Add ratelimiting on joins

changelog.d/8008.feature

@@ -0,0 +1 @@
+Add rate limiting to users joining rooms.

docs/sample_config.yaml

@@ -746,6 +746,10 @@ log_config: "CONFDIR/SERVERNAME.log.config"
 #   - 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.
+#   - two for ratelimiting number of rooms a user can join, "local" for when
+#     users are joining rooms the server is already in (this is cheap) vs
+#     "remote" for when users are trying to join rooms not on the server (which
+#     can be more expensive)
 #
 # The defaults are as shown below.
 #
@@ -771,6 +775,14 @@ log_config: "CONFDIR/SERVERNAME.log.config"
 #rc_admin_redaction:
 #  per_second: 1
 #  burst_count: 50
+#
+#rc_joins:
+#  local:
+#    per_second: 0.1
+#    burst_count: 3
+#  remote:
+#    per_second: 0.01
+#    burst_count: 3
 
 
 # Ratelimiting settings for incoming federation

synapse/config/ratelimiting.py

@@ -93,6 +93,15 @@ def read_config(self, config, **kwargs):
         if rc_admin_redaction:
             self.rc_admin_redaction = RateLimitConfig(rc_admin_redaction)
 
+        self.rc_joins_local = RateLimitConfig(
+            config.get("rc_joins", {}).get("local", {}),
+            defaults={"per_second": 0.1, "burst_count": 3},
+        )
+        self.rc_joins_remote = RateLimitConfig(
+            config.get("rc_joins", {}).get("remote", {}),
+            defaults={"per_second": 0.01, "burst_count": 3},
+        )
+
     def generate_config_section(self, **kwargs):
         return """\
         ## Ratelimiting ##
@@ -118,6 +127,10 @@ def generate_config_section(self, **kwargs):
         #   - 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.
+        #   - two for ratelimiting number of rooms a user can join, "local" for when
+        #     users are joining rooms the server is already in (this is cheap) vs
+        #     "remote" for when users are trying to join rooms not on the server (which
+        #     can be more expensive)
         #
         # The defaults are as shown below.
         #
@@ -143,6 +156,14 @@ def generate_config_section(self, **kwargs):
         #rc_admin_redaction:
         #  per_second: 1
         #  burst_count: 50
+        #
+        #rc_joins:
+        #  local:
+        #    per_second: 0.1
+        #    burst_count: 3
+        #  remote:
+        #    per_second: 0.01
+        #    burst_count: 3
 
 
         # Ratelimiting settings for incoming federation

synapse/handlers/room_member.py

@@ -22,7 +22,8 @@
 
 from synapse import types
 from synapse.api.constants import MAX_DEPTH, EventTypes, Membership
-from synapse.api.errors import AuthError, Codes, SynapseError
+from synapse.api.errors import AuthError, Codes, LimitExceededError, SynapseError
+from synapse.api.ratelimiting import Ratelimiter
 from synapse.api.room_versions import EventFormatVersions
 from synapse.crypto.event_signing import compute_event_reference_hash
 from synapse.events import EventBase
@@ -77,6 +78,17 @@ def __init__(self, hs):
         if self._is_on_event_persistence_instance:
             self.persist_event_storage = hs.get_storage().persistence
 
+        self._join_rate_limiter_local = Ratelimiter(
+            clock=self.clock,
+            rate_hz=hs.config.ratelimiting.rc_joins_local.per_second,
+            burst_count=hs.config.ratelimiting.rc_joins_local.burst_count,
+        )
+        self._join_rate_limiter_remote = Ratelimiter(
+            clock=self.clock,
+            rate_hz=hs.config.ratelimiting.rc_joins_remote.per_second,
+            burst_count=hs.config.ratelimiting.rc_joins_remote.burst_count,
+        )
+
         # This is only used to get at ratelimit function, and
         # maybe_kick_guest_users. It's fine there are multiple of these as
         # it doesn't store state.
@@ -441,7 +453,28 @@ async def _update_membership(
                     # so don't really fit into the general auth process.
                     raise AuthError(403, "Guest access not allowed")
 
-            if not is_host_in_room:
+            if is_host_in_room:
+                time_now_s = self.clock.time()
+                allowed, time_allowed = self._join_rate_limiter_local.can_do_action(
+                    requester.user.to_string(),
+                )
+
+                if not allowed:
+                    raise LimitExceededError(
+                        retry_after_ms=int(1000 * (time_allowed - time_now_s))
+                    )
+
+            else:
+                time_now_s = self.clock.time()
+                allowed, time_allowed = self._join_rate_limiter_remote.can_do_action(
+                    requester.user.to_string(),
+                )
+
+                if not allowed:
+                    raise LimitExceededError(
+                        retry_after_ms=int(1000 * (time_allowed - time_now_s))
+                    )
+
                 inviter = await self._get_inviter(target.to_string(), room_id)
                 if inviter and not self.hs.is_mine(inviter):
                     remote_room_hosts.append(inviter.domain)

tests/utils.py

@@ -154,6 +154,10 @@ def default_config(name, parse=False):
             "account": {"per_second": 10000, "burst_count": 10000},
             "failed_attempts": {"per_second": 10000, "burst_count": 10000},
         },
+        "rc_joins": {
+            "local": {"per_second": 10000, "burst_count": 10000},
+            "remote": {"per_second": 10000, "burst_count": 10000},
+        },
         "saml2_enabled": False,
         "public_baseurl": None,
         "default_identity_server": None,