[PATCH][RFC] __legitimize_mnt(): check for MNT_SYNC_UMOUNT should be under mount_lock

[PATCH][RFC] __legitimize_mnt(): check for MNT_SYNC_UMOUNT should be under mount_lock

[Al Viro]

[another catch from struct mount audit]
... or we risk stealing final mntput from sync umount - raising mnt_count
after umount(2) has verified that victim is not busy, but before it
has set MNT_SYNC_UMOUNT; in that case __legitimize_mnt() doesn't see
that it's safe to quietly undo mnt_count increment and leaves dropping
the reference to caller, where it'll be a full-blown mntput().

Check under mount_lock is needed; leaving the current one done before
taking that makes no sense - it's nowhere near common enough to bother
with.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
diff --git a/fs/namespace.c b/fs/namespace.c
index 98a5cd756e9a..eba4748388b1 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -790,12 +790,8 @@ int __legitimize_mnt(struct vfsmount *bastard, unsigned seq)
 	smp_mb();			// see mntput_no_expire()
 	if (likely(!read_seqretry(&mount_lock, seq)))
 		return 0;
-	if (bastard->mnt_flags & MNT_SYNC_UMOUNT) {
-		mnt_add_count(mnt, -1);
-		return 1;
-	}
 	lock_mount_hash();
-	if (unlikely(bastard->mnt_flags & MNT_DOOMED)) {
+	if (unlikely(bastard->mnt_flags & (MNT_SYNC_UMOUNT | MNT_DOOMED))) {
 		mnt_add_count(mnt, -1);
 		unlock_mount_hash();
 		return 1;

Re: [PATCH][RFC] __legitimize_mnt(): check for MNT_SYNC_UMOUNT should be under mount_lock

[Christian Brauner]

On Sun, Apr 27, 2025 at 08:50:02PM +0100, Al Viro wrote:
> [another catch from struct mount audit]
> ... or we risk stealing final mntput from sync umount - raising mnt_count
> after umount(2) has verified that victim is not busy, but before it
> has set MNT_SYNC_UMOUNT; in that case __legitimize_mnt() doesn't see
> that it's safe to quietly undo mnt_count increment and leaves dropping
> the reference to caller, where it'll be a full-blown mntput().
> 
> Check under mount_lock is needed; leaving the current one done before
> taking that makes no sense - it's nowhere near common enough to bother
> with.
> 
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---

Reviewed-by: Christian Brauner <brauner@kernel.org>