Date:   Thu, 19 May 2022 06:13:40 -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 5/19/22 3:26 AM, Lee Jones wrote:
> On Wed, 18 May 2022, Jens Axboe wrote:
> 
>> On 5/18/22 10:34 AM, Lee Jones wrote:
>>> On Wed, 18 May 2022, Jens Axboe wrote:
>>>
>>>> On 5/18/22 09:39, Lee Jones wrote:
>>>>> On Wed, 18 May 2022, Jens Axboe wrote:
>>>>>
>>>>>> On 5/18/22 9:14 AM, Lee Jones wrote:
>>>>>>> On Wed, 18 May 2022, Jens Axboe wrote:
>>>>>>>
>>>>>>>> On 5/18/22 6:54 AM, Jens Axboe wrote:
>>>>>>>>> On 5/18/22 6:52 AM, Jens Axboe wrote:
>>>>>>>>>> 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...
>>>>>>>>>
>>>>>>>>> Looks like I sent the one from the wrong directory, sorry about that.
>>>>>>>>> This one should be better:
>>>>>>>>
>>>>>>>> Nope, both are the right one. Maybe your mailer is mangling the patch?
>>>>>>>> I'll attach it gzip'ed here in case that helps.
>>>>>>>
>>>>>>> Okay, that applied, thanks.
>>>>>>>
>>>>>>> Unfortunately, I am still able to crash the kernel in the same way.
>>>>>>
>>>>>> Alright, maybe it's not enough. I can't get your reproducer to crash,
>>>>>> unfortunately. I'll try on a different box.
>>>>>
>>>>> You need to have fuzzing and kasan enabled.
>>>>
>>>> I do have kasan enabled. What's fuzzing?
>>>
>>> CONFIG_KCOV
>>
>> Ah ok - I don't think that's needed for this.
>>
>> Looking a bit deeper at this, I'm now convinced your bisect went off the
>> rails at some point. Probably because this can be timing specific.
>>
>> Can you try with this patch?
>>
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 4330603eae35..3ecf71151fb1 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -4252,12 +4252,8 @@ static int io_statx(struct io_kiocb *req, bool force_nonblock)
>>  	struct io_statx *ctx = &req->statx;
>>  	int ret;
>>  
>> -	if (force_nonblock) {
>> -		/* only need file table for an actual valid fd */
>> -		if (ctx->dfd == -1 || ctx->dfd == AT_FDCWD)
>> -			req->flags |= REQ_F_NO_FILE_TABLE;
>> +	if (force_nonblock)
>>  		return -EAGAIN;
>> -	}
>>  
>>  	ret = do_statx(ctx->dfd, ctx->filename, ctx->flags, ctx->mask,
>>  		       ctx->buffer);
> 
> This does appear to solve the issue. :)
> 
> Thanks so much for working on this.
> 
> What are the next steps?
> 
> Are you able to submit this to Stable?

Yes, I'll get it queued up for stable.

-- 
Jens Axboe

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