diff --git a/changelog.d/4814.feature b/changelog.d/4814.feature
new file mode 100644
index 0000000000..9433acd959
--- /dev/null
+++ b/changelog.d/4814.feature
@@ -0,0 +1 @@
+Add checks to incoming events over federation for events evading auth (aka "soft fail").
diff --git a/synapse/events/__init__.py b/synapse/events/__init__.py
index 20c1ab4203..bd130f8816 100644
--- a/synapse/events/__init__.py
+++ b/synapse/events/__init__.py
@@ -77,6 +77,20 @@ class _EventInternalMetadata(object):
"""
return getattr(self, "recheck_redaction", False)
+ def is_soft_failed(self):
+ """Whether the event has been soft failed.
+
+ Soft failed events should be handled as usual, except:
+ 1. They should not go down sync or event streams, or generally
+ sent to clients.
+ 2. They should not be added to the forward extremities (and
+ therefore not to current state).
+
+ Returns:
+ bool
+ """
+ return getattr(self, "soft_failed", False)
+
def _event_dict_property(key):
# We want to be able to use hasattr with the event dict properties.
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 72b63d64d0..9eaf2d3e18 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -45,6 +45,7 @@ from synapse.api.errors import (
SynapseError,
)
from synapse.crypto.event_signing import compute_event_signature
+from synapse.event_auth import auth_types_for_event
from synapse.events.validator import EventValidator
from synapse.replication.http.federation import (
ReplicationCleanRoomRestServlet,
@@ -1628,6 +1629,7 @@ class FederationHandler(BaseHandler):
origin, event,
state=state,
auth_events=auth_events,
+ backfilled=backfilled,
)
# reraise does not allow inlineCallbacks to preserve the stacktrace, so we
@@ -1672,6 +1674,7 @@ class FederationHandler(BaseHandler):
event,
state=ev_info.get("state"),
auth_events=ev_info.get("auth_events"),
+ backfilled=backfilled,
)
defer.returnValue(res)
@@ -1794,7 +1797,7 @@ class FederationHandler(BaseHandler):
)
@defer.inlineCallbacks
- def _prep_event(self, origin, event, state=None, auth_events=None):
+ def _prep_event(self, origin, event, state, auth_events, backfilled):
"""
Args:
@@ -1802,6 +1805,7 @@ class FederationHandler(BaseHandler):
event:
state:
auth_events:
+ backfilled (bool)
Returns:
Deferred, which resolves to synapse.events.snapshot.EventContext
@@ -1843,12 +1847,100 @@ class FederationHandler(BaseHandler):
context.rejected = RejectedReason.AUTH_ERROR
+ if not context.rejected:
+ yield self._check_for_soft_fail(event, state, backfilled)
+
if event.type == EventTypes.GuestAccess and not context.rejected:
yield self.maybe_kick_guest_users(event)
defer.returnValue(context)
@defer.inlineCallbacks
+ def _check_for_soft_fail(self, event, state, backfilled):
+ """Checks if we should soft fail the event, if so marks the event as
+ such.
+
+ Args:
+ event (FrozenEvent)
+ state (dict|None): The state at the event if we don't have all the
+ event's prev events
+ backfilled (bool): Whether the event is from backfill
+
+ Returns:
+ Deferred
+ """
+ # For new (non-backfilled and non-outlier) events we check if the event
+ # passes auth based on the current state. If it doesn't then we
+ # "soft-fail" the event.
+ do_soft_fail_check = not backfilled and not event.internal_metadata.is_outlier()
+ if do_soft_fail_check:
+ extrem_ids = yield self.store.get_latest_event_ids_in_room(
+ event.room_id,
+ )
+
+ extrem_ids = set(extrem_ids)
+ prev_event_ids = set(event.prev_event_ids())
+
+ if extrem_ids == prev_event_ids:
+ # If they're the same then the current state is the same as the
+ # state at the event, so no point rechecking auth for soft fail.
+ do_soft_fail_check = False
+
+ if do_soft_fail_check:
+ room_version = yield self.store.get_room_version(event.room_id)
+
+ # Calculate the "current state".
+ if state is not None:
+ # If we're explicitly given the state then we won't have all the
+ # prev events, and so we have a gap in the graph. In this case
+ # we want to be a little careful as we might have been down for
+ # a while and have an incorrect view of the current state,
+ # however we still want to do checks as gaps are easy to
+ # maliciously manufacture.
+ #
+ # So we use a "current state" that is actually a state
+ # resolution across the current forward extremities and the
+ # given state at the event. This should correctly handle cases
+ # like bans, especially with state res v2.
+
+ state_sets = yield self.store.get_state_groups(
+ event.room_id, extrem_ids,
+ )
+ state_sets = list(state_sets.values())
+ state_sets.append(state)
+ current_state_ids = yield self.state_handler.resolve_events(
+ room_version, state_sets, event,
+ )
+ current_state_ids = {
+ k: e.event_id for k, e in iteritems(current_state_ids)
+ }
+ else:
+ current_state_ids = yield self.state_handler.get_current_state_ids(
+ event.room_id, latest_event_ids=extrem_ids,
+ )
+
+ # Now check if event pass auth against said current state
+ auth_types = auth_types_for_event(event)
+ current_state_ids = [
+ e for k, e in iteritems(current_state_ids)
+ if k in auth_types
+ ]
+
+ current_auth_events = yield self.store.get_events(current_state_ids)
+ current_auth_events = {
+ (e.type, e.state_key): e for e in current_auth_events.values()
+ }
+
+ try:
+ self.auth.check(room_version, event, auth_events=current_auth_events)
+ except AuthError as e:
+ logger.warn(
+ "Failed current state auth resolution for %r because %s",
+ event, e,
+ )
+ event.internal_metadata.soft_failed = True
+
+ @defer.inlineCallbacks
def on_query_auth(self, origin, event_id, room_id, remote_auth_chain, rejects,
missing):
in_room = yield self.auth.check_host_in_room(
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index 06db9e56e6..990a5eaaae 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -537,6 +537,7 @@ class EventsStore(StateGroupWorkerStore, EventFederationStore, EventsWorkerStore
new_events = [
event for event, ctx in event_contexts
if not event.internal_metadata.is_outlier() and not ctx.rejected
+ and not event.internal_metadata.is_soft_failed()
]
# start with the existing forward extremities
diff --git a/synapse/visibility.py b/synapse/visibility.py
index efec21673b..16c40cd74c 100644
--- a/synapse/visibility.py
+++ b/synapse/visibility.py
@@ -67,6 +67,10 @@ def filter_events_for_client(store, user_id, events, is_peeking=False,
Returns:
Deferred[list[synapse.events.EventBase]]
"""
+ # Filter out events that have been soft failed so that we don't relay them
+ # to clients.
+ events = list(e for e in events if not e.internal_metadata.is_soft_failed())
+
types = (
(EventTypes.RoomHistoryVisibility, ""),
(EventTypes.Member, user_id),
|
