From: Eric Dumazet <edumazet@google.com>
Date: Tue, 17 Jul 2012 12:29:30 +0000 (+0000)
Subject: tcp: refine SYN handling in tcp_validate_incoming
X-Git-Tag: firefly_0821_release~7541^2~185
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=282190eab6442da4789ae3c3037c7b19d65eaf52;p=firefly-linux-kernel-4.4.55.git

tcp: refine SYN handling in tcp_validate_incoming

[ Upstream commit e371589917011efe6ff8c7dfb4e9e81934ac5855 ]

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@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Vijay Subramanian <subramanian.vijay@gmail.com>
Cc: Kiran Kumar Kella <kkiran@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index bf79a5aae24e..25988229f68f 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5214,8 +5214,11 @@ 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;
 	}
 
@@ -5245,6 +5248,7 @@ 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);