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Subject: Re: KASAN: slab-out-of-bounds Read in bpf_skb_change_proto
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     syzbot <syzbot+d2d729bdde65dee3eae6@syzkaller.appspotmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        David Miller <davem@davemloft.net>,
        LKML <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
References: <000000000000c64842056e4b4326@google.com>
 <76483220-226b-bf3c-d529-ea839d6624c6@iogearbox.net>
 <CACT4Y+boO990R6a9kGtx-UZpZPeB2bwhgvyUjPvEFOpbzxJKrg@mail.gmail.com>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <170ef50b-6895-d7fa-a9ae-80915741971b@iogearbox.net>
Date:   Mon, 11 Jun 2018 12:31:37 +0200
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On 06/11/2018 11:52 AM, Dmitry Vyukov wrote:
> On Mon, Jun 11, 2018 at 11:42 AM, Daniel Borkmann <daniel@iogearbox.net> wrote:
>> On 06/10/2018 05:27 PM, syzbot wrote:
>>> Hello,
>>>
>>> syzbot found the following crash on:
>>>
>>> HEAD commit:    a16afaf7928b Merge tag 'for-v4.18' of git://git.kernel.org..
>>> git tree:       upstream
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=1338f6bf800000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=314f2150f36c16ca
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=d2d729bdde65dee3eae6
>>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>>> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=1173381f800000
>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=171f90cf800000
>>>
>>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>>> Reported-by: syzbot+d2d729bdde65dee3eae6@syzkaller.appspotmail.com
>>
>> #syz fix: bpf: reject passing modified ctx to helper functions
> 
> On a related note, it seems that it still can unwind past
> bpf_skb_change_proto. I think the "net.core.bpf_jit_kallsyms = 1"
> sysctl should have been reached syzbot by the time of crash. Are you
> sure that's the only thing requires? We are using frame pointer
> unwinder just in case.

Nah, I know what happened; just sent a PR fix for syzkaller.

Thanks,
Daniel