tcp: refine SYN handling in tcp_validate_incoming

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>

1 files changed, 5 insertions, 1 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 8aaec5536111..fdd49f1b7a52 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5296,8 +5296,11 @@ 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;
         }
 
@@ -5327,6 +5330,7 @@ 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);