Implement IPv6 forwarding IPsec only.

IPsec gateways set the forwarding sysctl to 2.  While this worked
for IPv4 since a long time, adapt this feature for IPv6 now.  Set
sysctl net.inet6.ip6.forwarding=2 to forward only packets that have
been processed by IPsec.

Set IPV6_FORWARDING_IPSEC in ip6_input() and pass the flag down to
the call stack.  This provides consistent view on global variable
ip6_forwarding.  In ip6_output() or ip6_forward() drop packets that
do not match the policy.

OK denis@

5 files changed, 44 insertions, 19 deletions

diff --git a/sys/netinet6/ip6_forward.c b/sys/netinet6/ip6_forward.c
index 9daf51f92b6..c706c32c53c 100644
--- a/sys/netinet6/ip6_forward.c
+++ b/sys/netinet6/ip6_forward.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: ip6_forward.c,v 1.119 2024/06/20 19:25:42 bluhm Exp $	*/
+/*	$OpenBSD: ip6_forward.c,v 1.120 2024/07/04 12:50:08 bluhm Exp $	*/
 /*	$KAME: ip6_forward.c,v 1.75 2001/06/29 12:42:13 jinmei Exp $	*/
 
 /*
@@ -315,6 +315,15 @@ reroute:
 	}
 #endif
 
+#ifdef IPSEC
+	if (ISSET(flags, IPV6_FORWARDING) &&
+	    ISSET(flags, IPV6_FORWARDING_IPSEC) &&
+	    !ISSET(m->m_pkthdr.ph_tagsset, PACKET_TAG_IPSEC_IN_DONE)) {
+		error = EHOSTUNREACH;
+		goto senderr;
+	}
+#endif
+
 	error = if_output_tso(ifp, &m, dst, rt, ifp->if_mtu);
 	if (error)
 		ip6stat_inc(ip6s_cantforward);
diff --git a/sys/netinet6/ip6_input.c b/sys/netinet6/ip6_input.c
index de9468ecc3f..6952cf632f9 100644
--- a/sys/netinet6/ip6_input.c
+++ b/sys/netinet6/ip6_input.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: ip6_input.c,v 1.263 2024/06/20 19:25:42 bluhm Exp $	*/
+/*	$OpenBSD: ip6_input.c,v 1.264 2024/07/04 12:50:08 bluhm Exp $	*/
 /*	$KAME: ip6_input.c,v 1.188 2001/03/29 05:34:31 itojun Exp $	*/
 
 /*
@@ -416,8 +416,14 @@ ip6_input_if(struct mbuf **mp, int *offp, int nxt, int af, struct ifnet *ifp)
 		SET(flags, IPV6_REDIRECT);
 #endif
 
-	if (ip6_forwarding != 0)
+	switch (ip6_forwarding) {
+	case 2:
+		SET(flags, IPV6_FORWARDING_IPSEC);
+		/* FALLTHROUGH */
+	case 1:
 		SET(flags, IPV6_FORWARDING);
+		break;
+	}
 
 	/*
 	 * Without embedded scope ID we cannot find link-local
@@ -491,7 +497,7 @@ ip6_input_if(struct mbuf **mp, int *offp, int nxt, int af, struct ifnet *ifp)
 			 * must be discarded, else it may be accepted below.
 			 */
 			KERNEL_LOCK();
-			error = ip6_mforward(ip6, ifp, m);
+			error = ip6_mforward(ip6, ifp, m, flags);
 			KERNEL_UNLOCK();
 			if (error) {
 				ip6stat_inc(ip6s_cantforward);
@@ -1442,7 +1448,7 @@ const struct sysctl_bounded_args ipv6ctl_vars[] = {
 #ifdef MROUTING
 	{ IPV6CTL_MRTPROTO, &ip6_mrtproto, SYSCTL_INT_READONLY },
 #endif
-	{ IPV6CTL_FORWARDING, &ip6_forwarding, 0, 1 },
+	{ IPV6CTL_FORWARDING, &ip6_forwarding, 0, 2 },
 	{ IPV6CTL_SENDREDIRECTS, &ip6_sendredirects, 0, 1 },
 	{ IPV6CTL_DEFHLIM, &ip6_defhlim, 0, 255 },
 	{ IPV6CTL_MAXFRAGPACKETS, &ip6_maxfragpackets, 0, 1000 },
diff --git a/sys/netinet6/ip6_mroute.c b/sys/netinet6/ip6_mroute.c
index f2ab244ff7b..34bcb28ce6a 100644
--- a/sys/netinet6/ip6_mroute.c
+++ b/sys/netinet6/ip6_mroute.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: ip6_mroute.c,v 1.142 2024/06/07 08:37:59 jsg Exp $	*/
+/*	$OpenBSD: ip6_mroute.c,v 1.143 2024/07/04 12:50:08 bluhm Exp $	*/
 /*	$NetBSD: ip6_mroute.c,v 1.59 2003/12/10 09:28:38 itojun Exp $	*/
 /*	$KAME: ip6_mroute.c,v 1.45 2001/03/25 08:38:51 itojun Exp $	*/
 
