netdev - Re: net/ipv6: GPF in rt6

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Message-ID: <CAM_iQpWkUMVNb+MyQCMKFeXFo9TW5-TjzhKyugrVy=GBDNKBgg@mail.gmail.com>
Date:   Wed, 21 Jun 2017 14:01:03 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        James Morris <jmorris@...ei.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Patrick McHardy <kaber@...sh.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        David Ahern <dsa@...ulusnetworks.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: net/ipv6: GPF in rt6_ifdown

On Wed, Jun 21, 2017 at 1:08 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> On Wed, Jun 21, 2017 at 10:53 AM, Andrey Konovalov
> <andreyknvl@...gle.com> wrote:
>> On Wed, Jun 21, 2017 at 3:09 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
>>> On Wed, Jun 21, 2017 at 2:08 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
>>>> Hi,
>>>>
>>>> I've got the following error report while fuzzing the kernel with syzkaller.
>>>>
>>>> On commit 9705596d08ac87c18aee32cc97f2783b7d14624e (4.12-rc6+).
>>>>
>>>> It might be related to:
>>>> https://groups.google.com/forum/#!topic/syzkaller/ZJaqAiFLe3k
>>>>
>>>> I only have a reproducer in the form of a syzkaller program, attached
>>>> together with my .config.
>>>
>>> I now have a C reproducer as well, attached.
>>
>> And here's a much simpler reproducer.
>
> Thanks a lot for your reproducer!
>
> I added a few printk's, and find that we somehow have rt->rt6i_idev set
> to NULL but still keep it in the uncached list. ip6_dst_destroy() unlinks
> it before NULL'ing, so it should not be that case.

Heck... The loopback_dev->ip6_ptr is set to NULL because of
its mtu is set to smaller than IPV6_MIN_MTU, this is why we
have rt->rt6i_idev NULL after a rt6_uncached_list_flush_dev()...

I am thinking about a right fix.