netdev - Re: BUG: unable to handle kernel paging request at 0000000000609920 in networking code on 3.2.23.

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Date:	Fri, 25 Jan 2013 01:36:57 +0100 (CET)
From:	Jan Engelhardt <jengelh@...i.de>
To:	Florian Westphal <fw@...len.de>
cc:	Rafal Kupka <rkupka@...emetry.com>, netdev@...r.kernel.org,
	netfilter-devel <netfilter-devel@...r.kernel.org>
Subject: Re: BUG: unable to handle kernel paging request at 0000000000609920
 in networking code on 3.2.23.


On Friday 2013-01-25 00:08, Florian Westphal wrote:
>@@ -35,10 +35,18 @@ tcpoptstrip_mangle_packet(struct sk_buff *skb,
> {
> 	unsigned int optl, i, j;
> 	struct tcphdr *tcph;
>+	struct tcphdr _tcph;
> 	u_int16_t n, o;
> 	u_int8_t *opt;
> 
>-	if (!skb_make_writable(skb, skb->len))
>+	if (skb->len < minlen)
>+		return XT_CONTINUE;
>+
>+	tcph = skb_header_pointer(skb, tcphoff, sizeof(_tcph), &_tcph);
>+	if (!tcph)
>+		return XT_CONTINUE; /* no options -> nothing to do */

To the best of my analysis, the "no options" comment is incorrect here,
because you are not even looking at the options so far, but only tcph.
The prose should probably be something like:

	if (iph->frag_off & htons(IP_OFFSET)) != 0)
		/* not the first fragment - lost case */
		return XT_CONTINUE;
	if (iph->len < iph->ihl * 4 + sizeof(_tcph))
		/* fragment boundary within tcphdr */
		return XT_CONTINUE;
	tcph = skb_header_pointer(skb, tcphoff, sizeof(_tcph), &_tcph);
	if (tcph == NULL)
		/* packet way too short for some reason
		   (should not occur since we tested for fragment
		   boundary) */
		return NF_DROP;
	if (tcph->doff * 4 - sizeof(_tcph) == 0)
		/* no options */
		return XT_CONTINUE;
	if (iph->len < iph->ihl * 4 + sizeof(_tcph) + tcphoff))
		/* fragment boundary within tcpoptions */
		return XT_CONTINUE;

	skb_make_writable...
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