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

3 files changed, 36 insertions, 24 deletions

diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index d9e943c39c..7ce6540bdd 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -606,21 +606,6 @@ class Auth(object):
 
         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
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index eaeda7a5cb..6d7a987f13 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -23,6 +23,7 @@ from canonicaljson import encode_canonical_json, json
 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,
@@ -784,6 +785,20 @@ class EventCreationHandler(object):
                     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
@@ -791,18 +806,18 @@ class EventCreationHandler(object):
             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:
diff --git a/synapse/storage/events_worker.py b/synapse/storage/events_worker.py
index 1d969d70be..858fc755a1 100644
--- a/synapse/storage/events_worker.py
+++ b/synapse/storage/events_worker.py
@@ -259,6 +259,14 @@ class EventsWorkerStore(SQLBaseStore):
                     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 @@ class EventsWorkerStore(SQLBaseStore):
             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: