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Subject: Re: [PATCH-next] block: fix null-deref in percpu_ref_put
To:     Dennis Zhou <dennis@kernel.org>,
        Zhong Jinghua <zhongjinghua@huawei.com>,
        Ming Lei <ming.lei@redhat.com>
Cc:     tj@kernel.org, cl@linux.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, yi.zhang@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20221206090939.871239-1-zhongjinghua@huawei.com>
 <Y4/mzMd4evRg9yDi@fedora>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <4b826950-52a5-b50b-1086-c14422ca3039@huaweicloud.com>
Date:   Wed, 7 Dec 2022 21:10:33 +0800
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Hi,

?? 2022/12/07 9:05, Dennis Zhou д??:
> Hello,
> 
> On Tue, Dec 06, 2022 at 05:09:39PM +0800, Zhong Jinghua wrote:
>> A problem was find in stable 5.10 and the root cause of it like below.
>>
>> In the use of q_usage_counter of request_queue, blk_cleanup_queue using
>> "wait_event(q->mq_freeze_wq, percpu_ref_is_zero(&q->q_usage_counter))"
>> to wait q_usage_counter becoming zero. however, if the q_usage_counter
>> becoming zero quickly, and percpu_ref_exit will execute and ref->data
>> will be freed, maybe another process will cause a null-defef problem
>> like below:
>>
>> 	CPU0                             CPU1
>> blk_mq_destroy_queue
>>   blk_freeze_queue
>>    blk_mq_freeze_queue_wait
>> 				scsi_end_request
>> 				 percpu_ref_get
>> 				 ...
>> 				 percpu_ref_put
>> 				  atomic_long_sub_and_test
>>   blk_put_queue
>>    kobject_put
>>     kref_put
>>      blk_release_queue
>>       percpu_ref_exit
>>        ref->data -> NULL
>>     				   ref->data->release(ref) -> null-deref
>>
> 
> I remember thinking about this a while ago. I don't think this fix works
> as nicely as it may seem. Please correct me if I'm wrong.
> 
> q->q_usage_counter has the oddity that the lifetime of the percpu_ref
> object isn't managed by the release function. The freeing is handled by
> a separate path where it depends on the percpu_ref hitting 0. So here we
> have 2 concurrent paths racing to run with 1 destroying the object. We
> probably need blk_release_queue() to wait on percpu_ref's release
> finishing, not starting.
> 
> I think the above works in this specific case because there is a
> call_rcu() in blk_release_queue(). If there wasn't a call_rcu(),
> then by the same logic we could delay ref->data->release(ref) further
> and that could potentially lead to a use after free.
> 
> Ideally, I think fixing the race in q->q_usage_counter's pattern is
> better than masking it here as I think we're being saved by the
> call_rcu() call further down the object release path.

Agree.

BTW, Wensheng used to send a patch to fix this in block layer:

https://www.spinics.net/lists/kernel/msg4615696.html.

Thanks,
Kuai