netdev - Re: [PATCH] xt_TCPMSS: SYN packets are allowed to contain data

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Date:	Tue, 19 Jan 2010 12:43:46 -0000
From:	"Simon Arlott" <simon@...e.lp0.eu>
To:	"Patrick McHardy" <kaber@...sh.net>
Cc:	"William Allen Simpson" <william.allen.simpson@...il.com>,
	"netdev" <netdev@...r.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Netfilter Development Mailinglist" <netfilter-devel@...r.kernel.org>
Subject: Re: [PATCH] xt_TCPMSS: SYN packets are allowed to contain data

On Tue, January 19, 2010 09:30, Patrick McHardy wrote:
> William Allen Simpson wrote:
>> Simon Arlott wrote:
>>> This moves the check for (header size != packet size) to after
>>> attempting to modify an existing MSS option. Another check is
>>> needed before looking through the header to ensure it doesn't
>>> claim to be larger than the packet size.
>>>
>> What's the path from tcp_v[4,6]_rcv() to these tests?
>>
>> 1) Header larger than the packet is already tested in about 5 places,
>> and my patch "tcp: harmonize tcp_vx_rcv header length assumptions"
>> tries to get them all down to just *one* test.
>
> We're talking about a netfilter module here, which has to deal
> with forwarded traffic and can only rely on the IP header checks
> done in ip_rcv().

My gateway (where these error messages occur) is running 2.6.29,
and skb->len (from the prink) is 40 bytes.

If this is 20 (IPv4 Header) + 20 (TCP Header) = 40 bytes, then there
is no data and the header offset is wrong so it hasn't been checked.

-- 
Simon Arlott
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