Implement rechecking of redactions

synapse/api/auth.py

@@ -627,7 +627,7 @@ class Auth(object):
 
         defer.returnValue(auth_ids)
 
-    def check_redaction(self, event, auth_events):
+    def check_redaction(self, room_version, event, auth_events):
         """Check whether the event sender is allowed to redact the target event.
 
         Returns:
@@ -640,7 +640,7 @@ class Auth(object):
             AuthError if the event sender is definitely not allowed to redact
             the target event.
         """
-        return event_auth.check_redaction(event, auth_events)
+        return event_auth.check_redaction(room_version, event, auth_events)
 
     @defer.inlineCallbacks
     def check_can_change_room_list(self, room_id, user):

synapse/event_auth.py

@@ -20,7 +20,13 @@ from signedjson.key import decode_verify_key_bytes
 from signedjson.sign import SignatureVerifyException, verify_signed_json
 from unpaddedbase64 import decode_base64
 
-from synapse.api.constants import KNOWN_ROOM_VERSIONS, EventTypes, JoinRules, Membership
+from synapse.api.constants import (
+    KNOWN_ROOM_VERSIONS,
+    EventTypes,
+    JoinRules,
+    Membership,
+    RoomVersions,
+)
 from synapse.api.errors import AuthError, EventSizeError, SynapseError
 from synapse.types import UserID, get_domain_from_id
 
@@ -168,7 +174,7 @@ def check(room_version, event, auth_events, do_sig_check=True, do_size_check=Tru
         _check_power_levels(event, auth_events)
 
     if event.type == EventTypes.Redaction:
-        check_redaction(event, auth_events)
+        check_redaction(room_version, event, auth_events)
 
     logger.debug("Allowing! %s", event)
 
@@ -422,7 +428,7 @@ def _can_send_event(event, auth_events):
     return True
 
 
-def check_redaction(event, auth_events):
+def check_redaction(room_version, event, auth_events):
     """Check whether the event sender is allowed to redact the target event.
 
     Returns:
@@ -442,10 +448,16 @@ def check_redaction(event, auth_events):
     if user_level >= redact_level:
         return False
 
+    if room_version in (RoomVersions.V1, RoomVersions.V2, RoomVersions.VDH_TEST):
         redacter_domain = get_domain_from_id(event.event_id)
         redactee_domain = get_domain_from_id(event.redacts)
         if redacter_domain == redactee_domain:
             return True
+    elif room_version == RoomVersions.V3:
+        event.internal_metadata.recheck_redaction = True
+        return True
+    else:
+        raise RuntimeError("Unrecognized room version %r" % (room_version,))
 
     raise AuthError(
         403,

synapse/events/__init__.py

@@ -63,6 +63,9 @@ class _EventInternalMetadata(object):
         """
         return getattr(self, "send_on_behalf_of", None)
 
+    def need_to_check_redaction(self):
+        return getattr(self, "recheck_redaction", False)
+
 
 def _event_dict_property(key):
     # We want to be able to use hasattr with the event dict properties.

synapse/handlers/message.py

@@ -767,7 +767,8 @@ class EventCreationHandler(object):
             auth_events = {
                 (e.type, e.state_key): e for e in auth_events.values()
             }
-            if self.auth.check_redaction(event, auth_events=auth_events):
+            room_version = yield self.store.get_room_version(event.room_id)
+            if self.auth.check_redaction(room_version, event, auth_events=auth_events):
                 original_event = yield self.store.get_event(
                     event.redacts,
                     check_redacted=False,
@@ -781,6 +782,9 @@ class EventCreationHandler(object):
                         "You don't have permission to redact events"
                     )
 
+                # We've already checked.
+                event.internal_metadata.recheck_redaction = False
+
         if event.type == EventTypes.Create:
             prev_state_ids = yield context.get_prev_state_ids(self.store)
             if prev_state_ids:

synapse/storage/events_worker.py

@@ -21,13 +21,14 @@ from canonicaljson import json
 
 from twisted.internet import defer
 
-from synapse.api.constants import EventFormatVersions
+from synapse.api.constants import EventFormatVersions, EventTypes
 from synapse.api.errors import NotFoundError
 from synapse.events import FrozenEvent, event_type_from_format_version  # noqa: F401
 # these are only included to make the type annotations work
 from synapse.events.snapshot import EventContext  # noqa: F401
 from synapse.events.utils import prune_event
 from synapse.metrics.background_process_metrics import run_as_background_process
+from synapse.types import get_domain_from_id
 from synapse.util.logcontext import (
     LoggingContext,
     PreserveLoggingContext,
@@ -174,6 +175,29 @@ class EventsWorkerStore(SQLBaseStore):
             if not entry:
                 continue
 
+            # Some redactions in room version v3 need to be rechecked if we
+            # didn't have the redacted event at the time, so we recheck on read
+            # instead.
+            if not allow_rejected and entry.event.type == EventTypes.Redaction:
+                if entry.event.internal_metadata.need_to_check_redaction():
+                    orig = yield self.get_event(
+                        entry.event.redacts,
+                        allow_none=True,
+                        allow_rejected=True,
+                        get_prev_content=False,
+                    )
+                    expected_domain = get_domain_from_id(entry.event.sender)
+                    if orig and get_domain_from_id(orig.sender) == expected_domain:
+                        # This redaction event is allowed. Mark as not needing a
+                        # recheck.
+                        entry.event.recheck_redaction = False
+                    else:
+                        # We don't have the event that is being redacted, so we
+                        # assume that the event isn't authorized for now. (If we
+                        # later receive the event, then we will always redact
+                        # it anyway, since we have this redaction)
+                        continue
+
             if allow_rejected or not entry.event.rejected_reason:
                 if check_redacted and entry.redacted_event:
                     event = entry.redacted_event