linux-kernel - Re: [syzbot] UBSAN: array-index-out-of-bounds in io_submit

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Date:   Tue, 31 May 2022 02:56:46 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot <syzbot+b6c9b65b6753d333d833@...kaller.appspotmail.com>,
        asml.silence@...il.com, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] UBSAN: array-index-out-of-bounds in io_submit_sqes

On 5/31/22 2:52 AM, Dmitry Vyukov wrote:
> On Tue, 31 May 2022 at 10:45, Jens Axboe <axboe@...nel.dk> wrote:
>>
>> On 5/31/22 1:55 AM, syzbot wrote:
>>> Hello,
>>>
>>> syzbot found the following issue on:
>>>
>>> HEAD commit:    3b46e4e44180 Add linux-next specific files for 20220531
>>> git tree:       linux-next
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=16e151f5f00000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=ccb8d66fc9489ef
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=b6c9b65b6753d333d833
>>> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
>>>
>>> Unfortunately, I don't have any reproducer for this issue yet.
>>>
>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>> Reported-by: syzbot+b6c9b65b6753d333d833@...kaller.appspotmail.com
>>>
>>> ================================================================================
>>> ================================================================================
>>> UBSAN: array-index-out-of-bounds in fs/io_uring.c:8860:19
>>> index 75 is out of range for type 'io_op_def [47]'
>>
>> 'def' is just set here, it's not actually used after 'opcode' has been
>> verified.
> 
> An interesting thing about C is that now the compiler is within its
> rights to actually remove the check that is supposed to validate the
> index because indexing io_op_defs[opcode] implies that opcode is
> already within bounds, otherwise the program already has undefined
> behavior, so removing the check is that case is also OK ;)

I did fix this up as I think it's just a bug waiting to happen anyway.

-- 
Jens Axboe