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

["yukuai (C)" <yukuai3@huawei.com>]

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


Thanks,
Kuai