linux-kernel - Re: [REPORT] Use-after-free Read in __fdget

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Date:   Wed, 18 May 2022 06:52:36 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [REPORT] Use-after-free Read in __fdget_raw in v5.10.y

On 5/18/22 6:50 AM, Lee Jones wrote:
> On Tue, 17 May 2022, Jens Axboe wrote:
> 
>> On 5/17/22 7:00 AM, Lee Jones wrote:
>>> On Tue, 17 May 2022, Jens Axboe wrote:
>>>
>>>> On 5/17/22 6:36 AM, Lee Jones wrote:
>>>>> On Tue, 17 May 2022, Jens Axboe wrote:
>>>>>
>>>>>> On 5/17/22 6:24 AM, Lee Jones wrote:
>>>>>>> On Tue, 17 May 2022, Jens Axboe wrote:
>>>>>>>
>>>>>>>> On 5/17/22 5:41 AM, Lee Jones wrote:
>>>>>>>>> Good afternoon Jens, Pavel, et al.,
>>>>>>>>>
>>>>>>>>> Not sure if you are presently aware, but there appears to be a
>>>>>>>>> use-after-free issue affecting the io_uring worker driver (fs/io-wq.c)
>>>>>>>>> in Stable v5.10.y.
>>>>>>>>>
>>>>>>>>> The full sysbot report can be seen below [0].
>>>>>>>>>
>>>>>>>>> The C-reproducer has been placed below that [1].
>>>>>>>>>
>>>>>>>>> I had great success running this reproducer in an infinite loop.
>>>>>>>>>
>>>>>>>>> My colleague reverse-bisected the fixing commit to:
>>>>>>>>>
>>>>>>>>>   commit fb3a1f6c745ccd896afadf6e2d6f073e871d38ba
>>>>>>>>>   Author: Jens Axboe <axboe@...nel.dk>
>>>>>>>>>   Date:   Fri Feb 26 09:47:20 2021 -0700
>>>>>>>>>
>>>>>>>>>        io-wq: have manager wait for all workers to exit
>>>>>>>>>
>>>>>>>>>        Instead of having to wait separately on workers and manager, just have
>>>>>>>>>        the manager wait on the workers. We use an atomic_t for the reference
>>>>>>>>>        here, as we need to start at 0 and allow increment from that. Since the
>>>>>>>>>        number of workers is naturally capped by the allowed nr of processes,
>>>>>>>>>        and that uses an int, there is no risk of overflow.
>>>>>>>>>
>>>>>>>>>        Signed-off-by: Jens Axboe <axboe@...nel.dk>
>>>>>>>>>
>>>>>>>>>     fs/io-wq.c | 30 ++++++++++++++++++++++--------
>>>>>>>>>     1 file changed, 22 insertions(+), 8 deletions(-)
>>>>>>>>
>>>>>>>> Does this fix it:
>>>>>>>>
>>>>>>>> commit 886d0137f104a440d9dfa1d16efc1db06c9a2c02
>>>>>>>> Author: Jens Axboe <axboe@...nel.dk>
>>>>>>>> Date:   Fri Mar 5 12:59:30 2021 -0700
>>>>>>>>
>>>>>>>>     io-wq: fix race in freeing 'wq' and worker access
>>>>>>>>
>>>>>>>> Looks like it didn't make it into 5.10-stable, but we can certainly
>>>>>>>> rectify that.
>>>>>>>
>>>>>>> Thanks for your quick response Jens.
>>>>>>>
>>>>>>> This patch doesn't apply cleanly to v5.10.y.
>>>>>>
>>>>>> This is probably why it never made it into 5.10-stable :-/
>>>>>
>>>>> Right.  It doesn't apply at all unfortunately.
>>>>>
>>>>>>> I'll have a go at back-porting it.  Please bear with me.
>>>>>>
>>>>>> Let me know if you into issues with that and I can help out.
>>>>>
>>>>> I think the dependency list is too big.
>>>>>
>>>>> Too much has changed that was never back-ported.
>>>>>
>>>>> Actually the list of patches pertaining to fs/io-wq.c alone isn't so
>>>>> bad, I did start to back-port them all but some of the big ones have
>>>>> fs/io_uring.c changes incorporated and that list is huge (256 patches
>>>>> from v5.10 to the fixing patch mentioned above).
>>>>
>>>> The problem is that 5.12 went to the new worker setup, and this patch
>>>> landed after that even though it also applies to the pre-native workers.
>>>> Hence the dependency chain isn't really as long as it seems, probably
>>>> just a few patches backporting the change references and completions.
>>>>
>>>> I'll take a look this afternoon.
>>>
>>> Thanks Jens.  I really appreciate it.
>>
>> Can you see if this helps? Untested...
> 
> What base does this apply against please?
> 
> I tried Mainline and v5.10.116 and both failed.

It's against 5.10.116, so that's puzzling. Let me double check I sent
the right one...

-- 
Jens Axboe