net: dns: Check recursive pointers for CNAME handling

Make sure that the CNAME handling checks recursive name pointers
and fails the response if recursion is detected.
See RFC 9267 ch. 2 for details.

Signed-off-by: Jukka Rissanen <[email protected]>
(cherry picked from commit 5746f61)

Author: jukkar

subsys/net/lib/dns/dns_pack.c

@@ -5,6 +5,8 @@
  */
 
 #include <string.h>
+#include <zephyr/sys/bitarray.h>
+#include <zephyr/net/dns_resolve.h>
 #include <zephyr/net_buf.h>
 
 #include "dns_pack.h"
@@ -367,10 +369,11 @@ int dns_unpack_response_query(struct dns_msg_t *dns_msg)
 int dns_copy_qname(uint8_t *buf, uint16_t *len, uint16_t size,
 		   struct dns_msg_t *dns_msg, uint16_t pos)
 {
+	SYS_BITARRAY_DEFINE(visited, DNS_RESOLVER_MAX_BUF_SIZE);
 	uint16_t msg_size = dns_msg->msg_size;
 	uint8_t *msg = dns_msg->msg;
 	uint16_t lb_size;
-	int rc = -EINVAL;
+	int rc = -EINVAL, ret, prev;
 
 	*len = 0U;
 
@@ -394,6 +397,17 @@ int dns_copy_qname(uint8_t *buf, uint16_t *len, uint16_t size,
 			/* See: RFC 1035, 4.1.4. Message compression */
 			pos = ((msg[pos] & mask) << 8) + msg[pos + 1];
 
+			ret = sys_bitarray_test_and_set_bit(&visited, pos, &prev);
+			if (ret < 0) {
+				rc = -EINVAL;
+				break;
+			}
+
+			if (prev) {
+				rc = -ELOOP;
+				break;
+			}
+
 			continue;
 		}