OSPF6: Fix an undefined behavior

Handle ls_length shorter than sizeof(lsa_hdr) in the same way as OSPF.

Use a u_int32 to hold a loop variable initialized with GET_BE_U_4.

print-ospf6.c:815:46: runtime error: signed integer overflow:
-2147483648 - 1 cannot be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior print-ospf6.c:817:46

[Part of the PR the-tcpdump-group#1012]

print-ospf6.c

@@ -388,14 +388,23 @@ static int
 ospf6_print_lshdr(netdissect_options *ndo,
                   const struct lsa6_hdr *lshp, const u_char *dataend)
 {
+	u_int ls_length;
+
 	if ((const u_char *)(lshp + 1) > dataend)
 		goto trunc;
 
+	ls_length = GET_BE_U_2(lshp->ls_length);
+	if (ls_length < sizeof(struct lsa_hdr)) {
+		ND_PRINT("\n\t	  Bogus length %u < header (%zu)", ls_length,
+		    sizeof(struct lsa_hdr));
+		goto trunc;
+	}
+
 	ND_PRINT("\n\t  Advertising Router %s, seq 0x%08x, age %us, length %zu",
 		 GET_IPADDR_STRING(lshp->ls_router),
 		 GET_BE_U_4(lshp->ls_seq),
 		 GET_BE_U_2(lshp->ls_age),
-		 GET_BE_U_2(lshp->ls_length)-sizeof(struct lsa6_hdr));
+		 ls_length-sizeof(struct lsa6_hdr));
 
 	ospf6_print_ls_type(ndo, GET_BE_U_2(lshp->ls_type),
 			    &lshp->ls_stateid);
@@ -734,7 +743,7 @@ ospf6_decode_v3(netdissect_options *ndo,
 	const struct lsr6 *lsrp;
 	const struct lsa6_hdr *lshp;
 	const struct lsa6 *lsap;
-	int i;
+	uint32_t i;
 
 	switch (GET_U_1(op->ospf6_type)) {
 

tests/TESTLIST

@@ -922,3 +922,4 @@ pb_nhrp_1			pb_nhrp_1.pcap			pb_nhrp_1.out	-v
 ip-snmp-leftshift-unsigned ip-snmp-leftshift-unsigned.pcap ip-snmp-leftshift-unsigned.out
 ip6-snmp-oid-unsigned ip6-snmp-oid-unsigned.pcap ip6-snmp-oid-unsigned.out
 lwres-pointer-arithmetic-ub lwres-pointer-arithmetic-ub.pcap lwres-pointer-arithmetic-ub.out
+ospf-signed-integer-ubsan       ospf-signed-integer-ubsan.pcap          ospf-signed-integer-ubsan.out -vv

tests/ospf-signed-integer-ubsan.out

@@ -0,0 +1,3 @@
+    1  15:39:26.444985 IP6 (class 0xe0, hlim 1, next-header AH (51) payload length: 532) fe80::1 > fe80::2: AH(length=4(24-bytes),spi=0x00000100,seq=0x1e,icv=0x0a6ab0b271917e05f7a01c58): OSPFv3, LS-Update, length 508
+	Router-ID 1.1.1.108, Area 11.234.210.1, Instance 1
+		  Bogus length 0 < header (20) [|ospf3]