Improve validation for `send_{join,leave,knock}` (#10225)

The idea here is to stop people sending things that aren't joins/leaves/knocks through these endpoints: previously you could send anything you liked through them. I wasn't able to find any security holes from doing so, but it doesn't sound like a good thing.

6 files changed, 132 insertions, 183 deletions

diff --git a/changelog.d/10225.feature b/changelog.d/10225.feature
new file mode 100644
index 0000000000..d16f66ffe9
--- /dev/null
+++ b/changelog.d/10225.feature
@@ -0,0 +1 @@
+Improve validation on federation `send_{join,leave,knock}` endpoints.
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index 2b07f18529..341965047a 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -34,7 +34,7 @@ from twisted.internet import defer
 from twisted.internet.abstract import isIPAddress
 from twisted.python import failure
 
-from synapse.api.constants import EduTypes, EventTypes
+from synapse.api.constants import EduTypes, EventTypes, Membership
 from synapse.api.errors import (
     AuthError,
     Codes,
@@ -46,6 +46,7 @@ from synapse.api.errors import (
 )
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
 from synapse.events import EventBase
+from synapse.events.snapshot import EventContext
 from synapse.federation.federation_base import FederationBase, event_from_pdu_json
 from synapse.federation.persistence import TransactionActions
 from synapse.federation.units import Edu, Transaction
@@ -537,26 +538,21 @@ class FederationServer(FederationBase):
         return {"event": ret_pdu.get_pdu_json(time_now)}
 
     async def on_send_join_request(
-        self, origin: str, content: JsonDict
+        self, origin: str, content: JsonDict, room_id: str
     ) -> Dict[str, Any]:
-        logger.debug("on_send_join_request: content: %s", content)
-
-        assert_params_in_dict(content, ["room_id"])
-        room_version = await self.store.get_room_version(content["room_id"])
-        pdu = event_from_pdu_json(content, room_version)
-
-        origin_host, _ = parse_server_name(origin)
-        await self.check_server_matches_acl(origin_host, pdu.room_id)
-
-        logger.debug("on_send_join_request: pdu sigs: %s", pdu.signatures)
+        context = await self._on_send_membership_event(
+            origin, content, Membership.JOIN, room_id
+        )
 
-        pdu = await self._check_sigs_and_hash(room_version, pdu)
+        prev_state_ids = await context.get_prev_state_ids()
+        state_ids = list(prev_state_ids.values())
+        auth_chain = await self.store.get_auth_chain(room_id, state_ids)
+        state = await self.store.get_events(state_ids)
 
-        res_pdus = await self.handler.on_send_join_request(origin, pdu)
         time_now = self._clock.time_msec()
         return {
-            "state": [p.get_pdu_json(time_now) for p in res_pdus["state"]],
-            "auth_chain": [p.get_pdu_json(time_now) for p in res_pdus["auth_chain"]],
+            "state": [p.get_pdu_json(time_now) for p in state.values()],
+            "auth_chain": [p.get_pdu_json(time_now) for p in auth_chain],
         }
 
     async def on_make_leave_request(
@@ -571,21 +567,11 @@ class FederationServer(FederationBase):
         time_now = self._clock.time_msec()
         return {"event": pdu.get_pdu_json(time_now), "room_version": room_version}
 
-    async def on_send_leave_request(self, origin: str, content: JsonDict) -> dict:
+    async def on_send_leave_request(
+        self, origin: str, content: JsonDict, room_id: str
+    ) -> dict:
         logger.debug("on_send_leave_request: content: %s", content)
-
-        assert_params_in_dict(content, ["room_id"])
-        room_version = await self.store.get_room_version(content["room_id"])
-        pdu = event_from_pdu_json(content, room_version)
-
-        origin_host, _ = parse_server_name(origin)
-        await self.check_server_matches_acl(origin_host, pdu.room_id)
-
-        logger.debug("on_send_leave_request: pdu sigs: %s", pdu.signatures)
-
-        pdu = await self._check_sigs_and_hash(room_version, pdu)
-
-        await self.handler.on_send_leave_request(origin, pdu)
+        await self._on_send_membership_event(origin, content, Membership.LEAVE, room_id)
         return {}
 
     async def on_make_knock_request(
@@ -651,39 +637,76 @@ class FederationServer(FederationBase):
         Returns:
             The stripped room state.
         """
-        logger.debug("on_send_knock_request: content: %s", content)
+        event_context = await self._on_send_membership_event(
+            origin, content, Membership.KNOCK, room_id
+        )
+
+        # Retrieve stripped state events from the room and send them back to the remote
+        # server. This will allow the remote server's clients to display information
+        # related to the room while the knock request is pending.
+        stripped_room_state = (
+            await self.store.get_stripped_room_state_from_event_context(
+                event_context, self._room_prejoin_state_types
+            )
+        )
+        return {"knock_state_events": stripped_room_state}
+
+    async def _on_send_membership_event(
+        self, origin: str, content: JsonDict, membership_type: str, room_id: str
+    ) -> EventContext:
+        """Handle an on_send_{join,leave,knock} request
+
+        Does some preliminary validation before passing the request on to the
+        federation handler.
+
+        Args:
+            origin: The (authenticated) requesting server
+            content: The body of the send_* request - a complete membership event
+            membership_type: The expected membership type (join or leave, depending
+                on the endpoint)
+            room_id: The room_id from the request, to be validated against the room_id
+                in the event
+
+        Returns:
+            The context of the event after inserting it into the room graph.
+
+        Raises:
+            SynapseError if there is a problem with the request, including things like
+               the room_id not matching or the event not being authorized.
+        """
+        assert_params_in_dict(content, ["room_id"])
+        if content["room_id"] != room_id:
+            raise SynapseError(
+                400,
+                "Room ID in body does not match that in request path",
+                Codes.BAD_JSON,
+            )
 
         room_version = await self.store.get_room_version(room_id)
 
-        # Check that this room supports knocking as defined by its room version
-        if not room_version.msc2403_knocking:
+        if membership_type == Membership.KNOCK and not room_version.msc2403_knocking:
             raise SynapseError(
                 403,
                 "This room version does not support knocking",
                 errcode=Codes.FORBIDDEN,
             )
 
-        pdu = event_from_pdu_json(content, room_version)
+        event = event_from_pdu_json(content, room_version)
 
-        origin_host, _ = parse_server_name(origin)
-        await self.check_server_matches_acl(origin_host, pdu.room_id)
+        if event.type != EventTypes.Member or not event.is_state():
+            raise SynapseError(400, "Not an m.room.member event", Codes.BAD_JSON)
 
-        logger.debug("on_send_knock_request: pdu sigs: %s", pdu.signatures)
+        if event.content.get("membership") != membership_type:
+            raise SynapseError(400, "Not a %s event" % membership_type, Codes.BAD_JSON)
 
-        pdu = await self._check_sigs_and_hash(room_version, pdu)
+        origin_host, _ = parse_server_name(origin)
+        await self.check_server_matches_acl(origin_host, event.room_id)
 
-        # Handle the event, and retrieve the EventContext
-        event_context = await self.handler.on_send_knock_request(origin, pdu)
+        logger.debug("_on_send_membership_event: pdu sigs: %s", event.signatures)
 
