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H A D | namei.c | 20aac6c6 Mon Jul 04 16:26:29 CDT 2022 Al Viro <viro@zeniv.linux.org.uk> __follow_mount_rcu(): verify that mount_lock remains unchanged
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: da1ce0670c14 "vfs: add cross-rename" Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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