Security fix, CVE-2017-14491 DNS heap buffer overflow.

Fix heap overflow in DNS code. This is a potentially serious
security hole. It allows an attacker who can make DNS
requests to dnsmasq, and who controls the contents of
a domain, which is thereby queried, to overflow
(by 2 bytes) a heap buffer and either crash, or
even take control of, dnsmasq.

CHANGELOG

@@ -24,6 +24,18 @@ version 2.78
 	Juan Manuel Fernandez and Kevin Darbyshire-Bryant for
 	chasing this one down.  CVE-2017-13704 applies.
 
+	Fix heap overflow in DNS code. This is a potentially serious
+	security hole. It allows an attacker who can make DNS
+	requests to dnsmasq, and who controls the contents of
+	a domain, which is thereby queried, to overflow
+	(by 2 bytes) a heap buffer and either crash, or
+	even take control of, dnsmasq.
+	CVE-2017-14491 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/dnsmasq.h

@@ -1185,7 +1185,7 @@ u32 rand32(void);
 u64 rand64(void);
 int legal_hostname(char *name);
 char *canonicalise(char *in, int *nomem);
-unsigned char *do_rfc1035_name(unsigned char *p, char *sval);
+unsigned char *do_rfc1035_name(unsigned char *p, char *sval, char *limit);
 void *safe_malloc(size_t size);
 void safe_pipe(int *fd, int read_noblock);
 void *whine_malloc(size_t size);

src/dnssec.c

@@ -2230,7 +2230,7 @@ size_t dnssec_generate_query(struct dns_header *header, unsigned char *end, char
 
   p = (unsigned char *)(header+1);
 	
-  p = do_rfc1035_name(p, name);
+  p = do_rfc1035_name(p, name, NULL);
   *p++ = 0;
   PUTSHORT(type, p);
   PUTSHORT(class, p);

src/option.c

@@ -1415,7 +1415,7 @@ static int parse_dhcp_opt(char *errstr, char *arg, int flags)
 		    }
 		  
 		  p = newp;
-		  end = do_rfc1035_name(p + len, dom);
+		  end = do_rfc1035_name(p + len, dom, NULL);
 		  *end++ = 0;
 		  len = end - p;
 		  free(dom);

src/rfc1035.c

@@ -1062,6 +1062,7 @@ int check_for_ignored_address(struct dns_header *header, size_t qlen, struct bog
   return 0;
 }
 
+
 int add_resource_record(struct dns_header *header, char *limit, int *truncp, int nameoffset, unsigned char **pp, 
 			unsigned long ttl, int *offset, unsigned short type, unsigned short class, char *format, ...)
 {
@@ -1071,12 +1072,21 @@ int add_resource_record(struct dns_header *header, char *limit, int *truncp, int
   unsigned short usval;
   long lval;
   char *sval;
+#define CHECK_LIMIT(size) \
+  if (limit && p + (size) > (unsigned char*)limit) \
+    { \
+      va_end(ap); \
+      goto truncated; \
+    }
 
   if (truncp && *truncp)
     return 0;
- 
+
   va_start(ap, format);   /* make ap point to 1st unamed argument */
-  
+
+  /* nameoffset (1 or 2) + type (2) + class (2) + ttl (4) + 0 (2) */
+  CHECK_LIMIT(12);
+
   if (nameoffset > 0)
     {
       PUTSHORT(nameoffset | 0xc000, p);
@@ -1085,7 +1095,13 @@ int add_resource_record(struct dns_header *header, char *limit, int *truncp, int
     {
       char *name = va_arg(ap, char *);
       if (name)
-	p = do_rfc1035_name(p, name);
+	p = do_rfc1035_name(p, name, limit);
+        if (!p)
+          {
+            va_end(ap);
+            goto truncated;
+          }
+
       if (nameoffset < 0)
 	{
 	  PUTSHORT(-nameoffset | 0xc000, p);
@@ -1106,6 +1122,7 @@ int add_resource_record(struct dns_header *header, char *limit, int *truncp, int
       {
 #ifdef HAVE_IPV6
       case '6':
+        CHECK_LIMIT(IN6ADDRSZ);
 	sval = va_arg(ap, char *); 
 	memcpy(p, sval, IN6ADDRSZ);
 	p += IN6ADDRSZ;
@@ -1113,36 +1130,47 @@ int add_resource_record(struct dns_header *header, char *limit, int *truncp, int
 #endif
 	
