[PATCH 079/145] netfilter: xt_TCPMSS: Fix violation of RFC879 in absence of MSS option

From: Kamal Mostafa
Date: Wed Jul 17 2013 - 19:05:00 EST


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

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

From: Phil Oester <kernel@xxxxxxxxxxxx>

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@xxxxxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
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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