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

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Message-ID: <43ed2c81-7e86-a106-3592-7f1944ce0f25@huaweicloud.com>
Date: Wed, 6 Nov 2024 11:25:07 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Ming Lei <ming.lei@...hat.com>, Yu Kuai <yukuai1@...weicloud.com>
Cc: Christoph Hellwig <hch@...radead.org>, axboe@...nel.dk,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 yi.zhang@...wei.com, yangerkun@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH -next] block: fix uaf for flush rq while iterating tags

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@...wei.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):

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;
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()...

Thanks,
Kuai

> 
> Thanks,
> Ming
> 
> 
> .
>