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__follow_mount_rcu(): verify that mount_lock remains unchanged

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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@kernel.org
X-sky-is-falling: unlikely
Fixes: da1ce0670c "vfs: add cross-rename"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2022-07-04 17:26:29 -04:00
parent b13baccc38
commit 20aac6c609
1 changed files with 2 additions and 0 deletions
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2
fs/namei.c
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@ -1505,6 +1505,8 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
* 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))