@@ -122,8 +122,8 @@ int mcast6_debug = 1;
 	do { } while (0)
 #endif
 
-int ip6_mdq(struct mbuf *, struct ifnet *, struct rtentry *);
-void phyint_send6(struct ifnet *, struct ip6_hdr *, struct mbuf *);
+int ip6_mdq(struct mbuf *, struct ifnet *, struct rtentry *, int);
+void phyint_send6(struct ifnet *, struct ip6_hdr *, struct mbuf *, int);
 
 /*
  * Globals.  All but ip6_mrouter, ip6_mrtproto and mrt6stat could be static,
@@ -853,7 +853,7 @@ socket6_send(struct socket *so, struct mbuf *mm, struct sockaddr_in6 *src)
  * discard it.
  */
 int
-ip6_mforward(struct ip6_hdr *ip6, struct ifnet *ifp, struct mbuf *m)
+ip6_mforward(struct ip6_hdr *ip6, struct ifnet *ifp, struct mbuf *m, int flags)
 {
 	struct rtentry *rt;
 	struct mif6 *mifp;
@@ -902,7 +902,7 @@ ip6_mforward(struct ip6_hdr *ip6, struct ifnet *ifp, struct mbuf *m)
 
 	/* Entry exists, so forward if necessary */
 	if (rt) {
-		return (ip6_mdq(m, ifp, rt));
+		return (ip6_mdq(m, ifp, rt, flags));
 	} else {
 		/*
 		 * If we don't have a route for packet's origin,
@@ -997,7 +997,7 @@ mf6c_expire_route(struct rtentry *rt, u_int rtableid)
  * Packet forwarding routine once entry in the cache is made
  */
 int
-ip6_mdq(struct mbuf *m, struct ifnet *ifp, struct rtentry *rt)
+ip6_mdq(struct mbuf *m, struct ifnet *ifp, struct rtentry *rt, int flags)
 {
 	struct ip6_hdr *ip6 = mtod(m, struct ip6_hdr *);
 	struct mif6 *m6, *mifp = (struct mif6 *)ifp->if_mcast6;
@@ -1085,7 +1085,7 @@ ip6_mdq(struct mbuf *m, struct ifnet *ifp, struct rtentry *rt)
 		m6->m6_pkt_out++;
 		m6->m6_bytes_out += plen;
 
-		phyint_send6(ifn, ip6, m);
+		phyint_send6(ifn, ip6, m, flags);
 		if_put(ifn);
 	} while ((rt = rtable_iterate(rt)) != NULL);
 
@@ -1093,7 +1093,7 @@ ip6_mdq(struct mbuf *m, struct ifnet *ifp, struct rtentry *rt)
 }
 
 void
-phyint_send6(struct ifnet *ifp, struct ip6_hdr *ip6, struct mbuf *m)
+phyint_send6(struct ifnet *ifp, struct ip6_hdr *ip6, struct mbuf *m, int flags)
 {
 	struct mbuf *mb_copy;
 	struct sockaddr_in6 *dst6, sin6;
@@ -1126,8 +1126,8 @@ phyint_send6(struct ifnet *ifp, struct ip6_hdr *ip6, struct mbuf *m)
 		/* XXX: ip6_output will override ip6->ip6_hlim */
 		im6o.im6o_hlim = ip6->ip6_hlim;
 		im6o.im6o_loop = 1;
-		error = ip6_output(mb_copy, NULL, NULL, IPV6_FORWARDING, &im6o,
-		    NULL);
+		error = ip6_output(mb_copy, NULL, NULL, flags | IPV6_FORWARDING,
+		    &im6o, NULL);
 		return;
 	}
 
