ipvs: strip gre tunnel headers from icmp errors

Recognize GRE tunnels in received ICMP errors and
properly strip the tunnel headers.

Signed-off-by: Julian Anastasov <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/ipvs/ip_vs_core.c

···
 #include <net/udp.h>
 #include <net/icmp.h>                   /* for icmp_send */
 #include <net/gue.h>
+#include <net/gre.h>
 #include <net/route.h>
 #include <net/ip6_checksum.h>
 #include <net/netns/generic.h>		/* net_generic() */
···
 	return 0;
 }
 
+/* Check the GRE tunnel and return its header length */
+static int ipvs_gre_decap(struct netns_ipvs *ipvs, struct sk_buff *skb,
+			  unsigned int offset, __u16 af,
+			  const union nf_inet_addr *daddr, __u8 *proto)
+{
+	struct gre_base_hdr _greh, *greh;
+	struct ip_vs_dest *dest;
+
+	greh = skb_header_pointer(skb, offset, sizeof(_greh), &_greh);
+	if (!greh)
+		goto unk;
+	dest = ip_vs_find_tunnel(ipvs, af, daddr, 0);
+	if (!dest)
+		goto unk;
+	if (dest->tun_type == IP_VS_CONN_F_TUNNEL_TYPE_GRE) {
+		__be16 type;
+
+		/* Only support version 0 and C (csum) */
+		if ((greh->flags & ~GRE_CSUM) != 0)
+			goto unk;
+		type = greh->protocol;
+		/* Later we can support also IPPROTO_IPV6 */
+		if (type != htons(ETH_P_IP))
+			goto unk;
+		*proto = IPPROTO_IPIP;
+		return gre_calc_hlen(gre_flags_to_tnl_flags(greh->flags));
+	}
+
+unk:
+	return 0;
+}
+
 /*
  *	Handle ICMP messages in the outside-to-inside direction (incoming).
  *	Find any that might be relevant, check against existing connections,
···
 		if (cih == NULL)
 			return NF_ACCEPT; /* The packet looks wrong, ignore */
 		ipip = true;
-	} else if (cih->protocol == IPPROTO_UDP &&	/* Can be UDP encap */
+	} else if ((cih->protocol == IPPROTO_UDP ||	/* Can be UDP encap */
+		    cih->protocol == IPPROTO_GRE) &&	/* Can be GRE encap */
 		   /* Error for our tunnel must arrive at LOCAL_IN */
 		   (skb_rtable(skb)->rt_flags & RTCF_LOCAL)) {
 		__u8 iproto;
···
 		if (unlikely(cih->frag_off & htons(IP_OFFSET)))
 			return NF_ACCEPT;
 		offset2 = offset + cih->ihl * 4;
-		ulen = ipvs_udp_decap(ipvs, skb, offset2, AF_INET, raddr,
-				      &iproto);
+		if (cih->protocol == IPPROTO_UDP)
+			ulen = ipvs_udp_decap(ipvs, skb, offset2, AF_INET,
+					      raddr, &iproto);
+		else
+			ulen = ipvs_gre_decap(ipvs, skb, offset2, AF_INET,
+					      raddr, &iproto);
 		if (ulen > 0) {
-			/* Skip IP and UDP tunnel headers */
+			/* Skip IP and UDP/GRE tunnel headers */
 			offset = offset2 + ulen;
 			/* Now we should be at the original IP header */
 			cih = skb_header_pointer(skb, offset, sizeof(_ciph),