[PATCH v7 6/6] nbd: fix uaf in nbd_handle_reply()

[Yu Kuai]

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@xxxxxxxxxx>
---
 drivers/block/nbd.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 9a7bbf8ebe74..3e8b70b5d4f9 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -824,6 +824,7 @@ 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;
@@ -834,7 +835,24 @@ static void recv_work(struct work_struct *work)
 		if (nbd_read_reply(nbd, args->index, &reply))
 			break;
 
+		/*
+		 * Grab ref of q_usage_counter can prevent request being freed
+		 * during nbd_handle_reply(). If q_usage_counter is zero, then
+		 * no request is inflight, which means something is wrong since
+		 * we expect to find a request to complete here.
+		 */
+		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);
+		/*
+		 * It's safe to drop ref before request completion, inflight
+		 * request will ensure q_usage_counter won't be zero.
+		 */
+		percpu_ref_put(&q->q_usage_counter);
 		if (IS_ERR(cmd))
 			break;
 
-- 
2.31.1