Allow use of different ratelimits for admin redactions.

changelog.d/6015.feature

@@ -0,0 +1 @@
+Add config option to increase ratelimits for room admins redacting messages.

docs/sample_config.yaml

@@ -518,6 +518,9 @@ log_config: "CONFDIR/SERVERNAME.log.config"
 #   - one for login that ratelimits login requests based on the account the
 #     client is attempting to log into, based on the amount of failed login
 #     attempts for this account.
+#   - one for ratelimiting redactions by room admins. If this is not explicitly
+#     set then it uses the same ratelimiting as per rc_message. This is useful
+#     to allow room admins to deal with abuse quickly.
 #
 # The defaults are as shown below.
 #
@@ -539,6 +542,10 @@ log_config: "CONFDIR/SERVERNAME.log.config"
 #  failed_attempts:
 #    per_second: 0.17
 #    burst_count: 3
+#
+#rc_admin_redaction:
+#  per_second: 1
+#  burst_count: 50
 
 
 # Ratelimiting settings for incoming federation

synapse/config/ratelimiting.py

@@ -80,6 +80,12 @@ def read_config(self, config, **kwargs):
             "federation_rr_transactions_per_room_per_second", 50
         )
 
+        rc_admin_redaction = config.get("rc_admin_redaction")
+        if rc_admin_redaction:
+            self.rc_admin_redaction = RateLimitConfig(rc_admin_redaction)
+        else:
+            self.rc_admin_redaction = None
+
     def generate_config_section(self, **kwargs):
         return """\
         ## Ratelimiting ##
@@ -102,6 +108,9 @@ def generate_config_section(self, **kwargs):
         #   - one for login that ratelimits login requests based on the account the
         #     client is attempting to log into, based on the amount of failed login
         #     attempts for this account.
+        #   - one for ratelimiting redactions by room admins. If this is not explicitly
+        #     set then it uses the same ratelimiting as per rc_message. This is useful
+        #     to allow room admins to deal with abuse quickly.
         #
         # The defaults are as shown below.
         #
@@ -123,6 +132,10 @@ def generate_config_section(self, **kwargs):
         #  failed_attempts:
         #    per_second: 0.17
         #    burst_count: 3
+        #
+        #rc_admin_redaction:
+        #  per_second: 1
+        #  burst_count: 50
 
 
         # Ratelimiting settings for incoming federation

synapse/handlers/_base.py

@@ -45,6 +45,7 @@ def __init__(self, hs):
         self.state_handler = hs.get_state_handler()
         self.distributor = hs.get_distributor()
         self.ratelimiter = hs.get_ratelimiter()
+        self.admin_redaction_ratelimiter = hs.get_admin_redaction_ratelimiter()
         self.clock = hs.get_clock()
         self.hs = hs
 
@@ -53,7 +54,7 @@ def __init__(self, hs):
         self.event_builder_factory = hs.get_event_builder_factory()
 
     @defer.inlineCallbacks
-    def ratelimit(self, requester, update=True):
+    def ratelimit(self, requester, update=True, is_admin_redaction=False):
         """Ratelimits requests.
 
         Args:
@@ -62,6 +63,9 @@ def ratelimit(self, requester, update=True):
                 Set to False when doing multiple checks for one request (e.g.
                 to check up front if we would reject the request), and set to
                 True for the last call for a given request.
+            is_admin_redaction (bool): Whether this is a room admin/moderator
+                redacting an event. If so then we may apply different
+                ratelimits depending on config.
 