       case '4':
+        CHECK_LIMIT(INADDRSZ);
 	sval = va_arg(ap, char *); 
 	memcpy(p, sval, INADDRSZ);
 	p += INADDRSZ;
 	break;
 	
       case 'b':
+        CHECK_LIMIT(1);
 	usval = va_arg(ap, int);
 	*p++ = usval;
 	break;
 	
       case 's':
+        CHECK_LIMIT(2);
 	usval = va_arg(ap, int);
 	PUTSHORT(usval, p);
 	break;
 	
       case 'l':
+        CHECK_LIMIT(4);
 	lval = va_arg(ap, long);
 	PUTLONG(lval, p);
 	break;
 	
       case 'd':
-	/* get domain-name answer arg and store it in RDATA field */
-	if (offset)
-	  *offset = p - (unsigned char *)header;
-	p = do_rfc1035_name(p, va_arg(ap, char *));
-	*p++ = 0;
+        /* get domain-name answer arg and store it in RDATA field */
+        if (offset)
+          *offset = p - (unsigned char *)header;
+        p = do_rfc1035_name(p, va_arg(ap, char *), limit);
+        if (!p)
+          {
+            va_end(ap);
+            goto truncated;
+          }
+        CHECK_LIMIT(1);
+        *p++ = 0;
 	break;
 	
       case 't':
 	usval = va_arg(ap, int);
+        CHECK_LIMIT(usval);
 	sval = va_arg(ap, char *);
 	if (usval != 0)
 	  memcpy(p, sval, usval);
@@ -1154,20 +1182,24 @@ int add_resource_record(struct dns_header *header, char *limit, int *truncp, int
 	usval = sval ? strlen(sval) : 0;
 	if (usval > 255)
 	  usval = 255;
+        CHECK_LIMIT(usval + 1);
 	*p++ = (unsigned char)usval;
 	memcpy(p, sval, usval);
 	p += usval;
 	break;
       }
 
+#undef CHECK_LIMIT
   va_end(ap);	/* clean up variable argument pointer */
   
   j = p - sav - 2;
-  PUTSHORT(j, sav);     /* Now, store real RDLength */
+ /* this has already been checked against limit before */
+ PUTSHORT(j, sav);     /* Now, store real RDLength */
   
   /* check for overflow of buffer */
   if (limit && ((unsigned char *)limit - p) < 0)
     {
+truncated:
       if (truncp)
 	*truncp = 1;
       return 0;

src/rfc2131.c

@@ -2452,10 +2452,10 @@ static void do_options(struct dhcp_context *context,
 
 	      if (fqdn_flags & 0x04)
 		{
-		  p = do_rfc1035_name(p, hostname);
+		  p = do_rfc1035_name(p, hostname, NULL);
 		  if (domain)
 		    {
-		      p = do_rfc1035_name(p, domain);
+		      p = do_rfc1035_name(p, domain, NULL);
 		      *p++ = 0;
 		    }
 		}

src/rfc3315.c

@@ -1479,10 +1479,10 @@ static struct dhcp_netid *add_options(struct state *state, int do_refresh)
       if ((p = expand(len + 2)))
 	{
 	  *(p++) = state->fqdn_flags;
-	  p = do_rfc1035_name(p, state->hostname);
+	  p = do_rfc1035_name(p, state->hostname, NULL);
 	  if (state->send_domain)
 	    {
-	      p = do_rfc1035_name(p, state->send_domain);
+	      p = do_rfc1035_name(p, state->send_domain, NULL);
 	      *p = 0;
 	    }
 	}

src/util.c

@@ -240,15 +240,20 @@ char *canonicalise(char *in, int *nomem)
   return ret;
 }
 
-unsigned char *do_rfc1035_name(unsigned char *p, char *sval)
+unsigned char *do_rfc1035_name(unsigned char *p, char *sval, char *limit)
 {
   int j;
   
   while (sval && *sval)
     {
+      if (limit && p + 1 > (unsigned char*)limit)
+        return p;
+
       unsigned char *cp = p++;
       for (j = 0; *sval && (*sval != '.'); sval++, j++)
 	{
+          if (limit && p + 1 > (unsigned char*)limit)
+            return p;
 #ifdef HAVE_DNSSEC
 	  if (option_bool(OPT_DNSSEC_VALID) && *sval == NAME_ESCAPE)
 	    *p++ = (*(++sval))-1;