnt);
+	if (!ret)
+		mnt->mnt.mnt_flags |= MNT_READONLY;
+	mnt_unhold_writers(mnt);
+	return ret;
 }
 
 int sb_prepare_remount_readonly(struct super_block *sb)
@@ -547,6 +570,11 @@ int sb_prepare_remount_readonly(struct super_block *sb)
 
 static void free_vfsmnt(struct mount *mnt)
 {
+	struct user_namespace *mnt_userns;
+
+	mnt_userns = mnt_user_ns(&mnt->mnt);
+	if (mnt_userns != &init_user_ns)
+		put_user_ns(mnt_userns);
 	kfree_const(mnt->mnt_devname);
 #ifdef CONFIG_SMP
 	free_percpu(mnt->mnt_pcp);
@@ -1055,6 +1083,9 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
 	mnt->mnt.mnt_flags &= ~(MNT_WRITE_HOLD|MNT_MARKED|MNT_INTERNAL);
 
 	atomic_inc(&sb->s_active);
+	mnt->mnt.mnt_userns = mnt_user_ns(&old->mnt);
+	if (mnt->mnt.mnt_userns != &init_user_ns)
+		mnt->mnt.mnt_userns = get_user_ns(mnt->mnt.mnt_userns);
 	mnt->mnt.mnt_sb = sb;
 	mnt->mnt.mnt_root = dget(root);
 	mnt->mnt_mountpoint = mnt->mnt.mnt_root;
@@ -2514,20 +2545,15 @@ static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags)
 	if (readonly_request)
 		return mnt_make_readonly(mnt);
 
-	return __mnt_unmake_readonly(mnt);
+	mnt->mnt.mnt_flags &= ~MNT_READONLY;
+	return 0;
 }
 
-/*
- * Update the user-settable attributes on a mount.  The caller must hold
- * sb->s_umount for writing.
- */
 static void set_mount_attributes(struct mount *mnt, unsigned int mnt_flags)
 {
-	lock_mount_hash();
 	mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
 	mnt->mnt.mnt_flags = mnt_flags;
 	touch_mnt_namespace(mnt->mnt_ns);
-	unlock_mount_hash();
 }
 
 static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *mnt)
@@ -2572,11 +2598,17 @@ static int do_reconfigure_mnt(struct path *path, unsigned int mnt_flags)
 	if (!can_change_locked_flags(mnt, mnt_flags))
 		return -EPERM;
 
-	down_write(&sb->s_umount);
+	/*
+	 * We're only checking whether the superblock is read-only not
+	 * changing it, so only take down_read(&sb->s_umount).
+	 */
+	down_read(&sb->s_umount);
+	lock_mount_hash();
 	ret = change_mount_ro_state(mnt, mnt_flags);
 	if (ret == 0)
 		set_mount_attributes(mnt, mnt_flags);
-	up_write(&sb->s_umount);
+	unlock_mount_hash();
+	up_read(&sb->s_umount);
 
 	mnt_warn_timestamp_expiry(path, &mnt->mnt);
 
@@ -2616,8 +2648,11 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
 		err = -EPERM;
 		if (ns_capable(sb->s_user_ns, CAP_SYS_ADMIN)) {
 			err = reconfigure_super(fc);
-			if (!err)
+			if (!err) {
+				lock_mount_hash();
 				set_mount_attributes(mnt, mnt_flags);
+				unlock_mount_hash();
+			}
 		}
 		up_write(&sb->s_umount);
 	}
@@ -3440,6 +3475,33 @@ out_type:
 	return ret;
 }
 
+#define FSMOUNT_VALID_FLAGS \
+	(MOUNT_ATTR_RDONLY | MOUNT_ATTR_NOSUID | MOUNT_ATTR_NODEV | \
+	 MOUNT_ATTR_NOEXEC | MOUNT_ATTR__ATIME | MOUNT_ATTR_NODIRATIME)
+
+#define MOUNT_SETATTR_VALID_FLAGS (FSMOUNT_VALID_FLAGS | MOUNT_ATTR_IDMAP)
+
+#define MOUNT_SETATTR_PROPAGATION_FLAGS \
+	(MS_UNBINDABLE | MS_PRIVATE | MS_SLAVE | MS_SHARED)
+
+static unsigned int attr_flags_to_mnt_flags(u64 attr_flags)
+{
+	unsigned int mnt_flags = 0;
+
+	if (attr_flags & MOUNT_ATTR_RDONLY)
+		mnt_flags |= MNT_READONLY;
+	if (attr_flags & MOUNT_ATTR_NOSUID)
+		mnt_flags |= MNT_NOSUID;
+	if (attr_flags & MOUNT_ATTR_NODEV)
+		mnt_flags |= MNT_NODEV;
+	if (attr_flags & MOUNT_ATTR_NOEXEC)
+		mnt_flags |= MNT_NOEXEC;
+	if (attr_flags & MOUNT_ATTR_NODIRATIME)
+		mnt_flags |= MNT_NODIRATIME;
+
+	return mnt_flags;
+}
+
 /*
  * Create a kernel mount representation for a new, prepared superblock
  * (specified by fs_fd) and attach to an open_tree-like file descriptor.
@@ -3462,24 +3524,10 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags,
 	if ((flags & ~(FSMOUNT_CLOEXEC)) != 0)
 		return -EINVAL;
 
