tcp: mitigate a side channel for detection of TCP connections

If a blind attacker wants to guess by sending ACK segments if there
exists a TCP connection , this might trigger a challenge ACK on an
existing TCP connection. To make this hit non-observable for the
attacker, also increment the global counter, which would have been
incremented if it would have been a non-hit.
This issue was reported as issue number 11 in Keyu Man et al.:
SCAD: Towards a Universal and Automated Network Side-Channel
      Vulnerability Detection

Reviewed by:		Nick Banks, Peter Lei
MFC after:		1 week
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D51724

1 files changed, 8 insertions, 0 deletions

diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
index 26e7e53d540c..1fce7c591639 100644
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -82,6 +82,7 @@
 #include <netinet/ip.h>
 #include <netinet/ip_icmp.h>
 #include <netinet/ip_var.h>
+#include <netinet/icmp_var.h>
 #ifdef INET6
 #include <netinet/icmp6.h>
 #include <netinet/ip6.h>
@@ -2156,6 +2157,13 @@ tcp_send_challenge_ack(struct tcpcb *tp, struct tcphdr *th, struct mbuf *m)
 	sbintime_t now;
 	bool send_challenge_ack;
 
+	/*
+	 * The sending of a challenge ACK could be triggered by a blind attacker
+	 * to detect an existing TCP connection. To mitigate that, increment
+	 * also the global counter which would be incremented if the attacker
+	 * would have guessed wrongly.
+	 */
+	(void)badport_bandlim(BANDLIM_TCP_RST);
 	if (V_tcp_ack_war_time_window == 0 || V_tcp_ack_war_cnt == 0) {
 		/* ACK war protection is disabled. */
 		send_challenge_ack = true;