[PATCH 5.18 0978/1095] __follow_mount_rcu(): verify that mount_lock remains unchanged

[Greg Kroah-Hartman]

From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

commit 20aac6c60981f5bfacd66661d090d907bf1482f0 upstream.

Validate mount_lock seqcount as soon as we cross into mount in RCU
mode.  Sure, ->mnt_root is pinned and will remain so until we
do rcu_read_unlock() anyway, and we will eventually fail to unlazy if
the mount_lock had been touched, but we might run into a hard error
(e.g. -ENOENT) before trying to unlazy.  And it's possible to end
up with RCU pathwalk racing with rename() and umount() in a way
that would fail with -ENOENT while non-RCU pathwalk would've
succeeded with any timings.

Once upon a time we hadn't needed that, but analysis had been subtle,
brittle and went out of window as soon as RENAME_EXCHANGE had been
added.

It's narrow, hard to hit and won't get you anything other than
stray -ENOENT that could be arranged in much easier way with the
same priveleges, but it's a bug all the same.

Cc: stable@xxxxxxxxxx
X-sky-is-falling: unlikely
Fixes: da1ce0670c14 "vfs: add cross-rename"
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/namei.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1511,6 +1511,8 @@ static bool __follow_mount_rcu(struct na
 				 * becoming unpinned.
 				 */
 				flags = dentry->d_flags;
+				if (read_seqretry(&mount_lock, nd->m_seq))
+					return false;
 				continue;
 			}
 			if (read_seqretry(&mount_lock, nd->m_seq))