-	if (attr_flags & ~(MOUNT_ATTR_RDONLY |
-			   MOUNT_ATTR_NOSUID |
-			   MOUNT_ATTR_NODEV |
-			   MOUNT_ATTR_NOEXEC |
-			   MOUNT_ATTR__ATIME |
-			   MOUNT_ATTR_NODIRATIME))
+	if (attr_flags & ~FSMOUNT_VALID_FLAGS)
 		return -EINVAL;
 
-	if (attr_flags & MOUNT_ATTR_RDONLY)
-		mnt_flags |= MNT_READONLY;
-	if (attr_flags & MOUNT_ATTR_NOSUID)
-		mnt_flags |= MNT_NOSUID;
-	if (attr_flags & MOUNT_ATTR_NODEV)
-		mnt_flags |= MNT_NODEV;
-	if (attr_flags & MOUNT_ATTR_NOEXEC)
-		mnt_flags |= MNT_NOEXEC;
-	if (attr_flags & MOUNT_ATTR_NODIRATIME)
-		mnt_flags |= MNT_NODIRATIME;
+	mnt_flags = attr_flags_to_mnt_flags(attr_flags);
 
 	switch (attr_flags & MOUNT_ATTR__ATIME) {
 	case MOUNT_ATTR_STRICTATIME:
@@ -3787,6 +3835,362 @@ out0:
 	return error;
 }
 
