tcp: drop SYN+FIN messages

Denys Fedoryshchenko reported that SYN+FIN attacks were bringing his
linux machines to their limits.

Dont call conn_request() if the TCP flags includes SYN flag

Reported-by: Denys Fedoryshchenko <denys@visp.net.lb>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 0 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 78dd38cd5496..0cbb44076cfa 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5811,6 +5811,8 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
                         goto discard;
 
                 if (th->syn) {
+                        if (th->fin)
+                                goto discard;
                         if (icsk->icsk_af_ops->conn_request(sk, skb) < 0)
                                 return 1;