         Raises:
             LimitExceededError if the request should be ratelimited
@@ -90,16 +94,33 @@ def ratelimit(self, requester, update=True):
             messages_per_second = override.messages_per_second
             burst_count = override.burst_count
         else:
-            messages_per_second = self.hs.config.rc_message.per_second
-            burst_count = self.hs.config.rc_message.burst_count
-
-        allowed, time_allowed = self.ratelimiter.can_do_action(
-            user_id,
-            time_now,
-            rate_hz=messages_per_second,
-            burst_count=burst_count,
-            update=update,
-        )
+            # We default to different values if this is an admin redaction and
+            # the config is set
+            if is_admin_redaction and self.hs.config.rc_admin_redaction:
+                messages_per_second = self.hs.config.rc_admin_redaction.per_second
+                burst_count = self.hs.config.rc_admin_redaction.burst_count
+            else:
+                messages_per_second = self.hs.config.rc_message.per_second
+                burst_count = self.hs.config.rc_message.burst_count
+
+        if is_admin_redaction and self.hs.config.rc_admin_redaction:
+            # If we have separate config for admin redactions we use a separate
+            # ratelimiter
+            allowed, time_allowed = self.admin_redaction_ratelimiter.can_do_action(
+                user_id,
+                time_now,
+                rate_hz=messages_per_second,
+                burst_count=burst_count,
+                update=update,
+            )
+        else:
+            allowed, time_allowed = self.ratelimiter.can_do_action(
+                user_id,
+                time_now,
+                rate_hz=messages_per_second,
+                burst_count=burst_count,
+                update=update,
+            )
         if not allowed:
             raise LimitExceededError(
                 retry_after_ms=int(1000 * (time_allowed - time_now))

synapse/handlers/message.py

@@ -729,7 +729,27 @@ def persist_and_notify_client_event(
         assert not self.config.worker_app
 
         if ratelimit:
-            yield self.base_handler.ratelimit(requester)
+            # We check if this is a room admin redacting an event so that we
+            # can apply different ratelimiting. We do this by simply checking
+            # it's not a self-redaction (to avoid having to look up whether the
+            # user is actually admin or not).
+            is_admin_redaction = False
+            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,
+                )
+
+                is_admin_redaction = (
+                    original_event and event.sender != original_event.sender
+                )
+
+            yield self.base_handler.ratelimit(
+                requester, is_admin_redaction=is_admin_redaction
+            )
 
         yield self.base_handler.maybe_kick_guest_users(event, context)
 

synapse/server.py

@@ -221,6 +221,7 @@ def __init__(self, hostname, reactor=None, **kwargs):
         self.clock = Clock(reactor)
         self.distributor = Distributor()
         self.ratelimiter = Ratelimiter()
+        self.admin_redaction_ratelimiter = Ratelimiter()
         self.registration_ratelimiter = Ratelimiter()
 
         self.datastore = None
@@ -279,6 +280,9 @@ def get_ratelimiter(self):
     def get_registration_ratelimiter(self):
         return self.registration_ratelimiter
 
+    def get_admin_redaction_ratelimiter(self):
+        return self.admin_redaction_ratelimiter
+
     def build_federation_client(self):
         return FederationClient(self)
 

tests/rest/client/test_redactions.py

@@ -30,6 +30,14 @@ class RedactionsTestCase(HomeserverTestCase):
         sync.register_servlets,
     ]
 
+    def make_homeserver(self, reactor, clock):
+        config = self.default_config()
+
+        config["rc_message"] = {"per_second": 0.2, "burst_count": 10}
+        config["rc_admin_redaction"] = {"per_second": 1, "burst_count": 100}
+
+        return self.setup_test_homeserver(config=config)
+
     def prepare(self, reactor, clock, hs):
         # register a couple of users
         self.mod_user_id = self.register_user("user1", "pass")
@@ -177,3 +185,20 @@ def test_redact_create_event(self):
         self._redact_event(
             self.other_access_token, self.room_id, create_event_id, expect_code=403
         )
+
+    def test_redact_event_as_moderator_ratelimit(self):
+        """Tests that the correct ratelimiting is applied to redactions
+        """
+
+        message_ids = []
+        # as a regular user, send messages to redact
+        for _ in range(20):
+            b = self.helper.send(room_id=self.room_id, tok=self.other_access_token)
+            message_ids.append(b["event_id"])
+            self.reactor.advance(10)  # To get around ratelimits
+
+        # as the moderator, send a bunch of redactions
+        for msg_id in message_ids:
+            # These should all succeed, even though this would be denied by
+            # the standard message ratelimiter
+            self._redact_event(self.mod_access_token, self.room_id, msg_id)