-        # Retrieve stripped state events from the room and send them back to the remote
-        # server. This will allow the remote server's clients to display information
-        # related to the room while the knock request is pending.
-        stripped_room_state = (
-            await self.store.get_stripped_room_state_from_event_context(
-                event_context, self._room_prejoin_state_types
-            )
-        )
-        return {"knock_state_events": stripped_room_state}
+        event = await self._check_sigs_and_hash(room_version, event)
+
+        return await self.handler.on_send_membership_event(origin, event)
 
     async def on_event_auth(
         self, origin: str, room_id: str, event_id: str
diff --git a/synapse/federation/transport/server.py b/synapse/federation/transport/server.py
index bed47f8abd..676fbd3750 100644
--- a/synapse/federation/transport/server.py
+++ b/synapse/federation/transport/server.py
@@ -553,7 +553,7 @@ class FederationV1SendLeaveServlet(BaseFederationServerServlet):
     PATH = "/send_leave/(?P<room_id>[^/]*)/(?P<event_id>[^/]*)"
 
     async def on_PUT(self, origin, content, query, room_id, event_id):
-        content = await self.handler.on_send_leave_request(origin, content)
+        content = await self.handler.on_send_leave_request(origin, content, room_id)
         return 200, (200, content)
 
 
@@ -563,7 +563,7 @@ class FederationV2SendLeaveServlet(BaseFederationServerServlet):
     PREFIX = FEDERATION_V2_PREFIX
 
     async def on_PUT(self, origin, content, query, room_id, event_id):
-        content = await self.handler.on_send_leave_request(origin, content)
+        content = await self.handler.on_send_leave_request(origin, content, room_id)
         return 200, content
 
 
@@ -602,9 +602,9 @@ class FederationV1SendJoinServlet(BaseFederationServerServlet):
     PATH = "/send_join/(?P<room_id>[^/]*)/(?P<event_id>[^/]*)"
 
     async def on_PUT(self, origin, content, query, room_id, event_id):
-        # TODO(paul): assert that room_id/event_id parsed from path actually
+        # TODO(paul): assert that event_id parsed from path actually
         #   match those given in content
-        content = await self.handler.on_send_join_request(origin, content)
+        content = await self.handler.on_send_join_request(origin, content, room_id)
         return 200, (200, content)
 
 
@@ -614,9 +614,9 @@ class FederationV2SendJoinServlet(BaseFederationServerServlet):
     PREFIX = FEDERATION_V2_PREFIX
 
     async def on_PUT(self, origin, content, query, room_id, event_id):
-        # TODO(paul): assert that room_id/event_id parsed from path actually
+        # TODO(paul): assert that event_id parsed from path actually
         #   match those given in content
-        content = await self.handler.on_send_join_request(origin, content)
+        content = await self.handler.on_send_join_request(origin, content, room_id)
         return 200, content
 
 
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 74d169a2ac..12f3d85342 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -1711,80 +1711,6 @@ class FederationHandler(BaseHandler):
 
