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Subject: Re: general protection fault in fib_dump_info (2)
To:     syzbot <syzbot+a61aa19b0c14c8770bd9@syzkaller.appspotmail.com>,
        davem@davemloft.net, dsahern@gmail.com, kuba@kernel.org,
        kuznet@ms2.inr.ac.ru, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, syzkaller-bugs@googlegroups.com,
        yoshfuji@linux-ipv6.org
References: <00000000000039b10005ad64df20@google.com>
From:   Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Message-ID: <47e92c2b-c9c5-4c74-70c4-103e70e91630@cumulusnetworks.com>
Date:   Fri, 21 Aug 2020 19:00:18 +0300
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On 8/21/20 6:27 PM, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    da2968ff Merge tag 'pci-v5.9-fixes-1' of git://git.kernel...
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=137316ca900000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=a0437fdd630bee11
> dashboard link: https://syzkaller.appspot.com/bug?extid=a61aa19b0c14c8770bd9
> compiler:       gcc (GCC) 10.1.0-syz 20200507
> userspace arch: i386
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12707051900000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1150a046900000
> 
> The issue was bisected to:
> 
> commit 0b5e2e39739e861fa5fc84ab27a35dbe62a15330
> Author: David Ahern <dsahern@gmail.com>
> Date:   Tue May 26 18:56:16 2020 +0000
> 
>      nexthop: Expand nexthop_is_multipath in a few places
> 

This seems like a much older bug to me, the code allows to pass 0 groups and
thus we end up without any nh_grp_entry pointers. I reproduced it with a
modified iproute2 that sends an empty NHA_GROUP and then just uses the new
nexthop in any way (e.g. add a route with it). This is the same bug as the
earlier report for: "general protection fault in fib_check_nexthop"

I have a patch but I'll be able to send it tomorrow.

Cheers,
  Nik