netdev - Re: [BUG] null pointer dereference in tcp_gso

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Date:	Wed, 22 Jan 2014 14:18:45 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Arnaud Ebalard <arno@...isbad.org>
Cc:	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Daniel Borkmann <dborkman@...hat.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Willy Tarreau <w@....eu>, netdev@...r.kernel.org
Subject: Re: [BUG] null pointer dereference in tcp_gso_segment()

On Wed, 2014-01-22 at 23:02 +0100, Arnaud Ebalard wrote:
> Hi Eric,
> 
> Eric Dumazet <eric.dumazet@...il.com> writes:
> 
> >> Unless there is an assumption I missed somewhere in the function, the
> >> problem may occur during the first round of the loop, because (unlike
> >> the 'while' condition does at line 21) skb->next is not checked against
> >> null at lines 17 above before it is passed to tcp_hdr() at line 18.
> >> 
> >> To be honest, I am asking because I am not familiar w/ the code and it
> >> is somewhat old so I wonder why noone got hit before. AFAICT,
> >> f4c50d990dcf ([NET]: Add software TSOv4) added TSOv4 support in 2006 via
> >> introduction of tcp_tso_segmen() (with the same kind of deref but
> >> possibly different assumptions) which was more recently modified via
> >> 28850dc7c7 (net: tcp: move GRO/GSO functions to tcp_offload) to become
> >> tcp_gso_segment().
> >> 
> >> David, can you confirm the analysis and possibly comment on the
> >> conditions needed for the bug to manifest?
> >
> > A gso packet contains at least 2 segments.
> 
> By whom / where is it enforced?

For example, tcp_gso_segment() does the following check :

if (unlikely(skb->len <= mss))
	goto out;

If there was one segment, then skb->len should also be smaller than mss

Since TCP stack seemed to be the provider of the packet in your stack
trace, check tcp_set_skb_tso_segs()



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