+static unsigned int recalc_flags(struct mount_kattr *kattr, struct mount *mnt)
+{
+	unsigned int flags = mnt->mnt.mnt_flags;
+
+	/*  flags to clear */
+	flags &= ~kattr->attr_clr;
+	/* flags to raise */
+	flags |= kattr->attr_set;
+
+	return flags;
+}
+
+static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
+{
+	struct vfsmount *m = &mnt->mnt;
+
+	if (!kattr->mnt_userns)
+		return 0;
+
+	/*
+	 * Once a mount has been idmapped we don't allow it to change its
+	 * mapping. It makes things simpler and callers can just create
+	 * another bind-mount they can idmap if they want to.
+	 */
+	if (mnt_user_ns(m) != &init_user_ns)
+		return -EPERM;
+
+	/* The underlying filesystem doesn't support idmapped mounts yet. */
+	if (!(m->mnt_sb->s_type->fs_flags & FS_ALLOW_IDMAP))
+		return -EINVAL;
+
+	/* We're not controlling the superblock. */
+	if (!ns_capable(m->mnt_sb->s_user_ns, CAP_SYS_ADMIN))
+		return -EPERM;
+
+	/* Mount has already been visible in the filesystem hierarchy. */
+	if (!is_anon_ns(mnt->mnt_ns))
+		return -EINVAL;
+
+	return 0;
+}
+
+static struct mount *mount_setattr_prepare(struct mount_kattr *kattr,
+					   struct mount *mnt, int *err)
+{
+	struct mount *m = mnt, *last = NULL;
+
+	if (!is_mounted(&m->mnt)) {
+		*err = -EINVAL;
+		goto out;
+	}
+
+	if (!(mnt_has_parent(m) ? check_mnt(m) : is_anon_ns(m->mnt_ns))) {
+		*err = -EINVAL;
+		goto out;
+	}
+
+	do {
+		unsigned int flags;
+
+		flags = recalc_flags(kattr, m);
+		if (!can_change_locked_flags(m, flags)) {
+			*err = -EPERM;
+			goto out;
+		}
+
+		*err = can_idmap_mount(kattr, m);
+		if (*err)
+			goto out;
+
+		last = m;
+
+		if ((kattr->attr_set & MNT_READONLY) &&
+		    !(m->mnt.mnt_flags & MNT_READONLY)) {
+			*err = mnt_hold_writers(m);
+			if (*err)
+				goto out;
+		}
+	} while (kattr->recurse && (m = next_mnt(m, mnt)));
+
+out:
+	return last;
+}
+
+static void do_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
+{
+	struct user_namespace *mnt_userns;
+
+	if (!kattr->mnt_userns)
+		return;
+
+	mnt_userns = get_user_ns(kattr->mnt_userns);
+	/* Pairs with smp_load_acquire() in mnt_user_ns(). */
+	smp_store_release(&mnt->mnt.mnt_userns, mnt_userns);
+}
+
+static void mount_setattr_commit(struct mount_kattr *kattr,
+				 struct mount *mnt, struct mount *last,
+				 int err)
+{
+	struct mount *m = mnt;
+
+	do {
+		if (!err) {
+			unsigned int flags;
+
+			do_idmap_mount(kattr, m);
+			flags = recalc_flags(kattr, m);
+			WRITE_ONCE(m->mnt.mnt_flags, flags);
+		}
+
+		/*
+		 * We either set MNT_READONLY above so make it visible
+		 * before ~MNT_WRITE_HOLD or we failed to recursively
+		 * apply mount options.
+		 */
+		if ((kattr->attr_set & MNT_READONLY) &&
+		    (m->mnt.mnt_flags & MNT_WRITE_HOLD))
+			mnt_unhold_writers(m);
+
+		if (!err && kattr->propagation)
+			change_mnt_propagation(m, kattr->propagation);
+
+		/*
+		 * On failure, only cleanup until we found the first mount
+		 * we failed to handle.
+		 */
+		if (err && m == last)
+			break;
+	} while (kattr->recurse && (m = next_mnt(m, mnt)));
+
+	if (!err)
+		touch_mnt_namespace(mnt->mnt_ns);
+}
+
+static int do_mount_setattr(struct path *path, struct mount_kattr *kattr)
+{
+	struct mount *mnt = real_mount(path->mnt), *last = NULL;
+	int err = 0;
+
+	if (path->dentry != mnt->mnt.mnt_root)
+		return -EINVAL;
+
+	if (kattr->propagation) {
+		/*
+		 * Only take namespace_lock() if we're actually changing
+		 * propagation.
+		 */
+		namespace_lock();
+		if (kattr->propagation == MS_SHARED) {
+			err = invent_group_ids(mnt, kattr->recurse);
+			if (err) {
+				namespace_unlock();
+				return err;
+			}
+		}
+	}
+
+	lock_mount_hash();
+
+	/*
+	 * Get the mount tree in a shape where we can change mount
+	 * properties without failure.
+	 */
+	last = mount_setattr_prepare(kattr, mnt, &err);
+	if (last) /* Commit all changes or revert to the old state. */
+		mount_setattr_commit(kattr, mnt, last, err);
+
+	unlock_mount_hash();
+
+	if (kattr->propagation) {
+		namespace_unlock();
+		if (err)
+			cleanup_group_ids(mnt, NULL);
+	}
+
+	return err;
+}
+
+static int build_mount_idmapped(const struct mount_attr *attr, size_t usize,
+				struct mount_kattr *kattr, unsigned int flags)
+{
+	int err = 0;
+	struct ns_common *ns;
+	struct user_namespace *mnt_userns;
+	struct file *file;
+
+	if (!((attr->attr_set | attr->attr_clr) & MOUNT_ATTR_IDMAP))
+		return 0;
+
+	/*
+	 * We currently do not support clearing an idmapped mount. If this ever
