Re: net: GPF in eth_header

From: Eric Dumazet
Date: Sat Nov 26 2016 - 15:34:56 EST

2016-11-26 12:05 GMT-08:00 Eric Dumazet <erdlkml@xxxxxxxxx>:
>> 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...