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

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On 2021/09/14 17:08, yukuai (C) wrote:
On 2021/09/14 15:46, Ming Lei wrote:
On Tue, Sep 14, 2021 at 03:13:38PM +0800, yukuai (C) wrote:
On 2021/09/14 14:44, Ming Lei wrote:
On Tue, Sep 14, 2021 at 11:11:06AM +0800, yukuai (C) wrote:
On 2021/09/14 9:11, Ming Lei wrote:
On Thu, Sep 09, 2021 at 10:12:55PM +0800, Yu Kuai wrote:
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

Hi, Ming

Why can't blk_mq_find_and_get_req() fix it? I can't think of any
scenario that might have problem currently.

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;
+

We can't make sure freeze_queue is called before this, thus this approch
can't fix the problem, right?
  nbd_read_stat
     blk_mq_tag_to_rq
             elevator_switch
              blk_mq_freeze_queue(q);
              elevator_switch_mq
               elevator_exit
                blk_mq_sched_free_requests
     blk_mq_request_started -> UAF

Hi, Ming

I forgot that if percpu_ref_tryget succeed here, blk_mq_free_queue()
will block untill blk_queue_exit() in nbd_read_stat().

Thanks,
Kuai

Thanks,
Kuai

      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

.




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