tcp: mitigate ACK loops for connections as tcp_request_sock

In the SYN_RECV state, where the TCP connection is represented by
tcp_request_sock, we now rate-limit SYNACKs in response to a client's
retransmitted SYNs: we do not send a SYNACK in response to client SYN
if it has been less than sysctl_tcp_invalid_ratelimit (default 500ms)
since we last sent a SYNACK in response to a client's retransmitted
SYN.

This allows the vast majority of legitimate client connections to
proceed unimpeded, even for the most aggressive platforms, iOS and
MacOS, which actually retransmit SYNs 1-second intervals for several
times in a row. They use SYN RTO timeouts following the progression:
1,1,1,1,1,2,4,8,16,32.

Reported-by: Avery Fay <avery@mixpanel.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 1 deletions

diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index bc9216dc9de1..131aa4950d1c 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -605,7 +605,11 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
                  * Reset timer after retransmitting SYNACK, similar to
                  * the idea of fast retransmit in recovery.
                  */
-                if (!inet_rtx_syn_ack(sk, req))
+                if (!tcp_oow_rate_limited(sock_net(sk), skb,
+                                          LINUX_MIB_TCPACKSKIPPEDSYNRECV,
+                                          &tcp_rsk(req)->last_oow_ack_time) &&
+
+                    !inet_rtx_syn_ack(sk, req))
                         req->expires = min(TCP_TIMEOUT_INIT << req->num_timeout,
                                            TCP_RTO_MAX) + jiffies;
                 return NULL;