Re: [PATCH v5 5/6] nbd: convert to use blk_mq_find_and_get_req()

On Tue, Sep 14, 2021 at 03:13:38PM +0800, yukuai (C) wrote:

On 2021/09/14 14:44, Ming Lei wrote:

quoted

On Tue, Sep 14, 2021 at 11:11:06AM +0800, yukuai (C) wrote:

quoted

On 2021/09/14 9:11, Ming Lei wrote:

quoted

On Thu, Sep 09, 2021 at 10:12:55PM +0800, Yu Kuai wrote:

quoted

blk_mq_tag_to_rq() can only ensure to return valid request in
following situation:

1) client send request message to server first
submit_bio
...
   blk_mq_get_tag
   ...
   blk_mq_get_driver_tag
   ...
   nbd_queue_rq
    nbd_handle_cmd
     nbd_send_cmd

2) client receive respond message from server
recv_work
   nbd_read_stat
    blk_mq_tag_to_rq

If step 1) is missing, blk_mq_tag_to_rq() will return a stale
request, which might be freed. Thus convert to use
blk_mq_find_and_get_req() to make sure the returned request is not
freed.

But NBD_CMD_INFLIGHT has been added for checking if the reply is
expected, do we still need blk_mq_find_and_get_req() for covering
this issue? BTW, request and its payload is pre-allocated, so there
isn't real use-after-free.

Hi, Ming

Checking NBD_CMD_INFLIGHT relied on the request founded by tag is valid,
not the other way round.

nbd_read_stat
  req = blk_mq_tag_to_rq()
  cmd = blk_mq_rq_to_pdu(req)
  mutex_lock(cmd->lock)
  checking NBD_CMD_INFLIGHT

Request and its payload is pre-allocated, and either req->ref or cmd->lock can
serve the same purpose here. Once cmd->lock is held, you can check if the cmd is
inflight or not. If it isn't inflight, just return -ENOENT. Is there any
problem to handle in this way?

Hi, Ming

in nbd_read_stat:

1) get a request by tag first
2) get nbd_cmd by the request
3) hold cmd->lock and check if cmd is inflight

If we want to check if the cmd is inflight in step 3), we have to do
setp 1) and 2) first. As I explained in patch 0, blk_mq_tag_to_rq()
can't make sure the returned request is not freed:

nbd_read_stat
			blk_mq_sched_free_requests
			 blk_mq_free_rqs
  blk_mq_tag_to_rq
  -> get rq before clear mapping
			  blk_mq_clear_rq_mapping
			  __free_pages -> rq is freed
  blk_mq_request_started -> UAF

If the above can happen, blk_mq_find_and_get_req() may not fix it too, just
wondering why not take the following simpler way for avoiding the UAF?

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 5170a630778d..dfa5cce71f66 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c

@@ -795,9 +795,13 @@ 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_cmd *cmd;
 	struct request *rq;
 
+	if (!percpu_ref_tryget(&q->q_usage_counter))
+                return;
+
 	while (1) {
 		cmd = nbd_read_stat(nbd, args->index);
 		if (IS_ERR(cmd)) {

@@ -813,6 +817,7 @@ static void recv_work(struct work_struct *work)
 		if (likely(!blk_should_fake_timeout(rq->q)))
 			blk_mq_complete_request(rq);
 	}
+	blk_queue_exit(q);
 	nbd_config_put(nbd);
 	atomic_dec(&config->recv_threads);
 	wake_up(&config->recv_wq);

Thanks,
Ming