From 4f4f27e57fdab1d7cc6e275b8acabc785952205e Mon Sep 17 00:00:00 2001 From: Sean Quah <8349537+squahtx@users.noreply.github.com> Date: Fri, 17 Feb 2023 09:40:32 +0000 Subject: [PATCH] Mitigate a race where /make_join could 403 for restricted rooms (#15080) Previously, when creating a join event in /make_join, we would decide whether to include additional fields to satisfy restricted room checks based on the current state of the room. Then, when building the event, we would capture the forward extremities of the room to use as prev events. This is subject to race conditions. For example, when leaving and rejoining a room, the following sequence of events leads to a misleading 403 response: 1. /make_join reads the current state of the room and sees that the user is still in the room. It decides to omit the field required for restricted room joins. 2. The leave event is persisted and the room's forward extremities are updated. 3. /make_join builds the event, using the post-leave forward extremities. The event then fails the restricted room checks. To mitigate the race, we move the read of the forward extremities closer to the read of the current state. Ideally, we would compute the state based off the chosen prev events, but that can involve state resolution, which is expensive. Signed-off-by: Sean Quah --- changelog.d/15080.bugfix | 1 + synapse/handlers/federation.py | 16 +++++++++++++++- 2 files changed, 16 insertions(+), 1 deletion(-) create mode 100644 changelog.d/15080.bugfix diff --git a/changelog.d/15080.bugfix b/changelog.d/15080.bugfix new file mode 100644 index 000000000..965d0b921 --- /dev/null +++ b/changelog.d/15080.bugfix @@ -0,0 +1 @@ +Reduce the likelihood of a rare race condition where rejoining a restricted room over federation would fail. diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py index 1d0f6bcd6..5f2057269 100644 --- a/synapse/handlers/federation.py +++ b/synapse/handlers/federation.py @@ -952,7 +952,20 @@ class FederationHandler: # # Note that this requires the /send_join request to come back to the # same server. + prev_event_ids = None if room_version.msc3083_join_rules: + # Note that the room's state can change out from under us and render our + # nice join rules-conformant event non-conformant by the time we build the + # event. When this happens, our validation at the end fails and we respond + # to the requesting server with a 403, which is misleading — it indicates + # that the user is not allowed to join the room and the joining server + # should not bother retrying via this homeserver or any others, when + # in fact we've just messed up with building the event. + # + # To reduce the likelihood of this race, we capture the forward extremities + # of the room (prev_event_ids) just before fetching the current state, and + # hope that the state we fetch corresponds to the prev events we chose. + prev_event_ids = await self.store.get_prev_events_for_room(room_id) state_ids = await self._state_storage_controller.get_current_state_ids( room_id ) @@ -994,7 +1007,8 @@ class FederationHandler: event, unpersisted_context, ) = await self.event_creation_handler.create_new_client_event( - builder=builder + builder=builder, + prev_event_ids=prev_event_ids, ) except SynapseError as e: logger.warning("Failed to create join to %s because %s", room_id, e)