git: f0f6e5038896 - main - tcp: mitigate a side channel for detection of TCP connections

The branch main has been updated by tuexen:

URL: https://cgit.FreeBSD.org/src/commit/?id=f0f6e50388963cae44bb92bb69ed7a1135dd2eec

commit f0f6e50388963cae44bb92bb69ed7a1135dd2eec
Author:     Michael Tuexen <tuexen@FreeBSD.org>
AuthorDate: 2025-08-09 12:17:38 +0000
Commit:     Michael Tuexen <tuexen@FreeBSD.org>
CommitDate: 2025-08-09 12:17:38 +0000

    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
---
 sys/netinet/tcp_subr.c | 8 ++++++++
 1 file changed, 8 insertions(+)

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;