block: fix use-after-free in disk_part_iter_next

commit aebf5db upstream.

Make sure that bdgrab() is done on the 'block_device' instance before
referring to it for avoiding use-after-free.

Cc: <[email protected]>
Reported-by: [email protected]
Signed-off-by: Ming Lei <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Ming Lei

block/genhd.c

@@ -256,14 +256,17 @@ struct hd_struct *disk_part_iter_next(struct disk_part_iter *piter)
 		part = rcu_dereference(ptbl->part[piter->idx]);
 		if (!part)
 			continue;
+		get_device(part_to_dev(part));
+		piter->part = part;
 		if (!part_nr_sects_read(part) &&
 		    !(piter->flags & DISK_PITER_INCL_EMPTY) &&
 		    !(piter->flags & DISK_PITER_INCL_EMPTY_PART0 &&
-		      piter->idx == 0))
+		      piter->idx == 0)) {
+			put_device(part_to_dev(part));
+			piter->part = NULL;
 			continue;
+		}
 
-		get_device(part_to_dev(part));
-		piter->part = part;
 		piter->idx += inc;
 		break;
 	}