Reject malformed tunnel packets

meshtastic/tests/test_tunnel.py

@@ -123,6 +123,33 @@ def test_shouldFilterPacket_random(mock_platform_system, caplog, iface_with_node
             assert not ignore
 
 
+@pytest.mark.unit
+@patch("platform.system")
+@pytest.mark.parametrize(
+    "packet",
+    [
+        b"",
+        b"\x00" * 19,
+        "not-a-packet",
+        *[b"\x00" * 9 + bytes([protocol]) + b"\x00" * 10 for protocol in (1, 6, 17)],
+    ],
+)
+def test_shouldFilterPacket_rejects_malformed_packets(
+    mock_platform_system, caplog, iface_with_nodes, packet
+):
+    """Malformed mesh payloads must not crash the tunnel reader."""
+    iface = iface_with_nodes
+    iface.noProto = True
+    mock_platform_system.return_value = "Linux"
+
+    with caplog.at_level(logging.WARNING):
+        with patch("socket.socket"):
+            tun = Tunnel(iface)
+            assert tun._shouldFilterPacket(packet)
+
+    assert re.search(r"Ignoring (malformed|truncated)", caplog.text)
+
+
 @pytest.mark.unitslow
 @patch("platform.system")
 def test_shouldFilterPacket_in_blacklist(

meshtastic/tunnel.py

@@ -146,6 +146,14 @@ def onReceive(self, packet):
 
     def _shouldFilterPacket(self, p):
         """Given a packet, decode it and return true if it should be ignored"""
+        # Packets received from the mesh are untrusted.  IPv4 requires a
+        # 20-byte header, and the protocol-specific fields below require the
+        # first four bytes of the transport header.  Without these checks a
+        # truncated payload raises IndexError in the tunnel reader thread.
+        if not isinstance(p, (bytes, bytearray)) or len(p) < 20:
+            logger.warning("Ignoring malformed IP tunnel packet")
+            return True
+
         protocol = p[8 + 1]
         srcaddr = p[12:16]
         destAddr = p[16:20]
@@ -157,6 +165,9 @@ def _shouldFilterPacket(self, p):
                 self.LOG_TRACE, f"Ignoring blacklisted protocol 0x{protocol:02x}"
             )
         elif protocol == 0x01:  # ICMP
+            if len(p) < subheader + 4:
+                logger.warning("Ignoring truncated ICMP tunnel packet")
+                return True
             icmpType = p[20]
             icmpCode = p[21]
             checksum = p[22:24]
@@ -168,6 +179,9 @@ def _shouldFilterPacket(self, p):
             # pingback = p[:12]+p[16:20]+p[12:16]+p[20:]
             # tap.write(pingback)
         elif protocol == 0x11:  # UDP
+            if len(p) < subheader + 4:
+                logger.warning("Ignoring truncated UDP tunnel packet")
+                return True
             srcport = readnet_u16(p, subheader)
             destport = readnet_u16(p, subheader + 2)
             if destport in self.udpBlacklist:
@@ -176,6 +190,9 @@ def _shouldFilterPacket(self, p):
             else:
                 logger.debug(f"forwarding udp srcport={srcport}, destport={destport}")
         elif protocol == 0x06:  # TCP
+            if len(p) < subheader + 4:
+                logger.warning("Ignoring truncated TCP tunnel packet")
+                return True
             srcport = readnet_u16(p, subheader)
             destport = readnet_u16(p, subheader + 2)
             if destport in self.tcpBlacklist: