net: dns: Validate source buffer length properly

Make sure that when copying the qname, the source buffer is large
enough for the data.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

subsys/net/lib/dns/dns_pack.c

@@ -394,7 +394,7 @@ int dns_copy_qname(uint8_t *buf, uint16_t *len, uint16_t size,
 		/* validate that the label (i.e. size + elements),
 		 * fits the current msg buffer
 		 */
-		if (DNS_LABEL_LEN_SIZE + lb_size > size - *len) {
+		if (DNS_LABEL_LEN_SIZE + lb_size > MIN(size - *len, msg_size - pos)) {
 			rc = -ENOMEM;
 			break;
 		}