+	 * is a use-case we can revisit this but for now let's keep it simple
+	 * and not allow it.
+	 */
+	if (attr->attr_clr & MOUNT_ATTR_IDMAP)
+		return -EINVAL;
+
+	if (attr->userns_fd > INT_MAX)
+		return -EINVAL;
+
+	file = fget(attr->userns_fd);
+	if (!file)
+		return -EBADF;
+
+	if (!proc_ns_file(file)) {
+		err = -EINVAL;
+		goto out_fput;
+	}
+
+	ns = get_proc_ns(file_inode(file));
+	if (ns->ops->type != CLONE_NEWUSER) {
+		err = -EINVAL;
+		goto out_fput;
+	}
+
+	/*
+	 * The init_user_ns is used to indicate that a vfsmount is not idmapped.
+	 * This is simpler than just having to treat NULL as unmapped. Users
+	 * wanting to idmap a mount to init_user_ns can just use a namespace
+	 * with an identity mapping.
+	 */
+	mnt_userns = container_of(ns, struct user_namespace, ns);
+	if (mnt_userns == &init_user_ns) {
+		err = -EPERM;
+		goto out_fput;
+	}
+	kattr->mnt_userns = get_user_ns(mnt_userns);
+
+out_fput:
+	fput(file);
+	return err;
+}
+
+static int build_mount_kattr(const struct mount_attr *attr, size_t usize,
+			     struct mount_kattr *kattr, unsigned int flags)
+{
+	unsigned int lookup_flags = LOOKUP_AUTOMOUNT | LOOKUP_FOLLOW;
+
+	if (flags & AT_NO_AUTOMOUNT)
+		lookup_flags &= ~LOOKUP_AUTOMOUNT;
+	if (flags & AT_SYMLINK_NOFOLLOW)
+		lookup_flags &= ~LOOKUP_FOLLOW;
+	if (flags & AT_EMPTY_PATH)
+		lookup_flags |= LOOKUP_EMPTY;
+
+	*kattr = (struct mount_kattr) {
+		.lookup_flags	= lookup_flags,
+		.recurse	= !!(flags & AT_RECURSIVE),
+	};
+
+	if (attr->propagation & ~MOUNT_SETATTR_PROPAGATION_FLAGS)
+		return -EINVAL;
+	if (hweight32(attr->propagation & MOUNT_SETATTR_PROPAGATION_FLAGS) > 1)
+		return -EINVAL;
+	kattr->propagation = attr->propagation;
+
+	if ((attr->attr_set | attr->attr_clr) & ~MOUNT_SETATTR_VALID_FLAGS)
+		return -EINVAL;
+
+	kattr->attr_set = attr_flags_to_mnt_flags(attr->attr_set);
+	kattr->attr_clr = attr_flags_to_mnt_flags(attr->attr_clr);
+
+	/*
+	 * Since the MOUNT_ATTR_<atime> values are an enum, not a bitmap,
+	 * users wanting to transition to a different atime setting cannot
+	 * simply specify the atime setting in @attr_set, but must also
+	 * specify MOUNT_ATTR__ATIME in the @attr_clr field.
+	 * So ensure that MOUNT_ATTR__ATIME can't be partially set in
+	 * @attr_clr and that @attr_set can't have any atime bits set if
+	 * MOUNT_ATTR__ATIME isn't set in @attr_clr.
+	 */
+	if (attr->attr_clr & MOUNT_ATTR__ATIME) {
+		if ((attr->attr_clr & MOUNT_ATTR__ATIME) != MOUNT_ATTR__ATIME)
+			return -EINVAL;
+
+		/*
+		 * Clear all previous time settings as they are mutually
+		 * exclusive.
+		 */
+		kattr->attr_clr |= MNT_RELATIME | MNT_NOATIME;
+		switch (attr->attr_set & MOUNT_ATTR__ATIME) {
+		case MOUNT_ATTR_RELATIME:
+			kattr->attr_set |= MNT_RELATIME;
+			break;
+		case MOUNT_ATTR_NOATIME:
+			kattr->attr_set |= MNT_NOATIME;
+			break;
+		case MOUNT_ATTR_STRICTATIME:
+			break;
+		default:
+			return -EINVAL;
+		}
+	} else {
+		if (attr->attr_set & MOUNT_ATTR__ATIME)
+			return -EINVAL;
+	}
+
+	return build_mount_idmapped(attr, usize, kattr, flags);
+}
+
+static void finish_mount_kattr(struct mount_kattr *kattr)
+{
+	put_user_ns(kattr->mnt_userns);
+	kattr->mnt_userns = NULL;
+}
+
+SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
+		unsigned int, flags, struct mount_attr __user *, uattr,
+		size_t, usize)
+{
+	int err;
+	struct path target;
+	struct mount_attr attr;
+	struct mount_kattr kattr;
+
+	BUILD_BUG_ON(sizeof(struct mount_attr) != MOUNT_ATTR_SIZE_VER0);
+
+	if (flags & ~(AT_EMPTY_PATH |
+		      AT_RECURSIVE |
+		      AT_SYMLINK_NOFOLLOW |
+		      AT_NO_AUTOMOUNT))
+		return -EINVAL;
+
+	if (unlikely(usize > PAGE_SIZE))
+		return -E2BIG;
+	if (unlikely(usize < MOUNT_ATTR_SIZE_VER0))
+		return -EINVAL;
+
+	if (!may_mount())
+		return -EPERM;
+
+	err = copy_struct_from_user(&attr, sizeof(attr), uattr, usize);
+	if (err)
+		return err;
+
+	/* Don't bother walking through the mounts if this is a nop. */
+	if (attr.attr_set == 0 &&
+	    attr.attr_clr == 0 &&
+	    attr.propagation == 0)
+		return 0;
+
+	err = build_mount_kattr(&attr, usize, &kattr, flags);
+	if (err)
+		return err;
+
+	err = user_path_at(dfd, path, kattr.lookup_flags, &target);
+	if (err)
+		return err;
+
+	err = do_mount_setattr(&target, &kattr);
+	finish_mount_kattr(&kattr);
+	path_put(&target);
+	return err;
+}
+
 static void __init init_mount_tree(void)
 {
 	struct vfsmount *mnt;