Security fix, CVE-2017-14496, Integer underflow in DNS response creation.

Fix DoS in DNS. Invalid boundary checks in the
add_pseudoheader function allows a memcpy call with negative
size An attacker which can send malicious DNS queries
to dnsmasq can trigger a DoS remotely.
dnsmasq is vulnerable only if one of the following option is
specified: --add-mac, --add-cpe-id or --add-subnet.

CHANGELOG

@@ -60,6 +60,17 @@ version 2.78
 	and Kevin Hamacher of the Google Security Team for
 	finding this.
 	
+	Fix DoS in DNS. Invalid boundary checks in the
+	add_pseudoheader function allows a memcpy call with negative
+	size An attacker which can send malicious DNS queries
+	to dnsmasq can trigger a DoS remotely.
+	dnsmasq is vulnerable only if one of the following option is
+	specified: --add-mac, --add-cpe-id or --add-subnet.
+	CVE-2017-14496 applies.
+	Credit to Felix Wilhelm, Fermin J. Serna, Gabriel Campana
+	and Kevin Hamacher of the Google Security Team for
+	finding this.
+	
 	
 version 2.77
 	Generate an error when configured with a CNAME loop,

src/edns0.c

@@ -144,7 +144,7 @@ size_t add_pseudoheader(struct dns_header *header, size_t plen, unsigned char *l
 	  GETSHORT(len, p);
 	  
 	  /* malformed option, delete the whole OPT RR and start again. */
-	  if (i + len > rdlen)
+	  if (i + 4 + len > rdlen)
 	    {
 	      rdlen = 0;
 	      is_last = 0;
@@ -193,6 +193,8 @@ size_t add_pseudoheader(struct dns_header *header, size_t plen, unsigned char *l
 			     ntohs(header->ancount) + ntohs(header->nscount) + ntohs(header->arcount), 
 			     header, plen)))
 	return plen;
+      if (p + 11 > limit)
+       return plen; /* Too big */
       *p++ = 0; /* empty name */
       PUTSHORT(T_OPT, p);
       PUTSHORT(udp_sz, p); /* max packet length, 512 if not given in EDNS0 header */
@@ -204,6 +206,11 @@ size_t add_pseudoheader(struct dns_header *header, size_t plen, unsigned char *l
       /* Copy back any options */
       if (buff)
 	{
+          if (p + rdlen > limit)
+          {
+            free(buff);
+            return plen; /* Too big */
+          }
 	  memcpy(p, buff, rdlen);
 	  free(buff);
 	  p += rdlen;
@@ -220,8 +227,12 @@ size_t add_pseudoheader(struct dns_header *header, size_t plen, unsigned char *l
   /* Add new option */
   if (optno != 0 && replace != 2)
     {
+      if (p + 4 > limit)
+       return plen; /* Too big */
       PUTSHORT(optno, p);
       PUTSHORT(optlen, p);
+      if (p + optlen > limit)
+       return plen; /* Too big */
       memcpy(p, opt, optlen);
       p += optlen;  
       PUTSHORT(p - datap, lenp);