CVE-2017-12989/RESP: Make sure resp_get_length() advances the pointer…

… for invalid lengths.

Make sure that it always sends *endp before returning and that, for
invalid lengths where we don't like a character in the length string,
what it sets *endp to is past the character in question, so we don't
run the risk of infinitely looping (or doing something else random) if a
character in the length is invalid.

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).

print-resp.c

@@ -481,8 +481,10 @@ resp_get_length(netdissect_options *ndo, register const u_char *bp, int len, con
         ND_TCHECK(*bp);
         c = *bp;
         if (!(c >= '0' && c <= '9')) {
-            if (!saw_digit)
+            if (!saw_digit) {
+                bp++;
                 goto invalid;
+            }
             break;
         }
         c -= '0';
@@ -510,15 +512,19 @@ resp_get_length(netdissect_options *ndo, register const u_char *bp, int len, con
     if (len == 0)
         goto trunc;
     ND_TCHECK(*bp);
-    if (*bp != '\r')
+    if (*bp != '\r') {
+        bp++;
         goto invalid;
+    }
     bp++;
     len--;
     if (len == 0)
         goto trunc;
     ND_TCHECK(*bp);
-    if (*bp != '\n')
+    if (*bp != '\n') {
+        bp++;
         goto invalid;
+    }
     bp++;
     len--;
     *endp = bp;
@@ -531,8 +537,10 @@ resp_get_length(netdissect_options *ndo, register const u_char *bp, int len, con
     return (too_large ? -3 : result);
 
 trunc:
+    *endp = bp;
     return (-2);
 
 invalid:
+    *endp = bp;
     return (-5);
 }

tests/TESTLIST

@@ -468,6 +468,7 @@ zephyr-oobr		zephyr-oobr.pcap		zephyr-oobr.out		-vvv -e
 bgp-as-path-oobr	bgp-as-path-oobr.pcap		bgp-as-path-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
 
 # RTP tests
 # fuzzed pcap

tests/resp_4_infiniteloop.out

@@ -0,0 +1,2 @@
+00:50:56:b4:08:69 > 00:50:56:b4:4c:2a, ethertype IPv4 (0x0800), length 920: (tos 0x0, ttl 64, id 27576, offset 0, flags [DF], proto TCP (6), length 906)
+    172.16.8.77.33926 > 172.16.8.149.6379: Flags [P.], cksum 0xa129 (incorrect -> 0xaaa0), seq 3839414413:3839415267, ack 2526552240, win 229, options [nop,nop,TS val 2407226 ecr 24894817], length 854: RESP length negative and not -1 invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid "4" "EVAL" invalid invalid invalid invalid "GKMbNZq^@0" "stuubt.pack('<ivdMFG4294967245',^V ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''319', 2',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',',', '-1494241318543828858')'L')N))'r')')~D')')E)')')')')')')')'l')')')')')'M-`'o')')'Pp)U)" invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid invalid "1" [|RESP]