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

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Date:   Mon, 24 May 2021 11:16:40 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     "Zhang, Qiang" <Qiang.Zhang@...driver.com>,
        Hillf Danton <hdanton@...a.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>
Cc:     "syzbot+6cb11ade52aa17095297@...kaller.appspotmail.com" 
        <syzbot+6cb11ade52aa17095297@...kaller.appspotmail.com>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: 回复: [PATCH] io-wq: Fix UAF when wakeup wqe in hash waitqueue

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

-- 
Pavel Begunkov