mirror of
https://github.com/element-hq/synapse.git
synced 2026-11-11 18:47:54 +00:00
89 lines
4.6 KiB
Diff
89 lines
4.6 KiB
Diff
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
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index 52e5435793..26d74305f8 100644
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--- a/synapse/handlers/federation_event.py
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+++ b/synapse/handlers/federation_event.py
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@@ -270,23 +270,22 @@ class FederationEventHandler:
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) = await self._store.get_local_current_membership_for_user_in_room(
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pdu.state_key, pdu.room_id
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)
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- if (
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- membership == Membership.INVITE
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- and membership_event_id
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- and membership_event_id
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- in pdu.auth_event_ids() # The invite should be in the auth events of the rescission.
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- ):
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+ if membership == Membership.INVITE and membership_event_id:
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invite_event = await self._store.get_event(
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membership_event_id, allow_none=True
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)
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# We cannot fully auth the rescission event, but we can
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# check if the sender of the leave event is the same as the
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- # invite.
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+ # invite, and that it references the invite it rescinds.
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#
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# Technically, a room admin could rescind the invite, but we
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# have no way of knowing who is and isn't a room admin.
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- if invite_event and pdu.sender == invite_event.sender:
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+ if (
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+ invite_event
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+ and pdu.sender == invite_event.sender
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+ and rescinds_invite(pdu, invite_event)
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+ ):
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# Handle the rescission event
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pdu.internal_metadata.outlier = True
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pdu.internal_metadata.out_of_band_membership = True
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@@ -3172,6 +3171,23 @@ class FederationEventHandler:
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raise SynapseError(HTTPStatus.BAD_REQUEST, "Too many auth_events")
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+def rescinds_invite(pdu: EventBase, invite_event: EventBase) -> bool:
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+ """Returns True if the given leave event references the given invite.
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+
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+ We are not in the room, so we cannot authorise the leave event: the most we can do is
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+ check that it names the invite we are holding. Without this an old leave event could
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+ be replayed by anyone to cancel a newer invite.
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+ """
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+ if supports_msc4242_state_dag(pdu):
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+ # MSC4242 state DAG events don't list their auth events: they are calculated from
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+ # the state DAG, which we cannot do as we are not in the room. Instead, a leave
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+ # event which rescinds an invite must name the invite in its prev_state_events.
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+ return invite_event.event_id in pdu.prev_state_events
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+
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+ # The invite should be in the auth events of the rescission.
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+ return invite_event.event_id in pdu.auth_event_ids()
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+
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+
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def is_state_dag_connected(state_dag: Collection[MSC4242Event]) -> bool:
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"""Returns True if the given events form a connected state DAG (MSC4242).
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diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
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index b73e7114c0..faa8aab7ea 100644
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--- a/synapse/handlers/room_member.py
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+++ b/synapse/handlers/room_member.py
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@@ -1040,6 +1040,24 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
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"You cannot reject this invite",
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errcode=Codes.CANNOT_LEAVE_SERVER_NOTICE_ROOM,
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)
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+
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+ # If we are rescinding an invite to a user on a server which is not in the
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+ # room, that server cannot authorise the leave event: it has no state DAG,
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+ # so it cannot calculate the auth events which would otherwise tell it that
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+ # the leave supersedes the invite it is holding. Name the invite in
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+ # prev_state_events so that it can, see MSC4242.
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+ if (
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+ is_state_dags
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+ and prev_state_events is None
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+ and old_membership == Membership.INVITE
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+ and effective_membership_state == Membership.LEAVE
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+ and requester.user.to_string() != target.to_string()
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+ and not self.hs.is_mine(target)
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+ ):
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+ # `latest_event_ids` are the state DAG extremities for these rooms.
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+ prev_state_events = list(latest_event_ids)
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+ if old_state_id not in prev_state_events:
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+ prev_state_events.append(old_state_id)
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else:
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if action == "kick":
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raise AuthError(403, "The target user is not in the room")
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