From 6bcd38a50ce7d1b6767c22e8037234979f651903 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Fri, 10 Jan 2020 11:50:56 +0000
Subject: Check inbound to device messages for correct devices.

To aid debugging we want to sanity check the device direct messages sent
between servers to try and make sure that remote servers have the
correct device lists.
---
 synapse/handlers/devicemessage.py | 44 ++++++++++++++++++++++++++++++++++++---
 1 file changed, 41 insertions(+), 3 deletions(-)

diff --git a/synapse/handlers/devicemessage.py b/synapse/handlers/devicemessage.py
index 73b9e120f5..21ea89d279 100644
--- a/synapse/handlers/devicemessage.py
+++ b/synapse/handlers/devicemessage.py
@@ -19,6 +19,7 @@ from canonicaljson import json
 
 from twisted.internet import defer
 
+import synapse
 from synapse.api.errors import SynapseError
 from synapse.logging.opentracing import (
     get_active_span_text_map,
@@ -31,9 +32,11 @@ from synapse.util.stringutils import random_string
 
 logger = logging.getLogger(__name__)
 
+device_list_debugging_logger = logging.getLogger("synapse.devices.DEBUG_TRACKING")
+
 
 class DeviceMessageHandler(object):
-    def __init__(self, hs):
+    def __init__(self, hs: "synapse.server.HomeServer"):
         """
         Args:
             hs (synapse.server.HomeServer): server
@@ -65,6 +68,9 @@ class DeviceMessageHandler(object):
                 logger.warning("Request for keys for non-local user %s", user_id)
                 raise SynapseError(400, "Not a user here")
 
+            if not by_device:
+                continue
+
             messages_by_device = {
                 device_id: {
                     "content": message_content,
@@ -73,8 +79,40 @@ class DeviceMessageHandler(object):
                 }
                 for device_id, message_content in by_device.items()
             }
-            if messages_by_device:
-                local_messages[user_id] = messages_by_device
+            local_messages[user_id] = messages_by_device
+
+            # We expect the sending user to send the message to all the devices
+            # to the user, if they don't then that is potentially suspicious and
+            # so we log for debugging purposes.
+            if device_list_debugging_logger.isEnabledFor(logging.INFO):
+                expected_devices = yield self.store.get_devices_by_user(user_id)
+                expected_devices = set(expected_devices)
+                received_devices = set(by_device)
+                if received_devices != {"*"} and received_devices != expected_devices:
+                    # Devices that the remote didn't send to
+                    missed = expected_devices - received_devices
+
+                    # Devices the remote sent to that we don't know bout
+                    extraneous = received_devices - expected_devices
+
+                    # We try and pull out the `sender_key` from the first message,
+                    # if it has one. This just helps figure out which device the
+                    # message came from.
+                    sender_key = list(by_device.values())[0].get(
+                        "sender_key", "<unknown>"
+                    )
+
+                    device_list_debugging_logger.info(
+                        "Received direct message (%s, %s) from %s (%s) to %s with mismatched devices."
+                        " Missing: %s, extraneous: %s",
+                        message_type,
+                        message_id,
+                        sender_user_id,
+                        sender_key,
+                        user_id,
+                        missed,
+                        extraneous,
+                    )
 
         stream_id = yield self.store.add_messages_from_remote_to_device_inbox(
             origin, message_id, local_messages
-- 
cgit 1.4.1