linux-kernel - Re: [BUG] skb corruption and kernel panic at forwarding with fragmentation

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Message-ID: <CALYGNiOjeh4Jfvmc6fJHaZYJyinXmZjTHXcacNreBXvzB1ajtQ@mail.gmail.com>
Date:	Wed, 6 Jan 2016 23:11:41 +0300
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] skb corruption and kernel panic at forwarding with fragmentation

On Wed, Jan 6, 2016 at 10:59 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> On Wed, Jan 6, 2016 at 11:15 AM, Konstantin Khlebnikov <koct9i@...il.com> wrote:
>> Looks like this happens because ip_options_fragment() relies on
>> correct ip options length in ip control block in skb. But in
>> ip_finish_output_gso() control block in segments is reused by
>> skb_gso_segment(). following ip_fragment() sees some garbage.
>>
>> In my case there was no ip options but length becomes non-zero and
>> ip_options_fragment() picked some bytes from payload and decides to
>> fill huge range with IPOPT_NOOP (1). One of that ones flipped nr_frags
>> in skb_shared_info at the end of data =)
>>
>
> Hmm, it looks like SKB_GSO_CB should be cleared after skb_gso_segment()
> since all the gso information should be saved in shared_info after it finishes.
>
> Does a memset(0) on SKB_GSO_CB after skb_gso_segment() work as well?

This will break present logic around ip_options_fragment() - it clears
options from
second and following fragments. With zeroed cb it will do nothing.

ip_options_fragment() can get required information directly from ip header but
it also resets fields in IPCB -- probably it should stay valid here
and somebody else will use it later.
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