do_umount(): add missing barrier before refcount checks in sync case

[ Upstream commit 65781e19dcfcb4aed1167d87a3ffcc2a0c071d47 ]

do_umount() analogue of the race fixed in 119e1ef "fix
__legitimize_mnt()/mntput() race".  Here we want to make sure that
if __legitimize_mnt() doesn't notice our lock_mount_hash(), we will
notice their refcount increment.  Harder to hit than mntput_no_expire()
one, fortunately, and consequences are milder (sync umount acting
like umount -l on a rare race with RCU pathwalk hitting at just the
wrong time instead of use-after-free galore mntput_no_expire()
counterpart used to be hit).  Still a bug...

Fixes: 48a066e ("RCU'd vfsmounts")
Reviewed-by: Christian Brauner <[email protected]>
Signed-off-by: Al Viro <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
(cherry picked from commit 7ced6fd9d6f87adb4b0f0cbf96983be83f061b4d)
Signed-off-by: Vijayendra Suman <[email protected]>

Author: Al Viro

fs/namespace.c

@@ -586,7 +586,7 @@ int __legitimize_mnt(struct vfsmount *bastard, unsigned seq)
 		return 0;
 	mnt = real_mount(bastard);
 	mnt_add_count(mnt, 1);
-	smp_mb();			// see mntput_no_expire()
+	smp_mb();		// see mntput_no_expire() and do_umount()
 	if (likely(!read_seqretry(&mount_lock, seq)))
 		return 0;
 	if (bastard->mnt_flags & MNT_SYNC_UMOUNT) {
@@ -1664,6 +1664,7 @@ static int do_umount(struct mount *mnt, int flags)
 			umount_tree(mnt, UMOUNT_PROPAGATE);
 		retval = 0;
 	} else {
+		smp_mb(); // paired with __legitimize_mnt()
 		shrink_submounts(mnt);
 		retval = -EBUSY;
 		if (!propagate_mount_busy(mnt, 2)) {