linux-kernel - Re: [PATCH v5 5/6] nbd: convert to use blk_mq_find_and_get

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Date:   Tue, 14 Sep 2021 17:08:00 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Ming Lei <ming.lei@...hat.com>
CC:     <axboe@...nel.dk>, <josef@...icpanda.com>, <hch@...radead.org>,
        <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <nbd@...er.debian.org>, <yi.zhang@...wei.com>
Subject: Re: [PATCH v5 5/6] nbd: convert to use blk_mq_find_and_get_req()

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

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
> 
> .
>