[PATCH 08/21] namespace: never expire a locked mount

[Christian Brauner]

Locked mounts are special. They protect the underlying files and
directories from being revealed. do_umount() refuses to unmount locked
mounts but shrink_submounts() doesn't.

A shrinkable mount can become locked once the owner of a user namespace
puts it beneath a locked mount with MOVE_MOUNT_BENEATH. The locked
property now moves to the mount at the bottom making it possible to
unmount the top mount.

So umount() of an unlocked ancestor now expires the bottom mount first
and the covered directory is revealed:

  move_mount(c -> x/hidden, MOVE_MOUNT_BENEATH) = 0
  the cover after the lock moved:  umount2(x/hidden) = 0
  the holder of the lock:          umount2(x/hidden) = EINVAL
  the root of the copy, busy:      umount2(x) = EBUSY
  reads x/hidden/secret: "covered-by-root"

So leave a locked mount alone as it dies together with its parent. A
plain umount() of an unlocked mount with a locked one below it is EBUSY
from now on. It's the same for any other locked child. A lazy umount
still takes the whole tree.

mark_mounts_for_expiry() never sees a locked mount. lock_mnt_tree()
leaves a mount on an expiry list alone and nothing else puts a locked
one on such a list. Add an assert for this.

Fixes: 5ff9d8a65ce8 ("vfs: Lock in place mounts from more privileged users")
Fixes: c62a4766937e ("move_mount: transfer MNT_LOCKED")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>
---
 fs/namespace.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 27bf8665ed58..e74e63466c24 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4032,6 +4032,8 @@ void mark_mounts_for_expiry(struct list_head *mounts)
 	list_for_each_entry_safe(mnt, next, mounts, mnt_expire) {
 		if (!is_mounted(&mnt->mnt))
 			continue;
+		/* lock_mnt_tree() leaves expirable mounts alone */
+		VFS_WARN_ON_ONCE(IS_MNT_LOCKED(mnt));
 		if (!xchg(&mnt->mnt_expiry_mark, 1) ||
 			propagate_mount_busy(mnt, 1))
 			continue;
@@ -4058,7 +4060,8 @@ EXPORT_SYMBOL_GPL(mark_mounts_for_expiry);
  */
 static bool shrink_submount(struct mount *mnt)
 {
-	if (propagate_mount_busy(mnt, 1))
+	/* not the kernel's to remove either */
+	if (IS_MNT_LOCKED(mnt) || propagate_mount_busy(mnt, 1))
 		return false;
 	touch_mnt_namespace(mnt->mnt_ns);
 	umount_tree(mnt, UMOUNT_PROPAGATE|UMOUNT_SYNC);

-- 
2.53.0