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

["Zhang, Qiang" <Qiang.Zhang@windriver.com>]

________________________________________
发件人: Pavel Begunkov <asml.silence@gmail.com>
发送时间: 2021年5月24日 18:16
收件人: Zhang, Qiang; Hillf Danton; axboe@kernel.dk
抄送: syzbot+6cb11ade52aa17095297@syzkaller.appspotmail.com; io-uring@vger.kernel.org; linux-kernel@vger.kernel.org
主题: Re: 回复: [PATCH] io-wq: Fix UAF when wakeup wqe in hash waitqueue

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On 5/24/21 10:19 AM, Zhang, Qiang wrote:
[...]
>> Scratch scalp one inch off to work out how this is a cure given a) uaf makes
>> no sense without free and b) how io workers could survive
>> wait_for_completion(&wq->worker_done).
>>
>> If they could OTOH then this is not the pill for the leak in worker_refs.
>
> Hello Pavel Begunkov, Hillf Danton
>
> Sorry there is a problem with the calltrace described in my message. Please ignore this modification
>
>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?

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

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

Zhang
>
>--
>Pavel Begunkov