WireGuard: support Decode As and non-zero reserved bytes

Recognize Cloudflare Warp traffic which may use non-zero bytes for load
balancing purposes. This is an extension of the WireGuard protocol, it
is not understood by official implementations which require the reserved
bytes field to be zero.

Change-Id: Iff789b538ab8477d8b5014302569264823d92358
Reviewed-on: https://code.wireshark.org/review/35215
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>

epan/dissectors/packet-wireguard.c

@@ -24,6 +24,7 @@
 #include <epan/expert.h>
 #include <epan/prefs.h>
 #include <epan/proto_data.h>
+#include <epan/conversation.h>
 #include <epan/uat.h>
 #include <wsutil/file_util.h>
 #include <wsutil/filesystem.h>
@@ -83,6 +84,7 @@ static const char  *pref_keylog_file;
 
 static dissector_handle_t ip_handle;
 #endif /* WG_DECRYPTION_SUPPORTED */
+static dissector_handle_t wg_handle;
 
 
 // Length of AEAD authentication tag
@@ -1578,10 +1580,6 @@ dissect_wg(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data _U_)
     const char *message_type_str;
     wg_packet_info_t *wg_pinfo;
 
-    /* Heuristics check: check for reserved bits (zeros) and message type. */
-    if (tvb_reported_length(tvb) < 4 || tvb_get_ntoh24(tvb, 1) != 0)
-        return 0;
-
     message_type = tvb_get_guint8(tvb, 0);
     message_type_str = try_val_to_str(message_type, wg_type_names);
     if (!message_type_str)
@@ -1622,6 +1620,74 @@ dissect_wg(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data _U_)
     DISSECTOR_ASSERT_NOT_REACHED();
 }
 
+static gboolean
+dissect_wg_heur(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data _U_)
+{
+    /*
+     * Heuristics to detect the WireGuard protocol:
+     * - The first byte must be one of the valid four messages.
+     * - The total packet length depends on the message type, and is fixed for
+     *   three of them. The Data type has a minimum length however.
+     * - The next three bytes are reserved and zero in the official protocol.
+     *   Cloudflare's implementation however uses this field for load balancing
+     *   purposes, so this condition is not checked here for most messages.
+     *   It is checked for data messages to avoid false positives.
+     */
+    guint32     message_type;
+    gboolean    reserved_is_zeroes;
+
+    if (tvb_reported_length(tvb) < 4)
+        return FALSE;
+
+    message_type = tvb_get_guint8(tvb, 0);
+    reserved_is_zeroes = tvb_get_ntoh24(tvb, 1) == 0;
+
+    switch (message_type) {
+        case WG_TYPE_HANDSHAKE_INITIATION:
+            if (tvb_reported_length(tvb) != 148)
+                return FALSE;
+            break;
+
+        case WG_TYPE_HANDSHAKE_RESPONSE:
+            if (tvb_reported_length(tvb) != 92)
+                return FALSE;
+            break;
+
+        case WG_TYPE_COOKIE_REPLY:
+            if (tvb_reported_length(tvb) != 64)
+                return FALSE;
+            if (!reserved_is_zeroes)
+                return FALSE;
+            break;
+
+        case WG_TYPE_TRANSPORT_DATA:
+            if (tvb_reported_length(tvb) < 32)
+                return FALSE;
+            if (!reserved_is_zeroes)
+                return FALSE;
+            break;
+
+        default:
+            return FALSE;
+    }
+
+    /*
+     * Assuming that this is a new handshake, make sure that future messages are
+     * directed to our dissector. This ensures that cookie replies and data
+     * messages using non-zero reserved bytes are still properly recognized.
+     * An edge case occurs when the address or port change. In that case, Data
+     * messages using non-zero reserved bytes will not be recognized. The user
+     * can use Decode As for this case.
+     */
+    if (message_type == WG_TYPE_HANDSHAKE_INITIATION) {
+        conversation_t *conversation = find_or_create_conversation(pinfo);
+        conversation_set_dissector(conversation, wg_handle);
+    }
+
+    dissect_wg(tvb, pinfo, tree, NULL);
+    return TRUE;
+}
+
 static void
 wg_init(void)
 {
@@ -1645,7 +1711,7 @@ proto_register_wg(void)
         },
         { &hf_wg_reserved,
           { "Reserved", "wg.reserved",
-            FT_NONE, BASE_NONE, NULL, 0x0,
+            FT_BYTES, BASE_NONE, NULL, 0x0,
             NULL, HFILL }
         },
         { &hf_wg_sender,
@@ -1824,7 +1890,7 @@ proto_register_wg(void)
     expert_wg = expert_register_protocol(proto_wg);
     expert_register_field_array(expert_wg, ei, array_length(ei));
 
-    register_dissector("wg", dissect_wg, proto_wg);
+    wg_handle = register_dissector("wg", dissect_wg, proto_wg);
 
 #ifdef WG_DECRYPTION_SUPPORTED
     wg_module = prefs_register_protocol(proto_wg, NULL);
@@ -1880,7 +1946,8 @@ proto_register_wg(void)
 void
 proto_reg_handoff_wg(void)
 {
-    heur_dissector_add("udp", dissect_wg, "WireGuard", "wg", proto_wg, HEURISTIC_ENABLE);
+    dissector_add_uint_with_preference("udp.port", 0, wg_handle);
+    heur_dissector_add("udp", dissect_wg_heur, "WireGuard", "wg", proto_wg, HEURISTIC_ENABLE);
 
 #ifdef WG_DECRYPTION_SUPPORTED
     ip_handle = find_dissector("ip");