__legitimize_mnt(): check for MNT_SYNC_UMOUNT should be under mount_lock

... 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>

diff --git a/fs/namespace.c b/fs/namespace.c
index 98a5cd756e9ae44120f5f110fc233dc882cbc3f8..eba4748388b1d002dbe91ed651948c7554123701 100644 (file)
--- 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;