diff --git a/sys/netinet6/ip6_output.c b/sys/netinet6/ip6_output.c
index c65bfd0b9f4..c5c82e5fe12 100644
--- a/sys/netinet6/ip6_output.c
+++ b/sys/netinet6/ip6_output.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: ip6_output.c,v 1.291 2024/04/17 20:48:51 bluhm Exp $	*/
+/*	$OpenBSD: ip6_output.c,v 1.292 2024/07/04 12:50:08 bluhm Exp $	*/
 /*	$KAME: ip6_output.c,v 1.172 2001/03/25 09:55:56 itojun Exp $	*/
 
 /*
@@ -533,7 +533,7 @@ reroute:
 			 */
 			if (ip6_mforwarding && ip6_mrouter[ifp->if_rdomain] &&
 			    (flags & IPV6_FORWARDING) == 0) {
-				if (ip6_mforward(ip6, ifp, m) != 0) {
+				if (ip6_mforward(ip6, ifp, m, flags) != 0) {
 					m_freem(m);
 					goto done;
 				}
@@ -644,6 +644,15 @@ reroute:
 	}
 #endif
 
+#ifdef IPSEC
+	if (ISSET(flags, IPV6_FORWARDING) &&
+	    ISSET(flags, IPV6_FORWARDING_IPSEC) &&
+	    !ISSET(m->m_pkthdr.ph_tagsset, PACKET_TAG_IPSEC_IN_DONE)) {
+		error = EHOSTUNREACH;
+		goto bad;
+	}
+#endif
+
 	/*
 	 * If the packet is not going on the wire it can be destined
 	 * to any local address.  In this case do not clear its scopes
diff --git a/sys/netinet6/ip6_var.h b/sys/netinet6/ip6_var.h
index 47fca707dda..bbe59c744ac 100644
--- a/sys/netinet6/ip6_var.h
+++ b/sys/netinet6/ip6_var.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: ip6_var.h,v 1.118 2024/06/20 19:25:42 bluhm Exp $	*/
+/*	$OpenBSD: ip6_var.h,v 1.119 2024/07/04 12:50:08 bluhm Exp $	*/
 /*	$KAME: ip6_var.h,v 1.33 2000/06/11 14:59:20 jinmei Exp $	*/
 
 /*
@@ -270,6 +270,7 @@ ip6stat_add(enum ip6stat_counters c, uint64_t v)
 #define IPV6_FORWARDING		0x02	/* most of IPv6 header exists */
 #define IPV6_MINMTU		0x04	/* use minimum MTU (IPV6_USE_MIN_MTU) */
 #define IPV6_REDIRECT		0x08	/* redirected by pf */
+#define IPV6_FORWARDING_IPSEC	0x10	/* only packets processed by IPsec */
 
 extern int ip6_mtudisc_timeout;		/* mtu discovery */
 extern struct rttimer_queue icmp6_mtudisc_timeout_q;
@@ -316,7 +317,7 @@ int	ip6_unknown_opt(struct mbuf **, u_int8_t *, int);
 int	ip6_get_prevhdr(struct mbuf *, int);
 int	ip6_nexthdr(struct mbuf *, int, int, int *);
 int	ip6_lasthdr(struct mbuf *, int, int, int *);
-int	ip6_mforward(struct ip6_hdr *, struct ifnet *, struct mbuf *);
+int	ip6_mforward(struct ip6_hdr *, struct ifnet *, struct mbuf *, int);
 int	ip6_process_hopopts(struct mbuf **, u_int8_t *, int, u_int32_t *,
 	     u_int32_t *);
 void	ip6_savecontrol(struct inpcb *, struct mbuf *, struct mbuf **);