Check for space membership during a remote join of a restricted room (#9814)

When receiving a /send_join request for a room with join rules set to 'restricted',
check if the user is a member of the spaces defined in the 'allow' key of the join rules.

This only applies to an experimental room version, as defined in MSC3083.

6 files changed, 131 insertions, 68 deletions

diff --git a/changelog.d/9814.feature b/changelog.d/9814.feature
new file mode 100644
index 0000000000..9404ad2fc0
--- /dev/null
+++ b/changelog.d/9814.feature
@@ -0,0 +1 @@
+Update experimental support for [MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083): restricting room access via group membership.
diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index 872fd100cd..2d845d0d5c 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -65,6 +65,7 @@ class Auth:
     """
     FIXME: This class contains a mix of functions for authenticating users
     of our client-server API and authenticating events added to room graphs.
+    The latter should be moved to synapse.handlers.event_auth.EventAuthHandler.
     """
 
     def __init__(self, hs):
diff --git a/synapse/handlers/event_auth.py b/synapse/handlers/event_auth.py
new file mode 100644
index 0000000000..eff639f407
--- /dev/null
+++ b/synapse/handlers/event_auth.py
@@ -0,0 +1,86 @@
+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# 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.
+from typing import TYPE_CHECKING
+
+from synapse.api.constants import EventTypes, JoinRules
+from synapse.api.room_versions import RoomVersion
+from synapse.types import StateMap
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+
+class EventAuthHandler:
+    """
+    This class contains methods for authenticating events added to room graphs.
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        self._store = hs.get_datastore()
+
+    async def can_join_without_invite(
+        self, state_ids: StateMap[str], room_version: RoomVersion, user_id: str
+    ) -> bool:
+        """
+        Check whether a user can join a room without an invite.
+
+        When joining a room with restricted joined rules (as defined in MSC3083),
+        the membership of spaces must be checked during join.
+
+        Args:
+            state_ids: The state of the room as it currently is.
+            room_version: The room version of the room being joined.
+            user_id: The user joining the room.
+
+        Returns:
+            True if the user can join the room, false otherwise.
+        """
+        # This only applies to room versions which support the new join rule.
+        if not room_version.msc3083_join_rules:
+            return True
+
+        # If there's no join rule, then it defaults to invite (so this doesn't apply).
+        join_rules_event_id = state_ids.get((EventTypes.JoinRules, ""), None)
+        if not join_rules_event_id:
+            return True
+
+        # If the join rule is not restricted, this doesn't apply.
+        join_rules_event = await self._store.get_event(join_rules_event_id)
+        if join_rules_event.content.get("join_rule") != JoinRules.MSC3083_RESTRICTED:
+            return True
+
+        # If allowed is of the wrong form, then only allow invited users.
+        allowed_spaces = join_rules_event.content.get("allow", [])
+        if not isinstance(allowed_spaces, list):
+            return False
+
+        # Get the list of joined rooms and see if there's an overlap.
+        joined_rooms = await self._store.get_rooms_for_user(user_id)
+
+        # Pull out the other room IDs, invalid data gets filtered.
+        for space in allowed_spaces:
+            if not isinstance(space, dict):
+                continue
+
+            space_id = space.get("space")
+            if not isinstance(space_id, str):
+                continue
+
+            # The user was joined to one of the spaces specified, they can join
+            # this room!
+            if space_id in joined_rooms:
+                return True
+
+        # The user was not in any of the required spaces.
+        return False
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index dbdd7d2db3..9d867aaf4d 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -146,6 +146,7 @@ class FederationHandler(BaseHandler):
         self.is_mine_id = hs.is_mine_id
         self.spam_checker = hs.get_spam_checker()
         self.event_creation_handler = hs.get_event_creation_handler()
+        self._event_auth_handler = hs.get_event_auth_handler()
         self._message_handler = hs.get_message_handler()
         self._server_notices_mxid = hs.config.server_notices_mxid
         self.config = hs.config
@@ -1673,8 +1674,40 @@ class FederationHandler(BaseHandler):
         # 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)
-        context = await self._auth_and_persist_event(origin, event, context)
+
+        # 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)
+        newly_joined = True
+        user_is_invited = False
+        if prev_member_event_id:
+            prev_member_event = await self.store.get_event(prev_member_event_id)
+            newly_joined = prev_member_event.membership != Membership.JOIN
+            user_is_invited = prev_member_event.membership == Membership.INVITE
+
+        # If the member is not already in the room, and not invited, check if
+        # they should be allowed access via membership in a space.
+        if (
+            newly_joined
+            and not user_is_invited
+            and not await self._event_auth_handler.can_join_without_invite(
+                prev_state_ids,
+                event.room_version,
+                user_id,
+            )
+        ):
+            raise AuthError(
+                403,
+                "You do not belong to any of the required spaces to join this room.",
+            )
+
+        # 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",
@@ -1682,8 +1715,6 @@ class FederationHandler(BaseHandler):
             event.signatures,
         )
 
-        prev_state_ids = await context.get_prev_state_ids()
-
         state_ids = list(prev_state_ids.values())
         auth_chain = await self.store.get_auth_chain(event.room_id, state_ids)
 
@@ -2006,7 +2037,7 @@ class FederationHandler(BaseHandler):
         state: Optional[Iterable[EventBase]] = None,
         auth_events: Optional[MutableStateMap[EventBase]] = None,
         backfilled: bool = False,
-    ) -> EventContext:
+    ) -> None:
         """
         Process an event by performing auth checks and then persisting to the database.
 
