linux-kernel - [PATCH 47/98] netfilter: xt_TCPMSS: Fix violation of RFC879 in absence of MSS option

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Message-Id: <1373552708-15235-48-git-send-email-luis.henriques@canonical.com>
Date:	Thu, 11 Jul 2013 15:24:17 +0100
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Phil Oester <kernel@...uxace.com>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 47/98] netfilter: xt_TCPMSS: Fix violation of RFC879 in absence of MSS option

3.5.7.17 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Phil Oester <kernel@...uxace.com>

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@...uxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@...filter.org>
Signed-off-by: Luis Henriques <luis.henriques@...onical.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

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