Implement MSC2730: verifiable forwarded events

changelog.d/8078.feature

@@ -0,0 +1 @@
+Implement [MSC2730](https://github.com/matrix-org/matrix-doc/pull/2730): Verifiable forwarded events.

synapse/handlers/federation.py

@@ -54,6 +54,7 @@
 from synapse.events import EventBase
 from synapse.events.snapshot import EventContext
 from synapse.events.validator import EventValidator
+from synapse.federation.federation_base import FederationBase, event_from_pdu_json
 from synapse.handlers._base import BaseHandler
 from synapse.logging.context import (
     make_deferred_yieldable,
@@ -108,7 +109,7 @@ class _NewEventInfo:
     auth_events = attr.ib(type=Optional[MutableStateMap[EventBase]], default=None)
 
 
-class FederationHandler(BaseHandler):
+class FederationHandler(BaseHandler, FederationBase):
     """Handles events that originated from federation.
         Responsible for:
         a) handling received Pdus before handing them on as Events to the rest
@@ -683,6 +684,41 @@ async def _get_events_from_store_or_dest(
 
         return fetched_events
 
+    _forwarded_key = "net.maunium.msc2730.forwarded"
+
+    async def _validate_forwarded_event(
+        self, event: EventBase
+    ) -> Tuple[bool, Optional[str]]:
+        try:
+            source_evt_dict = {**event.content[self._forwarded_key]}
+            room_version_identifier = source_evt_dict["unsigned"]["room_version"]
+            source_evt_dict["type"] = event.type
+            source_evt_dict["content"] = {**event.content}
+            del source_evt_dict["unsigned"]
+            del source_evt_dict["content"][self._forwarded_key]
+        except (KeyError, TypeError):
+            return False, None
+
+        room_version = KNOWN_ROOM_VERSIONS.get(room_version_identifier)
+        if not room_version:
+            # We don't support the source room version, so we can't verify the event :(
+            return False, None
+
+        try:
+            source_evt = event_from_pdu_json(source_evt_dict, room_version)
+        except SynapseError:
+            return False, None
+
+        try:
+            checked_evt = await self._check_sigs_and_hash(room_version, source_evt)
+            # _check_sigs_and_hash returns a redacted event if hash validation failed and
+            # a SynapseError if signature validation failed. In both those cases, we want to
+            # mark the forward as invalid.
+            valid = not checked_evt.internal_metadata.is_redacted()
+        except SynapseError:
+            valid = False
+        return valid, source_evt.event_id
+
     async def _process_received_pdu(
         self, origin: str, event: EventBase, state: Optional[Iterable[EventBase]],
     ):
@@ -703,6 +739,17 @@ async def _process_received_pdu(
 
         logger.debug("[%s %s] Processing event: %s", room_id, event_id, event)
 
+        if (
+            not event.is_state()
+            and not event.redacts
+            and self._forwarded_key in event.content
+        ):
+            valid, forwarded_event_id = await self._validate_forwarded_event(event)
+            event.unsigned[self._forwarded_key] = {
+                "valid": valid,
+                "event_id": forwarded_event_id,
+            }
+
         try:
             await self._handle_new_event(origin, event, state=state)
         except AuthError as e:

synapse/rest/__init__.py

@@ -37,6 +37,7 @@
     capabilities,
     devices,
     filter,
+    forward_event,
     groups,
     keys,
     notifications,
@@ -109,6 +110,7 @@ def register_servlets(client_resource, hs):
         tags.register_servlets(hs, client_resource)
         account_data.register_servlets(hs, client_resource)
         report_event.register_servlets(hs, client_resource)
+        forward_event.register_servlets(hs, client_resource)
         openid.register_servlets(hs, client_resource)
         notifications.register_servlets(hs, client_resource)
         devices.register_servlets(hs, client_resource)

