From 27c1abc7b876ef3e73c8b418a89b987f86147829 Mon Sep 17 00:00:00 2001
From: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>
Date: Fri, 18 Sep 2020 14:51:11 +0100
Subject: [PATCH] Use _check_sigs_and_hash_and_fetch to validate backfill
 requests (#8350)

This is a bit of a hack, as `_check_sigs_and_hash_and_fetch` is intended
for attempting to pull an event from the database/(re)pull it from the
server that originally sent the event if checking the signature of the
event fails.

During backfill we *know* that we won't have the event in our database,
however it is still useful to be able to query the original sending
server as the server we're backfilling from may be acting maliciously.

The main benefit and reason for this change however is that
`_check_sigs_and_hash_and_fetch` will drop an event during backfill if
it cannot be successfully validated, whereas the current code will
simply fail the backfill request - resulting in the client's /messages
request silently being dropped.

This is a quick patch to fix backfilling rooms that contain malformed
events. A better implementation in planned in future.
---
 changelog.d/8350.bugfix                 | 1 +
 synapse/federation/federation_client.py | 8 +++-----
 2 files changed, 4 insertions(+), 5 deletions(-)
 create mode 100644 changelog.d/8350.bugfix

diff --git a/changelog.d/8350.bugfix b/changelog.d/8350.bugfix
new file mode 100644
index 000000000000..0e493c028214
--- /dev/null
+++ b/changelog.d/8350.bugfix
@@ -0,0 +1 @@
+Partially mitigate bug where newly joined servers couldn't get past events in a room when there is a malformed event.
\ No newline at end of file
diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py
index a2e8d96ea27c..d42930d1b94b 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -217,11 +217,9 @@ async def backfill(
             for p in transaction_data["pdus"]
         ]
 
-        # FIXME: We should handle signature failures more gracefully.
-        pdus[:] = await make_deferred_yieldable(
-            defer.gatherResults(
-                self._check_sigs_and_hashes(room_version, pdus), consumeErrors=True,
-            ).addErrback(unwrapFirstError)
+        # Check signatures and hash of pdus, removing any from the list that fail checks
+        pdus[:] = await self._check_sigs_and_hash_and_fetch(
+            dest, pdus, outlier=True, room_version=room_version
         )
 
         return pdus