add SpamChecker callback for silently dropping inbound federated events (#12744)

Signed-off-by: jesopo <github@lolnerd.net>

1 files changed, 40 insertions, 0 deletions

diff --git a/synapse/events/spamcheck.py b/synapse/events/spamcheck.py
index f30207376a..61bcbe2abe 100644
--- a/synapse/events/spamcheck.py
+++ b/synapse/events/spamcheck.py
@@ -44,6 +44,10 @@ CHECK_EVENT_FOR_SPAM_CALLBACK = Callable[
     ["synapse.events.EventBase"],
     Awaitable[Union[bool, str]],
 ]
+SHOULD_DROP_FEDERATED_EVENT_CALLBACK = Callable[
+    ["synapse.events.EventBase"],
+    Awaitable[Union[bool, str]],
+]
 USER_MAY_JOIN_ROOM_CALLBACK = Callable[[str, str, bool], Awaitable[bool]]
 USER_MAY_INVITE_CALLBACK = Callable[[str, str, str], Awaitable[bool]]
 USER_MAY_SEND_3PID_INVITE_CALLBACK = Callable[[str, str, str, str], Awaitable[bool]]
@@ -168,6 +172,9 @@ class SpamChecker:
         self.clock = hs.get_clock()
 
         self._check_event_for_spam_callbacks: List[CHECK_EVENT_FOR_SPAM_CALLBACK] = []
+        self._should_drop_federated_event_callbacks: List[
+            SHOULD_DROP_FEDERATED_EVENT_CALLBACK
+        ] = []
         self._user_may_join_room_callbacks: List[USER_MAY_JOIN_ROOM_CALLBACK] = []
         self._user_may_invite_callbacks: List[USER_MAY_INVITE_CALLBACK] = []
         self._user_may_send_3pid_invite_callbacks: List[
@@ -191,6 +198,9 @@ class SpamChecker:
     def register_callbacks(
         self,
         check_event_for_spam: Optional[CHECK_EVENT_FOR_SPAM_CALLBACK] = None,
+        should_drop_federated_event: Optional[
+            SHOULD_DROP_FEDERATED_EVENT_CALLBACK
+        ] = None,
         user_may_join_room: Optional[USER_MAY_JOIN_ROOM_CALLBACK] = None,
         user_may_invite: Optional[USER_MAY_INVITE_CALLBACK] = None,
         user_may_send_3pid_invite: Optional[USER_MAY_SEND_3PID_INVITE_CALLBACK] = None,
@@ -209,6 +219,11 @@ class SpamChecker:
         if check_event_for_spam is not None:
             self._check_event_for_spam_callbacks.append(check_event_for_spam)
 
+        if should_drop_federated_event is not None:
+            self._should_drop_federated_event_callbacks.append(
+                should_drop_federated_event
+            )
+
         if user_may_join_room is not None:
             self._user_may_join_room_callbacks.append(user_may_join_room)
 
@@ -268,6 +283,31 @@ class SpamChecker:
 
         return False
 
+    async def should_drop_federated_event(
+        self, event: "synapse.events.EventBase"
+    ) -> Union[bool, str]:
+        """Checks if a given federated event is considered "spammy" by this
+        server.
+
+        If the server considers an event spammy, it will be silently dropped,
+        and in doing so will split-brain our view of the room's DAG.
+
+        Args:
+            event: the event to be checked
+
+        Returns:
+            True if the event should be silently dropped
+        """
+        for callback in self._should_drop_federated_event_callbacks:
+            with Measure(
+                self.clock, "{}.{}".format(callback.__module__, callback.__qualname__)
+            ):
+                res: Union[bool, str] = await delay_cancellation(callback(event))
+            if res:
+                return res
+
+        return False
+
     async def user_may_join_room(
         self, user_id: str, room_id: str, is_invited: bool
     ) -> bool: