nbd: fix uaf in nbd_handle_reply()

There is a problem that nbd_handle_reply() might access freed request:

1) At first, a normal io is submitted and completed with scheduler:

internel_tag = blk_mq_get_tag -> get tag from sched_tags
 blk_mq_rq_ctx_init
  sched_tags->rq[internel_tag] = sched_tag->static_rq[internel_tag]
...
blk_mq_get_driver_tag
 __blk_mq_get_driver_tag -> get tag from tags
 tags->rq[tag] = sched_tag->static_rq[internel_tag]

So, both tags->rq[tag] and sched_tags->rq[internel_tag] are pointing
to the request: sched_tags->static_rq[internal_tag]. Even if the
io is finished.

2) nbd server send a reply with random tag directly:

recv_work
 nbd_handle_reply
  blk_mq_tag_to_rq(tags, tag)
   rq = tags->rq[tag]

3) if the sched_tags->static_rq is freed:

blk_mq_sched_free_requests
 blk_mq_free_rqs(q->tag_set, hctx->sched_tags, i)
  -> step 2) access rq before clearing rq mapping
  blk_mq_clear_rq_mapping(set, tags, hctx_idx);
  __free_pages() -> rq is freed here

4) Then, nbd continue to use the freed request in nbd_handle_reply

Fix the problem by get 'q_usage_counter' before blk_mq_tag_to_rq(),
thus request is ensured not to be freed because 'q_usage_counter' is
not zero.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20210916141810.2325276-1-yukuai3@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -830,6 +830,7 @@ static void recv_work(struct work_struct *work)
 						     work);
 	struct nbd_device *nbd = args->nbd;
 	struct nbd_config *config = nbd->config;
+	struct request_queue *q = nbd->disk->queue;
 	struct nbd_sock *nsock;
 	struct nbd_cmd *cmd;
 	struct request *rq;
@@ -840,13 +841,28 @@ static void recv_work(struct work_struct *work)
 		if (nbd_read_reply(nbd, args->index, &reply))
 			break;
 
-		cmd = nbd_handle_reply(nbd, args->index, &reply);
-		if (IS_ERR(cmd))
+		/*
+		 * Grab .q_usage_counter so request pool won't go away, then no
+		 * request use-after-free is possible during nbd_handle_reply().
+		 * If queue is frozen, there won't be any inflight requests, we
+		 * needn't to handle the incoming garbage message.
+		 */
+		if (!percpu_ref_tryget(&q->q_usage_counter)) {
+			dev_err(disk_to_dev(nbd->disk), "%s: no io inflight\n",
+				__func__);
 			break;
+		}
+
+		cmd = nbd_handle_reply(nbd, args->index, &reply);
+		if (IS_ERR(cmd)) {
+			percpu_ref_put(&q->q_usage_counter);
+			break;
+		}
 
 		rq = blk_mq_rq_from_pdu(cmd);
 		if (likely(!blk_should_fake_timeout(rq->q)))
 			blk_mq_complete_request(rq);
+		percpu_ref_put(&q->q_usage_counter);
 	}
 
 	nsock = config->socks[args->index];