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

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Date:   Wed, 15 Sep 2021 09:54:09 +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 22:37, Ming Lei wrote:
> On Tue, Sep 14, 2021 at 05:19:31PM +0800, yukuai (C) wrote:
>> On 在 2021/09/14 15:46, Ming Lei wrote:
>>
>>> 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);
>>>
>>
>> Hi, Ming
>>
>> This apporch is wrong.
>>
>> If blk_mq_freeze_queue() is called, and nbd is waiting for all
>> request to complete. percpu_ref_tryget() will fail here, and deadlock
>> will occur because request can't complete in recv_work().
> 
> No, percpu_ref_tryget() won't fail until ->q_usage_counter is zero, when
> it is perfectly fine to do nothing in recv_work().
> 

Hi Ming

This apporch is a good idea, however we should not get q_usage_counter
in reccv_work(), because It will block freeze queue.

How about get q_usage_counter in nbd_read_stat(), and put in error path
or after request completion?

Thanks
Kuai