linux-kernel - Re: KASAN: slab-out-of-bounds Read in bpf_skb_change

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Date:   Mon, 11 Jun 2018 11:52:00 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     syzbot <syzbot+d2d729bdde65dee3eae6@...kaller.appspotmail.com>,
        Alexei Starovoitov <ast@...nel.org>,
        David Miller <davem@...emloft.net>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: KASAN: slab-out-of-bounds Read in bpf_skb_change_proto

On Mon, Jun 11, 2018 at 11:42 AM, Daniel Borkmann <daniel@...earbox.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@...kaller.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.