CVE-2017-12995/Check for DNS compression pointers that don't point ba…

…ckwards.

This is what BIND 9.11.0-P2 does; it not only detects pointers that
loop, as "point backwards" means "point before anything we've processed
so far, including what we're processing right now", so the pointer can't
point to itself (as that's what we're processing right now).

This fixes an infinite loop discovered by Forcepoint's security
researchers Otto Airamo & Antti Levomäki.

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

Also, add some infinite-pointer-loop captures.

More checks should be done.  We might, for example, want to make sure
the upper 2 bits of the label length/pointer byte are 00 or 11, and that
if we encounter a pointer and jump backwards to what we think is a label
the label ends before the beginning of the last label we processed, to
make sure the pointer doesn't point backwards into the *middle* of a
label, and also make sure the entire name is <= 255 bytes long.

print-domain.c

@@ -151,15 +151,14 @@ ns_nprint(netdissect_options *ndo,
 	register u_int i, l;
 	register const u_char *rp = NULL;
 	register int compress = 0;
-	int chars_processed;
 	int elt;
-	int data_size = ndo->ndo_snapend - bp;
+	u_int offset, max_offset;
 
 	if ((l = labellen(ndo, cp)) == (u_int)-1)
 		return(NULL);
 	if (!ND_TTEST2(*cp, 1))
 		return(NULL);
-	chars_processed = 1;
+	max_offset = (u_int)(cp - bp);
 	if (((i = *cp++) & INDIR_MASK) != INDIR_MASK) {
 		compress = 0;
 		rp = cp + l;
@@ -174,24 +173,28 @@ ns_nprint(netdissect_options *ndo,
 				}
 				if (!ND_TTEST2(*cp, 1))
 					return(NULL);
-				cp = bp + (((i << 8) | *cp) & 0x3fff);
+				offset = (((i << 8) | *cp) & 0x3fff);
+				/*
+				 * This must move backwards in the packet.
+				 * No RFC explicitly says that, but BIND's
+				 * name decompression code requires it,
+				 * as a way of preventing infinite loops
+				 * and other bad behavior, and it's probably
+				 * what was intended (compress by pointing
+				 * to domain name suffixes already seen in
+				 * the packet).
+				 */
+				if (offset >= max_offset) {
+					ND_PRINT((ndo, "<BAD PTR>"));
+					return(NULL);
+				}
+				max_offset = offset;
+				cp = bp + offset;
 				if ((l = labellen(ndo, cp)) == (u_int)-1)
 					return(NULL);
 				if (!ND_TTEST2(*cp, 1))
 					return(NULL);
 				i = *cp++;
-				chars_processed++;
-
-				/*
-				 * If we've looked at every character in
-				 * the message, this pointer will make
-				 * us look at some character again,
-				 * which means we're looping.
-				 */
-				if (chars_processed >= data_size) {
-					ND_PRINT((ndo, "<LOOP>"));
-					return (NULL);
-				}
 				continue;
 			}
 			if ((i & INDIR_MASK) == EDNS0_MASK) {
@@ -212,14 +215,12 @@ ns_nprint(netdissect_options *ndo,
 			}
 
 			cp += l;
-			chars_processed += l;
 			ND_PRINT((ndo, "."));
 			if ((l = labellen(ndo, cp)) == (u_int)-1)
 				return(NULL);
 			if (!ND_TTEST2(*cp, 1))
 				return(NULL);
 			i = *cp++;
-			chars_processed++;
 			if (!compress)
 				rp += l + 1;
 		}

tests/TESTLIST

@@ -474,6 +474,7 @@ zephyr-oobr		zephyr-oobr.pcap		zephyr-oobr.out		-vvv -e
 isakmp-no-none-np	isakmp-no-none-np.pcap		isakmp-no-none-np.out	-vvv -e
 telnet-iac-check-oobr	telnet-iac-check-oobr.pcap	telnet-iac-check-oobr.out	-vvv -e
 resp_4_infiniteloop	resp_4_infiniteloop.pcap	resp_4_infiniteloop.out	-vvv -e
+dns_fwdptr		dns_fwdptr.pcap			dns_fwdptr.out		-vvv -e
 
 # RTP tests
 # fuzzed pcap
@@ -483,3 +484,11 @@ rtp-seg-fault-2  rtp-seg-fault-2.pcap  rtp-seg-fault-2.out  -v -T rtp
 # NFS tests
 # fuzzed pcap
 nfs-seg-fault-1  nfs-seg-fault-1.pcap  nfs-seg-fault-1.out
+
+# DNS infinite loop tests
+#
+# See http://marc.info/?l=tcpdump-workers&m=95552439022555
+#
+dns-zlip-1		dns-zlip-1.pcap		dns-zlip-1.out
+dns-zlip-2		dns-zlip-2.pcap		dns-zlip-2.out
+dns-zlip-3		dns-zlip-3.pcap		dns-zlip-3.out

tests/dns-zlip-1.out

@@ -0,0 +1 @@
+IP 10.0.0.1.1024 > 146.84.28.88.53: 60777 Type49159 (Class 49168)? <BAD PTR>[|domain]

tests/dns-zlip-2.out

@@ -0,0 +1 @@
+IP 10.0.0.1.1024 > 146.84.28.88.53: 18992 Type49164 (Class 49168)? <BAD PTR>[|domain]

tests/dns-zlip-3.out

@@ -0,0 +1 @@
+IP 10.0.0.1.1024 > 146.84.28.88.53: 65483 Type49164 (Class 49164)? thisleetostringwillcrashyourlittlenameserverforsurehahahahahah.<BAD PTR>[|domain]

tests/dns_fwdptr.out

@@ -0,0 +1,2 @@
+be:af:ca:ce:ff:ff > de:ad:be:ef:00:01, ethertype IPv4 (0x0800), length 63207: (tos 0x0, ttl 128, id 36039, offset 0, flags [none], proto UDP (17), length 63193)
+    156.118.17.235.53 > 156.118.27.229.500: [udp sum ok] 51584 zoneRef NoChange*|$ [64259q] q: Type507 (Class 769)? M-{.^AM-{^C.M-{.^AM-{^C.M-{.^AM-{^C.M-{.^AM-{^C.M-{.^AM-{ .M-{^C^A.<BAD PTR>[|domain]