netdev - Re: BUG: net/netfilter: KASAN: stack-out-of-bounds in tcp

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Date:	Mon, 28 Mar 2016 14:29:57 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
Cc:	Jan Engelhardt <jengelh@...i.de>,
	David Miller <davem@...emloft.net>, sploving1@...il.com,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Patrick McHardy <kaber@...sh.net>,
	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: BUG: net/netfilter: KASAN: stack-out-of-bounds in tcp_packet

On Mon, 2016-03-28 at 23:11 +0200, Jozsef Kadlecsik wrote:

> In net/netfilter/nf_conntrack_proto_tcp.c we copy the options into a 
> buffer with skb_header_pointer(), so it's not a false positive there and 
> the KASAN report referred to that part.
> 

Although the out of bound could be one extra byte,
if skb_header_bpointer() had to copy something (since it also might
return a pointer inside skb->head)

No arch would possibly fault here.

So reading one byte on the stack is fooling KASAN, but no ill effect
would actually happen.

If the read byte is < 2, the function would return because of

	 if (opsize < 2)
		return;

If the read byte is >= 2, the function would return because of
	if (opsize > length)
		return; /* don't parse partial options */


(Since we care here of the case where length == 1)

No big deal, it is probably better to 'fix' the code so that it pleases
dynamic checkers.