Ignore redactions of m.room.create events (#5701)

changelog.d/5701.bugfix

@@ -0,0 +1 @@
+Ignore redactions of m.room.create events.

synapse/api/auth.py

@@ -606,21 +606,6 @@ def compute_auth_events(self, event, current_state_ids, for_verification=False):
 
         defer.returnValue(auth_ids)
 
-    def check_redaction(self, room_version, event, auth_events):
-        """Check whether the event sender is allowed to redact the target event.
-
-        Returns:
-            True if the the sender is allowed to redact the target event if the
-                target event was created by them.
-            False if the sender is allowed to redact the target event with no
-                further checks.
-
-        Raises:
-            AuthError if the event sender is definitely not allowed to redact
-                the target event.
-        """
-        return event_auth.check_redaction(room_version, event, auth_events)
-
     @defer.inlineCallbacks
     def check_can_change_room_list(self, room_id, user):
         """Check if the user is allowed to edit the room's entry in the

synapse/handlers/message.py

@@ -23,6 +23,7 @@
 from twisted.internet import defer
 from twisted.internet.defer import succeed
 
+from synapse import event_auth
 from synapse.api.constants import EventTypes, Membership, RelationTypes
 from synapse.api.errors import (
     AuthError,
@@ -784,25 +785,39 @@ def is_inviter_member_event(e):
                     event.signatures.update(returned_invite.signatures)
 
         if event.type == EventTypes.Redaction:
+            original_event = yield self.store.get_event(
+                event.redacts,
+                check_redacted=False,
+                get_prev_content=False,
+                allow_rejected=False,
+                allow_none=True,
+                check_room_id=event.room_id,
+            )
+
+            # we can make some additional checks now if we have the original event.
+            if original_event:
+                if original_event.type == EventTypes.Create:
+                    raise AuthError(403, "Redacting create events is not permitted")
+
             prev_state_ids = yield context.get_prev_state_ids(self.store)
             auth_events_ids = yield self.auth.compute_auth_events(
                 event, prev_state_ids, for_verification=True
             )
             auth_events = yield self.store.get_events(auth_events_ids)
             auth_events = {(e.type, e.state_key): e for e in auth_events.values()}
             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,
-                    get_prev_content=False,
-                    allow_rejected=False,
-                    allow_none=False,
-                )
+
+            if event_auth.check_redaction(room_version, event, auth_events=auth_events):
+                # this user doesn't have 'redact' rights, so we need to do some more
+                # checks on the original event. Let's start by checking the original
+                # event exists.
+                if not original_event:
+                    raise NotFoundError("Could not find event %s" % (event.redacts,))
+
                 if event.user_id != original_event.user_id:
                     raise AuthError(403, "You don't have permission to redact events")
 
-                # We've already checked.
+                # all the checks are done.
                 event.internal_metadata.recheck_redaction = False
 
         if event.type == EventTypes.Create:

synapse/storage/events_worker.py

@@ -259,6 +259,14 @@ def get_events_as_list(
                     continue
 
                 original_event = original_event_entry.event
+                if original_event.type == EventTypes.Create:
+                    # we never serve redactions of Creates to clients.
+                    logger.info(
+                        "Withholding redaction %s of create event %s",
+                        event_id,
+                        redacted_event_id,
+                    )
+                    continue
 
                 if entry.event.internal_metadata.need_to_check_redaction():
                     original_domain = get_domain_from_id(original_event.sender)
@@ -617,6 +625,10 @@ def _maybe_redact_event_row(self, original_ev, redactions):
             Deferred[EventBase|None]: if the event should be redacted, a pruned
                 event object. Otherwise, None.
         """
+        if original_ev.type == "m.room.create":
+            # we choose to ignore redactions of m.room.create events.
+            return None
+
         redaction_map = yield self._get_events_from_cache_or_db(redactions)
 
         for redaction_id in redactions:

tests/rest/client/test_redactions.py

@@ -157,3 +157,23 @@ def test_redact_nonexistent_event(self):
         self.assertEqual(timeline[-2]["event_id"], msg_id)
         self.assertEqual(timeline[-2]["unsigned"]["redacted_by"], redaction_id)
         self.assertEqual(timeline[-2]["content"], {})
+
+    def test_redact_create_event(self):
+        # control case: an existing event
+        b = self.helper.send(room_id=self.room_id, tok=self.mod_access_token)
+        msg_id = b["event_id"]
+        self._redact_event(self.mod_access_token, self.room_id, msg_id)
+
+        # sync the room, to get the id of the create event
+        timeline = self._sync_room_timeline(self.other_access_token, self.room_id)
+        create_event_id = timeline[0]["event_id"]
+
+        # room moderators cannot send redactions for create events
+        self._redact_event(
+            self.mod_access_token, self.room_id, create_event_id, expect_code=403
+        )
+
+        # and nor can normals
+        self._redact_event(
+            self.other_access_token, self.room_id, create_event_id, expect_code=403
+        )