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

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