This is a note to let you know that I've just added the patch titled __follow_mount_rcu(): verify that mount_lock remains unchanged to the 5.19-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: __follow_mount_rcu-verify-that-mount_lock-remains-unchanged.patch and it can be found in the queue-5.19 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 20aac6c60981f5bfacd66661d090d907bf1482f0 Mon Sep 17 00:00:00 2001 From: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Date: Mon, 4 Jul 2022 17:26:29 -0400 Subject: __follow_mount_rcu(): verify that mount_lock remains unchanged 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 @@ -1505,6 +1505,8 @@ 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)) Patches currently in stable-queue which might be from viro@xxxxxxxxxxxxxxxxxx are queue-5.19/fs-check-fmode_lseek-to-control-internal-pipe-splici.patch queue-5.19/epoll-autoremove-wakers-even-more-aggressively.patch queue-5.19/proc-fix-a-dentry-lock-race-between-release_task-and.patch queue-5.19/vfs-check-the-truncate-maximum-size-in-inode_newsize_ok.patch queue-5.19/__follow_mount_rcu-verify-that-mount_lock-remains-unchanged.patch queue-5.19/fix-short-copy-handling-in-copy_mc_pipe_to_iter.patch