Re: [PATCH] block: fix null pointer dereference in blk_mq_rq_timed_out()

[Yufen Yu <yuyufen@huawei.com>]

On 2019/9/12 18:07, Ming Lei wrote:
> On Thu, Sep 12, 2019 at 04:49:15PM +0800, Yufen Yu wrote:
>>
>> On 2019/9/12 12:16, Ming Lei wrote:
>>> On Thu, Sep 12, 2019 at 11:29:18AM +0800, Yufen Yu wrote:
>>>> On 2019/9/12 10:46, Ming Lei wrote:
>>>>> On Sat, Sep 07, 2019 at 06:24:50PM +0800, Yufen Yu wrote:
>>>>>> There is a race condition between timeout check and completion for
>>>>>> flush request as follow:
>>>>>>
>>>>>> timeout_work    issue flush      issue flush
>>>>>>                    blk_insert_flush
>>>>>>                                     blk_insert_flush
>>>>>> blk_mq_timeout_work
>>>>>>                    blk_kick_flush
>>>>>>
>>>>>> blk_mq_queue_tag_busy_iter
>>>>>> blk_mq_check_expired(flush_rq)
>>>>>>
>>>>>>                    __blk_mq_end_request
>>>>>>                   flush_end_io
>>>>>>                   blk_kick_flush
>>>>>>                   blk_rq_init(flush_rq)
>>>>>>                   memset(flush_rq, 0)
>>>>> Not see there is memset(flush_rq, 0) in block/blk-flush.c
>>>> Call path as follow:
>>>>
>>>> blk_kick_flush
>>>>       blk_rq_init
>>>>           memset(rq, 0, sizeof(*rq));
>>> Looks I miss this one in blk_rq_init(), sorry for that.
>>>
>>> Given there are only two users of blk_rq_init(), one simple fix could be
>>> not clearing queue in blk_rq_init(), something like below?
>>>
>>> diff --git a/block/blk-core.c b/block/blk-core.c
>>> index 77807a5d7f9e..25e6a045c821 100644
>>> --- a/block/blk-core.c
>>> +++ b/block/blk-core.c
>>> @@ -107,7 +107,9 @@ EXPORT_SYMBOL_GPL(blk_queue_flag_test_and_set);
>>>    void blk_rq_init(struct request_queue *q, struct request *rq)
>>>    {
>>> -	memset(rq, 0, sizeof(*rq));
>>> +	const int offset = offsetof(struct request, q);
>>> +
>>> +	memset((void *)rq + offset, 0, sizeof(*rq) - offset);
>>>    	INIT_LIST_HEAD(&rq->queuelist);
>>>    	rq->q = q;
>>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>>> index 1ac790178787..382e71b8787d 100644
>>> --- a/include/linux/blkdev.h
>>> +++ b/include/linux/blkdev.h
>>> @@ -130,7 +130,7 @@ enum mq_rq_state {
>>>     * especially blk_mq_rq_ctx_init() to take care of the added fields.
>>>     */
>>>    struct request {
>>> -	struct request_queue *q;
>>> +	struct request_queue *q;	/* Must be the 1st field */
>>>    	struct blk_mq_ctx *mq_ctx;
>>>    	struct blk_mq_hw_ctx *mq_hctx;
>> Not set req->q as '0' can just avoid BUG_ON for NULL pointer deference.
>>
>> However, the root problem is that 'flush_rq' have been reused while
>> timeout function handle it currently. That means mq_ops->timeout() may
>> access old values remained by the last flush request and make the wrong
>> decision.
>>
>> Take the race condition in the patch as an example.
>>
>> blk_mq_check_expired
>>      blk_mq_rq_timed_out
>>          req->q->mq_ops->timeout  // Driver timeout handle may read old data
>>      refcount_dec_and_test(&rq)
>>      __blk_mq_free_request   // If rq have been reset has '1' in
>> blk_rq_init(), it will be free here.
>>
>> So, I think we should solve this problem completely. Just like normal
>> request,
>> we can prevent flush request to call end_io when timeout handle the request.
> Seems it isn't specific for 'flush_rq', and it should be one generic issue
> for any request which implements .end_io.
>
> For requests without defining .end_io, rq->ref is applied for protecting
> its lifetime. However, rq->end_io() is still called even if rq->ref doesn't
> drop to zero.
>
> If the above is correct, we need to let rq->ref to cover rq->end_io().

Thanks for catching what I have ignored. I am trying to fix the problem.

Thanks,
Yufen