[PATCH 5.10] md: fix NULL pointer dereference at reregister_snapshot()

[Alexandr Sapozhnkiov <alsp705@gmail.com>]

From: Alexandr Sapozhnikov <alsp705@gmail.com>

Return value of a function '__lookup_origin' is dereferenced 
at dm-snap.c:596 without checking for NULL, but it is usually 
checked for this function

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
Signed-off-by: Alexandr Sapozhnikov <alsp705@gmail.com>

---
 drivers/md/dm-snap.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 4668b2cd98f4..105f6e768ad1 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -590,12 +590,15 @@ static int register_snapshot(struct dm_snapshot *snap)
  */
 static void reregister_snapshot(struct dm_snapshot *s)
 {
+	struct origin *o;
 	struct block_device *bdev = s->origin->bdev;
 
 	down_write(&_origins_lock);
+	o = __lookup_origin(s->origin->bdev);
 
 	list_del(&s->list);
-	__insert_snapshot(__lookup_origin(bdev), s);
+	if (o)
+		__insert_snapshot(o, s);
 
 	up_write(&_origins_lock);
 }
-- 
2.43.0