         return event
 
-    async def on_send_join_request(self, origin: str, pdu: EventBase) -> JsonDict:
-        """We have received a join event for a room. Fully process it and
-        respond with the current state and auth chains.
-        """
-        event = pdu
-
-        logger.debug(
-            "on_send_join_request from %s: Got event: %s, signatures: %s",
-            origin,
-            event.event_id,
-            event.signatures,
-        )
-
-        if get_domain_from_id(event.sender) != origin:
-            logger.info(
-                "Got /send_join request for user %r from different origin %s",
-                event.sender,
-                origin,
-            )
-            raise SynapseError(403, "User not from origin", Codes.FORBIDDEN)
-
-        event.internal_metadata.outlier = False
-        # Send this event on behalf of the origin server.
-        #
-        # The reasons we have the destination server rather than the origin
-        # server send it are slightly mysterious: the origin server should have
-        # all the necessary state once it gets the response to the send_join,
-        # so it could send the event itself if it wanted to. It may be that
-        # doing it this way reduces failure modes, or avoids certain attacks
-        # where a new server selectively tells a subset of the federation that
-        # it has joined.
-        #
-        # The fact is that, as of the current writing, Synapse doesn't send out
-        # the join event over federation after joining, and changing it now
-        # would introduce the danger of backwards-compatibility problems.
-        event.internal_metadata.send_on_behalf_of = origin
-
-        # Calculate the event context.
-        context = await self.state_handler.compute_event_context(event)
-
-        # Get the state before the new event.
-        prev_state_ids = await context.get_prev_state_ids()
-
-        # Check if the user is already in the room or invited to the room.
-        user_id = event.state_key
-        prev_member_event_id = prev_state_ids.get((EventTypes.Member, user_id), None)
-        prev_member_event = None
-        if prev_member_event_id:
-            prev_member_event = await self.store.get_event(prev_member_event_id)
-
-        # Check if the member should be allowed access via membership in a space.
-        await self._event_auth_handler.check_restricted_join_rules(
-            prev_state_ids,
-            event.room_version,
-            user_id,
-            prev_member_event,
-        )
-
-        # Persist the event.
-        await self._auth_and_persist_event(origin, event, context)
-
-        logger.debug(
-            "on_send_join_request: After _auth_and_persist_event: %s, sigs: %s",
-            event.event_id,
-            event.signatures,
-        )
-
-        state_ids = list(prev_state_ids.values())
-        auth_chain = await self.store.get_auth_chain(event.room_id, state_ids)
-
-        state = await self.store.get_events(list(prev_state_ids.values()))
-
-        return {"state": list(state.values()), "auth_chain": auth_chain}
-
     async def on_invite_request(
         self, origin: str, event: EventBase, room_version: RoomVersion
     ) -> EventBase:
@@ -1960,44 +1886,6 @@ class FederationHandler(BaseHandler):
 
         return event
 
-    async def on_send_leave_request(self, origin: str, pdu: EventBase) -> None:
-        """We have received a leave event for a room. Fully process it."""
-        event = pdu
-
-        logger.debug(
-            "on_send_leave_request: Got event: %s, signatures: %s",
-            event.event_id,
-            event.signatures,
-        )
-
-        if get_domain_from_id(event.sender) != origin:
-            logger.info(
-                "Got /send_leave request for user %r from different origin %s",
-                event.sender,
-                origin,
-            )
-            raise SynapseError(403, "User not from origin", Codes.FORBIDDEN)
-
-        event.internal_metadata.outlier = False
-
-        # Send this event on behalf of the other server.
-        #
-        # The remote server isn't a full participant in the room at this point, so
-        # may not have an up-to-date list of the other homeservers participating in
-        # the room, so we send it on their behalf.
-        event.internal_metadata.send_on_behalf_of = origin
-
-        context = await self.state_handler.compute_event_context(event)
-        await self._auth_and_persist_event(origin, event, context)
-
-        logger.debug(
-            "on_send_leave_request: After _auth_and_persist_event: %s, sigs: %s",
-            event.event_id,
-            event.signatures,
-        )
-
-        return None
-
     @log_function
     async def on_make_knock_request(
         self, origin: str, room_id: str, user_id: str
@@ -2061,34 +1949,38 @@ class FederationHandler(BaseHandler):
         return event
 
     @log_function
-    async def on_send_knock_request(
+    async def on_send_membership_event(
         self, origin: str, event: EventBase
     ) -> EventContext:
         """
-        We have received a knock event for a room. Verify that event and send it into the room
-        on the knocking homeserver's behalf.
+        We have received a join/leave/knock event for a room.
+
+        Verify that event and send it into the room on the remote homeserver's behalf.
 
         Args:
-            origin: The remote homeserver of the knocking user.
-            event: The knocking member event that has been signed by the remote homeserver.
+            origin: The homeserver of the remote (joining/invited/knocking) user.
+            event: The member event that has been signed by the remote homeserver.
 
