[PATCH AUTOSEL 5.4 3/3] __legitimize_mnt(): check for MNT_SYNC_UMOUNT should be under mount_lock

[Sasha Levin <sashal@kernel.org>]

From: Al Viro <viro@zeniv.linux.org.uk>

[ Upstream commit 250cf3693060a5f803c5f1ddc082bb06b16112a9 ]

... 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.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/namespace.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 281f08eaba5b9..c1aa46f780f16 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -572,12 +572,8 @@ 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;
-- 
2.39.5