netdev - Re: net: GPF in eth

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Date:   Sat, 26 Nov 2016 12:34:38 -0800
From:   Eric Dumazet <erdlkml@...il.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     syzkaller <syzkaller@...glegroups.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        David Miller <davem@...emloft.net>,
        Tom Herbert <tom@...bertland.com>,
        Alexander Duyck <aduyck@...antis.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Jiri Benc <jbenc@...hat.com>,
        Sabrina Dubroca <sd@...asysnail.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: net: GPF in eth_header

2016-11-26 12:05 GMT-08:00 Eric Dumazet <erdlkml@...il.com>:
>> Hi Eric,
>>
>> The crash happens when the kernel tries to access shadow for nonmapped memory.
>>
>> The issue here is an integer overflow which happens in neigh_resolve_output().
>> skb_network_offset(skb) can return negative number, but __skb_pull()
>> accepts unsigned int as len.
>> As a result, the least significat bit in higher 32 bits of skb->data
>> gets set and we get an out-of-bounds with offset of 4 GB.
>>
>> I've attached a short reproducer, but you either need KASAN or to add
>> a BUG_ON to see the crash.
>> In this reproducer skb_network_offset() becomes negative after merging
>> two ipv6 fragments.
>>
>> I actually see multiple places where skb_network_offset() is used as
>> an argument to skb_pull().
>> So I guess every place can potentially be buggy.
>
> Well, I think the intent is to accept a negative number.
>
> This definitely was assumed by commit e1f165032c8bade authors !
>
> I guess they were using a 32bit kernel for their tests.

Correct fix would be to use

skb_push(skb, -skb_network_offset(skb));

As done in other locations...