Fix OOB-read crash in get_rdata().

get_rdata() contains another instance of a crash caused by
a false value of RDLEN in a resource-record.

An RR containing name which is longer than the rdlen of the RR can
get get_rdata into a state where it calculates the length of the data
following the name as a negative number which is then promoted
to a very large positive number, and causing SIGSEGV.

This can be triggered by crafted packets from malicous DNS servers,
but only when DNSSEC validation is enabled. The attack packets do not
have to be correctly DNSSEC signed.

Thanks to Tristan G. for spotting this problem.

Also fix error handling for RRs with a fixed size fields in the specification
where the rrsize indicates that there are zero bytes in that position.

src/dnssec.c

@@ -177,7 +177,7 @@ static int get_rdata(struct dns_header *header, size_t plen, struct rdata_state
 	  if ((state->c = state->end - state->ip) != 0)
 	    {
 	      state->op = state->ip;
-	      state->ip = state->end;;
+	      state->ip = state->end;
 	    }
 	  else
 	    return 0;
@@ -189,24 +189,28 @@ static int get_rdata(struct dns_header *header, size_t plen, struct rdata_state
 	  if (d == (u16)0)
 	    {
 	      /* domain-name, canonicalise */
-	      int len;
 	      
+	      /* If the name is malformed, or runs beyond the end of the RR, abort.
+		 under no circumstances should state->ip be beyond state->end since
+		 that will cause the run-out code above to turn the negative
+		 calculted length into a very large postive value for state->c
+		 and overrun the buffer. */
 	      if (!extract_name(header, plen, &state->ip, state->buff, EXTR_NAME_EXTRACT, 0) ||
-		  (len = to_wire(state->buff)) == 0)
-		continue;
-	      
-	      state->c = len;
+		  state->ip > state->end)
+		return 0;
+
+	      /* to_wire() always returns length > 0 */
+	      state->c = to_wire(state->buff);
 	      state->op = (unsigned char *)state->buff;
 	    }
 	  else
 	    {
-	      /* plain data preceding a domain-name, don't run off the end of the data */
-	      if ((state->end - state->ip) < d)
-		d = state->end - state->ip;
-	      
-	      if (d == 0)
-		continue;
-		  
+	      /* Plain data preceding a domain-name.
+		 Handle short RR, if there are no bytes left, return finished. */
+	      if ((state->end - state->ip) < d &&
+		  (d = state->end - state->ip) == 0)
+		return 0;
+				  
 	      state->op = state->ip;
 	      state->c = d;
 	      state->ip += d;