From 0f2ecb961e580d5d039360edf041720680f8ad8c Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Fri, 26 Jul 2019 06:36:48 +0000
Subject: [PATCH] Fix DoS when there is a cycle in redaction events

Make sure that synapse doesn't explode when a redaction redacts itself, or
there is a larger cycle.
---
 synapse/storage/events_worker.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/synapse/storage/events_worker.py b/synapse/storage/events_worker.py
index 7dbb5df09a..06379281b6 100644
--- a/synapse/storage/events_worker.py
+++ b/synapse/storage/events_worker.py
@@ -637,6 +637,10 @@ class EventsWorkerStore(SQLBaseStore):
             # we choose to ignore redactions of m.room.create events.
             return None
 
+        if original_ev.type == "m.room.redaction":
+            # ... and redaction events
+            return None
+
         redaction_map = yield self._get_events_from_cache_or_db(redactions)
 
         for redaction_id in redactions: