CVE-2017-13043/BGP: fix decoding of MVPN route types 6 and 7

RFC 6514 Section 4.6 defines the structure for Shared Tree Join (6) and
Source Tree Join (7) multicast VPN route types. decode_multicast_vpn()
didn't implement the Source AS field of that structure properly, adjust
the offsets to put it right.

This fixes a buffer over-read discovered by Bhargava Shastry,
SecT/TU Berlin.

Add a test using the capture file supplied by the reporter(s).

print-bgp.c

@@ -965,13 +965,13 @@ decode_multicast_vpn(netdissect_options *ndo,
 
         case BGP_MULTICAST_VPN_ROUTE_TYPE_SHARED_TREE_JOIN: /* fall through */
         case BGP_MULTICAST_VPN_ROUTE_TYPE_SOURCE_TREE_JOIN:
-            ND_TCHECK2(pptr[0], BGP_VPN_RD_LEN);
+            ND_TCHECK2(pptr[0], BGP_VPN_RD_LEN + 4);
             offset = strlen(buf);
 	    snprintf(buf + offset, buflen - offset, ", RD: %s, Source-AS %s",
 		bgp_vpn_rd_print(ndo, pptr),
 		as_printf(ndo, astostr, sizeof(astostr),
 		EXTRACT_32BITS(pptr + BGP_VPN_RD_LEN)));
-            pptr += BGP_VPN_RD_LEN;
+            pptr += BGP_VPN_RD_LEN + 4;
 
             bgp_vpn_sg_print(ndo, pptr, buf, buflen);
             break;

tests/TESTLIST

@@ -564,6 +564,7 @@ hncp_dhcpv6data-oobr	hncp_dhcpv6data-oobr.pcap	hncp_dhcpv6data-oobr.out -v -c1
 hncp_dhcpv4data-oobr	hncp_dhcpv4data-oobr.pcap	hncp_dhcpv4data-oobr.out -v -c1
 vqp-oobr		vqp-oobr.pcap			vqp-oobr.out		-v -c1
 bgp_pmsi_tunnel-oobr	bgp_pmsi_tunnel-oobr.pcap	bgp_pmsi_tunnel-oobr.out -v -c1
+bgp_mvpn_6_and_7	bgp_mvpn_6_and_7.pcap		bgp_mvpn_6_and_7.out	-v -c1
 
 # bad packets from Katie Holly
 mlppp-oobr		mlppp-oobr.pcap			mlppp-oobr.out

tests/bgp_mvpn_6_and_7.out

@@ -0,0 +1,13 @@
+IP (tos 0xc, ttl 254, id 21263, offset 0, flags [rsvd], proto TCP (6), length 517, bad cksum 8e15 (->99c9)!)
+    241.0.93.20.179 > 255.247.0.1.200: Flags [none], seq 2146691977:2146692450, win 56026, options [unknown-161,eol], length 473: BGP
+	Update Message (2), length: 45
+	  Withdrawn routes: 3 bytes
+	  Attribute Set (128), length: 7, Flags [OTPE+f]: 
+	    Origin AS: 148
+	      Multi-Protocol Reach NLRI (14), length: 71, Flags [T+6]: 
+	    AFI: IPv4 (1), SAFI: Multicast VPN (5), nh-length: 0, no SNPA
+	      Route-Type: Intra-AS I-PMSI (1), length: 0, RD: 62209:33686018 (= 2.2.2.2), Originator bogus address length 4294967232
+	      Route-Type: Unknown (0), length: 0
+	      Route-Type: Unknown (243), length: 1
+	      Route-Type: Inter-AS I-PMSI (2), length: 2, RD: unknown RD format, Source-AS 368115805
+	      Route-Type: Inter-AS I-PMSI (2), length: 2, RD: unknown RD format, Source-AS 335544320[|BGP]