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

On Tue, Sep 14, 2021 at 05:08:00PM +0800, yukuai (C) wrote:

On 2021/09/14 15:46, Ming Lei wrote:

quoted

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

quoted

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

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.

The principle behind blk_mq_find_and_get_req() is that if one request's
ref is grabbed, the queue's usage counter is guaranteed to be grabbed,
and this way isn't straight-forward.

Yeah, it can fix the issue, but I don't think it is good to call it in
fast path cause tags->lock is required.

quoted

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

No, blk_mq_freeze_queue() waits until .q_usage_counter becomes zero, so
there won't be any concurrent nbd_read_stat() during switching elevator
if ->q_usage_counter is grabbed in recv_work().

Thanks,
Ming