net-tcp: Fast Open client - cookie-less mode

In trusted networks, e.g., intranet, data-center, the client does not
need to use Fast Open cookie to mitigate DoS attacks. In cookie-less
mode, sendmsg() with MSG_FASTOPEN flag will send SYN-data regardless
of cookie availability.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 11 insertions, 3 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c49a4fc175b..e67d685a6c0 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5650,7 +5650,7 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
                                     struct tcp_fastopen_cookie *cookie)
 {
         struct tcp_sock *tp = tcp_sk(sk);
-        struct sk_buff *data = tcp_write_queue_head(sk);
+        struct sk_buff *data = tp->syn_data ? tcp_write_queue_head(sk) : NULL;
         u16 mss = tp->rx_opt.mss_clamp;
         bool syn_drop;
 
@@ -5665,6 +5665,9 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
                 mss = opt.mss_clamp;
         }
 
+        if (!tp->syn_fastopen)  /* Ignore an unsolicited cookie */
+                cookie->len = -1;
+
         /* The SYN-ACK neither has cookie nor acknowledges the data. Presumably
          * the remote receives only the retransmitted (regular) SYNs: either
          * the original SYN-data or the corresponding SYN-ACK is lost.
@@ -5816,7 +5819,8 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 
                 tcp_finish_connect(sk, skb);
 
-                if (tp->syn_fastopen && tcp_rcv_fastopen_synack(sk, skb, &foc))
+                if ((tp->syn_fastopen || tp->syn_data) &&
+                    tcp_rcv_fastopen_synack(sk, skb, &foc))
                         return -1;
 
                 if (sk->sk_write_pending ||
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index c5cfd5ec318..27a32acfdb6 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2864,6 +2864,7 @@ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
         struct sk_buff *syn_data = NULL, *data;
         unsigned long last_syn_loss = 0;
 
+        tp->rx_opt.mss_clamp = tp->advmss;  /* If MSS is not cached */
         tcp_fastopen_cache_get(sk, &tp->rx_opt.mss_clamp, &fo->cookie,
                                &syn_loss, &last_syn_loss);
         /* Recurring FO SYN losses: revert to regular handshake temporarily */
@@ -2873,7 +2874,9 @@ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
                 goto fallback;
         }
 
-        if (fo->cookie.len <= 0)
+        if (sysctl_tcp_fastopen & TFO_CLIENT_NO_COOKIE)
+                fo->cookie.len = -1;
+        else if (fo->cookie.len <= 0)
                 goto fallback;
 
         /* MSS for SYN-data is based on cached MSS and bounded by PMTU and
@@ -2916,6 +2919,7 @@ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
         fo->copied = data->len;
 
         if (tcp_transmit_skb(sk, syn_data, 0, sk->sk_allocation) == 0) {
+                tp->syn_data = (fo->copied > 0);
                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPFASTOPENACTIVE);
                 goto done;
         }