Re: 回复: 回复: [PATCH] io-wq: Fix UAF when wakeup wqe in hash waitqueue

[Zhang, Qiang]

________________________________________
From: Pavel Begunkov <asml.silence@xxxxxxxxx>
Sent: Tuesday, 8 June 2021 01:38
To: Zhang, Qiang; Hillf Danton; axboe@xxxxxxxxx
Cc: syzbot+6cb11ade52aa17095297@xxxxxxxxxxxxxxxxxxxxxxxxx; io-uring@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
Subject: Re: 回复: 回复: [PATCH] io-wq: Fix UAF when wakeup wqe in hash waitqueue

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On 5/25/21 3:01 AM, Zhang, Qiang wrote:
[...]
>> Haven't looked at the trace and description, but I do think
>> there is a problem it solves.
>>
>> 1) io_wait_on_hash() -> __add_wait_queue(&hash->wait, &wqe->wait);
>> 2) (note: wqe is a worker) wqe's workers exit dropping refs
>> 3) refs are zero, free io-wq
>> 4) @hash is shared, so other task/wq does wake_up(&wq->hash->wait);
>> 5) it wakes freed wqe
>>
>> step 4) is a bit more trickier than that, tl;dr;
>> wq3:worker1     | locks bit1
>> wq1:worker2     | waits bit1
>> wq2:worker1     | waits bit1
>> wq1:worker3     | waits bit1
>>
>> wq3:worker1     | drop  bit1
>> wq1:worker2     | locks bit1
>> wq1:worker2     | completes all wq1 bit1 work items
>> wq1:worker2     | drop  bit1, exit and free io-wq
>>
>> wq2:worker1     | locks bit1
>> wq1             | free complete
>> wq2:worker1     | drops bit1
>> wq1:worker3     | waked up, even though freed
>>
>> Can be simplified, don't want to waste time on that
>
> Thanks Pavel
>
> Your description is better.  I have another question: under what circumstances will three io-wq(wq1, wq2, wq3) be created to share this @hash?

>Oops, missed the email. It's created by io_uring, and passed to
>io-wq, which is per-task and created on demand by io_uring.
>
>Can be achieved by a snippet just below, where threads
>haven't had io_uring instances before.
>
>thread1: ring = create_io_uring();
>thread2: submit_sqes(ring);
>thread3: submit_sqes(ring);

 Thank you for your explanation, Pavel

>
> This kind of problem also occurs between two io-wq(wq1, wq2). Is the following description OK？

>Yep, and I feel like there are cases simpler (and
>more likely) than the one I described.

>
> wq1:worker2     | locks bit1
> wq2:worker1     | waits bit1
> wq1:worker3     | waits bit1
>
> wq1:worker2     | completes all wq1 bit1 work items
> wq1:worker2     | drop  bit1, exit and free io-wq
>
> wq2:worker1     | locks bit1
> wq1                       | free complete
> wq2:worker1     | drops bit1
> wq1:worker3     | waked up, even though freed


>--
>Pavel Begunkov