@@ -2028,9 +2059,6 @@ class FederationHandler(BaseHandler):
                 event is an outlier), may be the auth events claimed by the remote
                 server.
             backfilled: True if the event was backfilled.
-
-        Returns:
-             The event context.
         """
         context = await self._check_event_auth(
             origin,
@@ -2060,8 +2088,6 @@ class FederationHandler(BaseHandler):
             )
             raise
 
-        return context
-
     async def _auth_and_persist_events(
         self,
         origin: str,
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 2bbfac6471..2c5bada1d8 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -19,7 +19,7 @@ from http import HTTPStatus
 from typing import TYPE_CHECKING, Iterable, List, Optional, Tuple
 
 from synapse import types
-from synapse.api.constants import AccountDataTypes, EventTypes, JoinRules, Membership
+from synapse.api.constants import AccountDataTypes, EventTypes, Membership
 from synapse.api.errors import (
     AuthError,
     Codes,
@@ -28,7 +28,6 @@ from synapse.api.errors import (
     SynapseError,
 )
 from synapse.api.ratelimiting import Ratelimiter
-from synapse.api.room_versions import RoomVersion
 from synapse.events import EventBase
 from synapse.events.snapshot import EventContext
 from synapse.types import JsonDict, Requester, RoomAlias, RoomID, StateMap, UserID
@@ -64,6 +63,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         self.profile_handler = hs.get_profile_handler()
         self.event_creation_handler = hs.get_event_creation_handler()
         self.account_data_handler = hs.get_account_data_handler()
+        self.event_auth_handler = hs.get_event_auth_handler()
 
         self.member_linearizer = Linearizer(name="member")
 
@@ -178,62 +178,6 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
 
         await self._invites_per_user_limiter.ratelimit(requester, invitee_user_id)
 
-    async def _can_join_without_invite(
-        self, state_ids: StateMap[str], room_version: RoomVersion, user_id: str
-    ) -> bool:
-        """
-        Check whether a user can join a room without an invite.
-
-        When joining a room with restricted joined rules (as defined in MSC3083),
-        the membership of spaces must be checked during join.
-
-        Args:
-            state_ids: The state of the room as it currently is.
-            room_version: The room version of the room being joined.
-            user_id: The user joining the room.
-
-        Returns:
-            True if the user can join the room, false otherwise.
-        """
-        # This only applies to room versions which support the new join rule.
-        if not room_version.msc3083_join_rules:
-            return True
-
-        # If there's no join rule, then it defaults to public (so this doesn't apply).
-        join_rules_event_id = state_ids.get((EventTypes.JoinRules, ""), None)
-        if not join_rules_event_id:
-            return True
-
-        # If the join rule is not restricted, this doesn't apply.
-        join_rules_event = await self.store.get_event(join_rules_event_id)
-        if join_rules_event.content.get("join_rule") != JoinRules.MSC3083_RESTRICTED:
-            return True
-
-        # If allowed is of the wrong form, then only allow invited users.
-        allowed_spaces = join_rules_event.content.get("allow", [])
-        if not isinstance(allowed_spaces, list):
-            return False
-
-        # Get the list of joined rooms and see if there's an overlap.
-        joined_rooms = await self.store.get_rooms_for_user(user_id)
-
-        # Pull out the other room IDs, invalid data gets filtered.
-        for space in allowed_spaces:
-            if not isinstance(space, dict):
-                continue
-
-            space_id = space.get("space")
-            if not isinstance(space_id, str):
-                continue
-
-            # The user was joined to one of the spaces specified, they can join
-            # this room!
-            if space_id in joined_rooms:
-                return True
-
-        # The user was not in any of the required spaces.
-        return False
-
     async def _local_membership_update(
         self,
         requester: Requester,
@@ -302,7 +246,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
             if (
                 newly_joined
                 and not user_is_invited
-                and not await self._can_join_without_invite(
+                and not await self.event_auth_handler.can_join_without_invite(
                     prev_state_ids, event.room_version, user_id
                 )
             ):
diff --git a/synapse/server.py b/synapse/server.py
index 59ae91b503..67598fffe3 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -77,6 +77,7 @@ from synapse.handlers.devicemessage import DeviceMessageHandler
 from synapse.handlers.directory import DirectoryHandler
 from synapse.handlers.e2e_keys import E2eKeysHandler
 from synapse.handlers.e2e_room_keys import E2eRoomKeysHandler
+from synapse.handlers.event_auth import EventAuthHandler
 from synapse.handlers.events import EventHandler, EventStreamHandler
 from synapse.handlers.federation import FederationHandler
 from synapse.handlers.groups_local import GroupsLocalHandler, GroupsLocalWorkerHandler
@@ -747,6 +748,10 @@ class HomeServer(metaclass=abc.ABCMeta):
         return SpaceSummaryHandler(self)
 
     @cache_in_self
+    def get_event_auth_handler(self) -> EventAuthHandler:
+        return EventAuthHandler(self)
+
+    @cache_in_self
     def get_external_cache(self) -> ExternalCache:
         return ExternalCache(self)