[ 3.5.y.z extended stable ] Patch "tcp: refine SYN handling in tcp_validate_incoming" has been added to staging queue

[Herton Ronaldo Krzesinski]

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

    tcp: refine SYN handling in tcp_validate_incoming

to the linux-3.5.y-queue branch of the 3.5.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-3.5.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 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Herton

------

>From 4077d72bcd6e39f715b95a126db7d82e97171e82 Mon Sep 17 00:00:00 2001
From: Eric Dumazet <edumazet at google.com>
Date: Tue, 17 Jul 2012 12:29:30 +0000
Subject: [PATCH] tcp: refine SYN handling in tcp_validate_incoming

commit e371589917011efe6ff8c7dfb4e9e81934ac5855 upstream.

Followup of commit 0c24604b68fc (tcp: implement RFC 5961 4.2)

As reported by Vijay Subramanian, we should send a challenge ACK
instead of a dup ack if a SYN flag is set on a packet received out of
window.

This permits the ratelimiting to work as intended, and to increase
correct SNMP counters.

Suggested-by: Vijay Subramanian <subramanian.vijay at gmail.com>
Signed-off-by: Eric Dumazet <edumazet at google.com>
Acked-by: Vijay Subramanian <subramanian.vijay at gmail.com>
Cc: Kiran Kumar Kella <kkiran at broadcom.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski at canonical.com>
---
 net/ipv4/tcp_input.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index e859775..afb436d 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5477,8 +5477,11 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 		 * an acknowledgment should be sent in reply (unless the RST
 		 * bit is set, if so drop the segment and return)".
 		 */
-		if (!th->rst)
+		if (!th->rst) {
+			if (th->syn)
+				goto syn_challenge;
 			tcp_send_dupack(sk, skb);
+		}
 		goto discard;
 	}

@@ -5508,6 +5511,7 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 	 * RFC 5691 4.2 : Send a challenge ack
 	 */
 	if (th->syn) {
+syn_challenge:
 		if (syn_inerr)
 			TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);
--
1.7.9.5