Re: [PATCH -next] block: fix uaf for flush rq while iterating tags

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2024/11/06 11:36, Ming Lei 写道:
> On Wed, Nov 06, 2024 at 11:25:07AM +0800, Yu Kuai wrote:
>> Hi,
>>
>> 在 2024/11/06 11:11, Ming Lei 写道:
>>> On Wed, Nov 06, 2024 at 10:58:40AM +0800, Yu Kuai wrote:
>>>> Hi,Ming and Christoph
>>>>
>>>> 在 2024/11/05 19:19, Christoph Hellwig 写道:
>>>>> On Mon, Nov 04, 2024 at 07:00:05PM +0800, Yu Kuai wrote:
>>>>>> From: Yu Kuai <yukuai3@huawei.com>
>>>>>>
>>>>>> blk_mq_clear_flush_rq_mapping() is not called during scsi probe, by
>>>>>> checking blk_queue_init_done(). However, QUEUE_FLAG_INIT_DONE is cleared
>>>>>> in del_gendisk by commit aec89dc5d421 ("block: keep q_usage_counter in
>>>>>> atomic mode after del_gendisk"), hence for disk like scsi, following
>>>>>> blk_mq_destroy_queue() will not clear flush rq from tags->rqs[] as well,
>>>>>> cause following uaf that is found by our syzkaller for v6.6:
>>>>>
>>>>> Which means we leave the flush request lingering after del_gendisk,
>>>>> which sounds like the real bug.  I suspect we just need to move the
>>>>> call to blk_mq_clear_flush_rq_mapping so that it is called from
>>>>> del_gendisk and doesn't leave the flush tag lingering around.
>>>>>
>>>>
>>>> This remind me that del_gendisk is still too late to do that. Noted that
>>>> flush_rq can acquire different tags, so if the multiple flush_rq is done
>>>> and those tags are not reused, the flush_rq can exist in multiple
>>>> entries in tags->rqs[]. The consequence I can think of is that iterating
>>>> tags can found the same flush_rq multiple times, and the flush_rq can be
>>>> inflight.
>>>
>>> How can that be one problem?
>>>
>>> Please look at
>>>
>>> commit 364b61818f65 ("blk-mq: clearing flush request reference in tags->rqs[]")
>>> commit bd63141d585b ("blk-mq: clear stale request in tags->rq[] before freeing one request pool")
>>>
>>> and understand the motivation.
>>>
>>> That also means it is just fine to delay blk_mq_clear_flush_rq_mapping()
>>> after disk is deleted.
>>
>> I do understand what you mean. Let's see if you want this to be avoided,
>> for example(no disk is deleted):
> 
> It is definitely another issue, and not supposed to be covered by
> blk_mq_clear_flush_rq_mapping().
> 
>>
>> 1) issue a flush, and tag 0 is used, after the flush is done,
>> tags->rqs[0] = flush_rq
>> 2) issue another flush, and tag 1 is used, after the flush is done,
>> tags->rqs[1] = flush_rq
>> 3) issue a flush again, and tag 2 is used, and the flush_rq is
>> dispatched to disk;
> 
> Yes, this kind of thing exists since blk-mq begins, and you can't expect
> blk_mq_in_flight_rw() to get accurate inflight requests.
> 
>> 4) Then in this case, blk_mq_in_flight_rw() will found the same flush_rq
>> 3 times and think there are 3 inflight request, same for
>> hctx_busy_show()...
> 
> But we have tried to avoid it, such as in blk_mq_find_and_get_req()
> req->tag is checked against the current 'bitnr' of sbitmap when walking
> over tags. Then the same flush_rq won't be counted 3 times.

Yes, I missed that req->tag is checked. Looks like there is no porblem
now. :)

Just to make sure, we're still on the same page for this patch, right?

Thanks for the explanation.
Kuai

> 
> 
> 
> Thanks,
> Ming
> 
> 
> .
>