[ 3.8.y.z extended stable ] Patch "netfilter: xt_TCPMSS: Fix violation of RFC879 in absence of" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    netfilter: xt_TCPMSS: Fix violation of RFC879 in absence of

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From b00a45085e067d8e584759806dbe57eacb165a30 Mon Sep 17 00:00:00 2001
From: Phil Oester <kernel at linuxace.com>
Date: Tue, 4 Jun 2013 05:09:27 +0000
Subject: [PATCH] netfilter: xt_TCPMSS: Fix violation of RFC879 in absence of
 MSS option

commit 409b545ac10d9548929557a75ad86540f59a2c83 upstream.

The clamp-mss-to-pmtu option of the xt_TCPMSS target can cause issues
connecting to websites if there was no MSS option present in the
original SYN packet from the client. In these cases, it may add a
MSS higher than the default specified in RFC879. Fix this by never
setting a value > 536 if no MSS option was specified by the client.

This closes netfilter's bugzilla #662.

Signed-off-by: Phil Oester <kernel at linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo at netfilter.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 net/netfilter/xt_TCPMSS.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
index 71a266d..4960b66 100644
--- a/net/netfilter/xt_TCPMSS.c
+++ b/net/netfilter/xt_TCPMSS.c
@@ -124,6 +124,12 @@ tcpmss_mangle_packet(struct sk_buff *skb,

 	skb_put(skb, TCPOLEN_MSS);

+	/* RFC 879 states that the default MSS is 536 without specific
+	 * knowledge that the destination host is prepared to accept larger.
+	 * Since no MSS was provided, we MUST NOT set a value > 536.
+	 */
+	newmss = min(newmss, (u16)536);
+
 	opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
 	memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));

--
1.8.1.2