         Returns:
             The context of the event after inserting it into the room graph.
         """
         logger.debug(
-            "on_send_knock_request: Got event: %s, signatures: %s",
+            "on_send_membership_event: Got event: %s, signatures: %s",
             event.event_id,
             event.signatures,
         )
 
         if get_domain_from_id(event.sender) != origin:
             logger.info(
-                "Got /send_knock request for user %r from different origin %s",
+                "Got send_membership request for user %r from different origin %s",
                 event.sender,
                 origin,
             )
             raise SynapseError(403, "User not from origin", Codes.FORBIDDEN)
 
+        if event.sender != event.state_key:
+            raise SynapseError(400, "state_key and sender must match", Codes.BAD_JSON)
+
         event.internal_metadata.outlier = False
 
         # Send this event on behalf of the other server.
@@ -2100,19 +1992,52 @@ class FederationHandler(BaseHandler):
 
         context = await self.state_handler.compute_event_context(event)
 
-        event_allowed = await self.third_party_event_rules.check_event_allowed(
-            event, context
-        )
-        if not event_allowed:
-            logger.info("Sending of knock %s forbidden by third-party rules", event)
-            raise SynapseError(
-                403, "This event is not allowed in this context", Codes.FORBIDDEN
+        # for joins, we need to check the restrictions of restricted rooms
+        if event.membership == Membership.JOIN:
+            await self._check_join_restrictions(context, event)
+
+        # for knock events, we run the third-party event rules. It's not entirely clear
+        # why we don't do this for other sorts of membership events.
+        if event.membership == Membership.KNOCK:
+            event_allowed = await self.third_party_event_rules.check_event_allowed(
+                event, context
             )
+            if not event_allowed:
+                logger.info("Sending of knock %s forbidden by third-party rules", event)
+                raise SynapseError(
+                    403, "This event is not allowed in this context", Codes.FORBIDDEN
+                )
 
         await self._auth_and_persist_event(origin, event, context)
 
         return context
 
+    async def _check_join_restrictions(
+        self, context: EventContext, event: EventBase
+    ) -> None:
+        """Check that restrictions in restricted join rules are matched
+
+        Called when we receive a join event via send_join.
+
+        Raises an auth error if the restrictions are not matched.
+        """
+        prev_state_ids = await context.get_prev_state_ids()
+
+        # Check if the user is already in the room or invited to the room.
+        user_id = event.state_key
+        prev_member_event_id = prev_state_ids.get((EventTypes.Member, user_id), None)
+        prev_member_event = None
+        if prev_member_event_id:
+            prev_member_event = await self.store.get_event(prev_member_event_id)
+
+        # Check if the member should be allowed access via membership in a space.
+        await self._event_auth_handler.check_restricted_join_rules(
+            prev_state_ids,
+            event.room_version,
+            user_id,
+            prev_member_event,
+        )
+
     async def get_state_for_pdu(self, room_id: str, event_id: str) -> List[EventBase]:
         """Returns the state at the event. i.e. not including said event."""
 
diff --git a/tests/handlers/test_federation.py b/tests/handlers/test_federation.py
index 8796af45ed..ba8cf44f46 100644
--- a/tests/handlers/test_federation.py
+++ b/tests/handlers/test_federation.py
@@ -251,7 +251,7 @@ class FederationTestCase(unittest.HomeserverTestCase):
         join_event.signatures[other_server] = {"x": "y"}
         with LoggingContext("send_join"):
             d = run_in_background(
-                self.handler.on_send_join_request, other_server, join_event
+                self.handler.on_send_membership_event, other_server, join_event
             )
         self.get_success(d)
 
diff --git a/tests/replication/test_federation_sender_shard.py b/tests/replication/test_federation_sender_shard.py
index 584da58371..a0c710f855 100644
--- a/tests/replication/test_federation_sender_shard.py
+++ b/tests/replication/test_federation_sender_shard.py
@@ -228,7 +228,7 @@ class FederationSenderTestCase(BaseMultiWorkerStreamTestCase):
             builder.build(prev_event_ids=prev_event_ids, auth_event_ids=None)
         )
 
-        self.get_success(federation.on_send_join_request(remote_server, join_event))
+        self.get_success(federation.on_send_membership_event(remote_server, join_event))
         self.replicate()
 
         return room