synapse/rest/client/v2_alpha/forward_event.py

@@ -0,0 +1,134 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 Tulir Asokan <[email protected]>
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+import logging
+
+from synapse.api.errors import AuthError, Codes, SynapseError
+from synapse.http.servlet import RestServlet, parse_integer
+from synapse.logging.opentracing import set_tag
+
+from ._base import client_patterns
+
+logger = logging.getLogger(__name__)
+
+
+class RoomEventForwardServlet(RestServlet):
+    """
+    PUT /net.maunium.msc2730/rooms/{room_id}/event/{event_id}/forward/{target_room_id}/{txn_id}
+    """
+
+    PATTERNS = client_patterns(
+        (
+            "/net.maunium.msc2730/rooms/(?P<room_id>[^/]*)/event/(?P<event_id>[^/]*)"
+            "/forward/(?P<target_room_id>[^/]*)/(?P<txn_id>.*)"
+        ),
+        releases=(),  # This is an unstable feature
+    )
+
+    _data_key = "net.maunium.msc2730.forwarded"
+    _err_not_forwardable = "NET.MAUNIUM.MSC2730_NOT_FORWARDABLE"
+
+    def __init__(self, hs):
+        super().__init__()
+        self.event_creation_handler = hs.get_event_creation_handler()
+        self.event_handler = hs.get_event_handler()
+        self.store = hs.get_datastore()
+        self.auth = hs.get_auth()
+
+    async def on_PUT(self, request, room_id, event_id, target_room_id, txn_id):
+        requester = await self.auth.get_user_by_req(request, allow_guest=True)
+
+        try:
+            event = await self.event_handler.get_event(
+                requester.user, room_id, event_id
+            )
+        except AuthError:
+            event = None
+        if not event:
+            raise SynapseError(404, "Event not found.", errcode=Codes.NOT_FOUND)
+
+        if event.is_state():
+            raise SynapseError(
+                401,
+                "State events cannot be forwarded.",
+                errcode=self._err_not_forwardable,
+            )
+        elif event.redacts:
+            raise SynapseError(
+                401,
+                "Redaction events cannot be forwarded.",
+                errcode=self._err_not_forwardable,
+            )
+        elif event.internal_metadata.is_redacted():
+            raise SynapseError(
+                401,
+                "Redacted events cannot be forwarded.",
+                errcode=self._err_not_forwardable,
+            )
+
+        event_id = event.event_id
+        event_dict = event.get_dict()
+
+        content = event_dict.pop("content")
+        unsigned = event_dict.pop("unsigned", {})
+        event_type = event_dict.pop("type")
+        has_forward_meta = self._data_key in content
+        try:
+            is_valid_forward = has_forward_meta and unsigned[self._data_key]["valid"]
+        except (KeyError, TypeError):
+            is_valid_forward = False
+
+        if has_forward_meta:
+            if not is_valid_forward:
+                raise SynapseError(
+                    401,
+                    "Event contains invalid forward metadata.",
+                    errcode=self._err_not_forwardable,
+                )
+            # Pass through the old event ID to the new unsigned data
+            event_id = unsigned[self._data_key]["event_id"]
+        elif not has_forward_meta:
+            content[self._data_key] = event_dict
+            room_version = await self.store.get_room_version(event.room_id)
+            content[self._data_key]["unsigned"] = {
+                "room_version": room_version.identifier,
+                # TODO add sender profile info here
+            }
+
+        forward_event_dict = {
+            "type": event_type,
+            "content": content,
+            "room_id": target_room_id,
+            "sender": requester.user.to_string(),
+            "unsigned": {self._data_key: {"valid": True, "event_id": event_id}},
+        }
+
+        if b"ts" in request.args and requester.app_service:
+            forward_event_dict["origin_server_ts"] = parse_integer(request, "ts", 0)
+
+        (
+            forwarded_event,
+            _,
+        ) = await self.event_creation_handler.create_and_send_nonmember_event(
+            requester, forward_event_dict, txn_id=txn_id
+        )
+
+        set_tag("event_id", forwarded_event.event_id)
+        return 200, {"event_id": forwarded_event.event_id}
+
+
+def register_servlets(hs, http_server):
+    RoomEventForwardServlet(hs).register(http_server)

synapse/rest/client/versions.py

@@ -75,6 +75,8 @@ def on_GET(self, request):
                     "org.matrix.e2e_cross_signing": True,
                     # Implements additional endpoints as described in MSC2432
                     "org.matrix.msc2432": True,
+                    # Implements endpoint and forwarded event validation as described in MSC2730
+                    "net.maunium.msc2730": True,
                     # Implements additional endpoints as described in MSC2666
                     "uk.half-shot.msc2666": True,
                     # Whether new rooms will be set to encrypted or not (based on presets).