pf: ensure we have the correct source/destination IP address in ICMP errors

When we route-to a packet that later turns out to not fit in the
outbound interface MTU we generate an ICMP error.
However, if we've already changed those (i.e. we've passed through a NAT
rule) we have to undo the transformation first.

Obtained from:	pfSense
MFC after:	3 weeks
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D32571

sys/netpfil/pf/pf.c

@@ -320,6 +320,8 @@ static u_int		 pf_purge_expired_states(u_int, int);
 static void		 pf_purge_unlinked_rules(void);
 static int		 pf_mtag_uminit(void *, int, int);
 static void		 pf_mtag_free(struct m_tag *);
+static void		 pf_packet_rework_nat(struct mbuf *, struct pf_pdesc *,
+			    int, struct pf_state_key *);
 #ifdef INET
 static void		 pf_route(struct mbuf **, struct pf_krule *, int,
 			    struct ifnet *, struct pf_kstate *,
@@ -341,6 +343,16 @@ extern struct proc *pf_purge_proc;
 
 VNET_DEFINE(struct pf_limit, pf_limits[PF_LIMIT_MAX]);
 
+#define	PACKET_UNDO_NAT(_m, _pd, _off, _s, _dir)		\
+	do {								\
+		struct pf_state_key *nk;				\
+		if ((_dir) == PF_OUT)					\
+			nk = (_s)->key[PF_SK_STACK];			\
+		else							\
+			nk = (_s)->key[PF_SK_WIRE];			\
+		pf_packet_rework_nat(_m, _pd, _off, nk);		\
+	} while (0)
+
 #define	PACKET_LOOPED(pd)	((pd)->pf_mtag &&			\
 				 (pd)->pf_mtag->flags & PF_PACKET_LOOPED)
 
@@ -446,6 +458,83 @@ pf_addr_cmp(struct pf_addr *a, struct pf_addr *b, sa_family_t af)
 	return (0);
 }
 
+static void
+pf_packet_rework_nat(struct mbuf *m, struct pf_pdesc *pd, int off,
+	struct pf_state_key *nk)
+{
+
+	switch (pd->proto) {
+	case IPPROTO_TCP: {
+		struct tcphdr *th = &pd->hdr.tcp;
+
+		if (PF_ANEQ(pd->src, &nk->addr[pd->sidx], pd->af))
+			pf_change_ap(m, pd->src, &th->th_sport, pd->ip_sum,
+			    &th->th_sum, &nk->addr[pd->sidx],
+			    nk->port[pd->sidx], 0, pd->af);
+		if (PF_ANEQ(pd->dst, &nk->addr[pd->didx], pd->af))
+			pf_change_ap(m, pd->dst, &th->th_dport, pd->ip_sum,
+			    &th->th_sum, &nk->addr[pd->didx],
+			    nk->port[pd->didx], 0, pd->af);
+		m_copyback(m, off, sizeof(*th), (caddr_t)th);
+		break;
+	}
+	case IPPROTO_UDP: {
+		struct udphdr *uh = &pd->hdr.udp;
+
+		if (PF_ANEQ(pd->src, &nk->addr[pd->sidx], pd->af))
+			pf_change_ap(m, pd->src, &uh->uh_sport, pd->ip_sum,
+			    &uh->uh_sum, &nk->addr[pd->sidx],
+			    nk->port[pd->sidx], 1, pd->af);
+		if (PF_ANEQ(pd->dst, &nk->addr[pd->didx], pd->af))
+			pf_change_ap(m, pd->dst, &uh->uh_dport, pd->ip_sum,
+			    &uh->uh_sum, &nk->addr[pd->didx],
+			    nk->port[pd->didx], 1, pd->af);
+		m_copyback(m, off, sizeof(*uh), (caddr_t)uh);
+		break;
+	}
+	case IPPROTO_ICMP: {
+		struct icmp *ih = &pd->hdr.icmp;
+
+		if (nk->port[pd->sidx] != ih->icmp_id) {
+			pd->hdr.icmp.icmp_cksum = pf_cksum_fixup(
+			    ih->icmp_cksum, ih->icmp_id,
+			    nk->port[pd->sidx], 0);
+			ih->icmp_id = nk->port[pd->sidx];
+			pd->sport = &ih->icmp_id;
+
+			m_copyback(m, off, ICMP_MINLEN, (caddr_t)ih);
+		}
+		/* FALLTHROUGH */
+	}
+	default:
+		if (PF_ANEQ(pd->src, &nk->addr[pd->sidx], pd->af)) {
+			switch (pd->af) {
+			case AF_INET:
+				pf_change_a(&pd->src->v4.s_addr,
+				    pd->ip_sum, nk->addr[pd->sidx].v4.s_addr,
+				    0);
+				break;
+			case AF_INET6:
+				PF_ACPY(pd->src, &nk->addr[pd->sidx], pd->af);
+				break;
+			}
+		}
+		if (PF_ANEQ(pd->dst, &nk->addr[pd->didx], pd->af)) {
+			switch (pd->af) {
+			case AF_INET:
+				pf_change_a(&pd->dst->v4.s_addr,
+				    pd->ip_sum, nk->addr[pd->didx].v4.s_addr,
+				    0);
+				break;
+			case AF_INET6:
+				PF_ACPY(pd->dst, &nk->addr[pd->didx], pd->af);
+				break;
+			}
+		}
+		break;
+	}
+}
+
 static __inline uint32_t
 pf_hashkey(struct pf_state_key *sk)
 {
@@ -5937,6 +6026,11 @@ pf_route(struct mbuf **m, struct pf_krule *r, int dir, struct ifnet *oifp,
 		error = EMSGSIZE;
 		KMOD_IPSTAT_INC(ips_cantfrag);
 		if (r->rt != PF_DUPTO) {
+			if (s && pd->nat_rule != NULL)
+				PACKET_UNDO_NAT(m0, pd,
+				    (ip->ip_hl << 2) + (ip_off & IP_OFFMASK),
+				    s, dir);
+
 			icmp_error(m0, ICMP_UNREACH, ICMP_UNREACH_NEEDFRAG, 0,
 			    ifp->if_mtu);
 			goto done;
@@ -6104,9 +6198,14 @@ pf_route6(struct mbuf **m, struct pf_krule *r, int dir, struct ifnet *oifp,
 		nd6_output_ifp(ifp, ifp, m0, &dst, NULL);
 	else {
 		in6_ifstat_inc(ifp, ifs6_in_toobig);
-		if (r->rt != PF_DUPTO)
+		if (r->rt != PF_DUPTO) {
+			if (s && pd->nat_rule != NULL)
+				PACKET_UNDO_NAT(m0, pd,
+				    ((caddr_t)ip6 - m0->m_data) +
+				    sizeof(struct ip6_hdr), s, dir);
+
 			icmp6_error(m0, ICMP6_PACKET_TOO_BIG, 0, ifp->if_mtu);
-		else
+		} else
 			goto bad;
 	}