Ratelimit set_presence updates

changelog.d/18000.bugfix

@@ -0,0 +1 @@
+Add ratelimit `rc_set_presence.per_user` to prevent load from excessive presence updates sent by clients. Contributed by @rda0.

docs/usage/configuration/config_documentation.md

@@ -1866,6 +1866,22 @@ rc_federation:
   concurrent: 5
 ```
 ---
+### `rc_set_presence`
+
+This option sets ratelimiting for presence.
+
+The `rc_set_presence.per_user` sets ratelimiting how often a specific users' presence
+updates are evaluated. Ratelimited presence updates are ignored.
+`per_user` defaults to `per_second: 0.1`, `burst_count: 1`.
+
+Example configuration:
+```yaml
+rc_set_presence:
+  per_user:
+    per_second: 0.1
+    burst_count: 1
+```
+---
 ### `federation_rr_transactions_per_room_per_second`
 
 Sets outgoing federation transaction frequency for sending read-receipts,

synapse/config/ratelimiting.py

@@ -228,3 +228,9 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
                 config.get("remote_media_download_burst_count", "500M")
             ),
         )
+
+        self.rc_set_presence_per_user = RatelimitSettings.parse(
+            config,
+            "rc_set_presence.per_user",
+            defaults={"per_second": 0.1, "burst_count": 1},
+        )

synapse/rest/client/sync.py

@@ -24,9 +24,10 @@
 from typing import TYPE_CHECKING, Any, Dict, List, Mapping, Optional, Tuple, Union
 
 from synapse.api.constants import AccountDataTypes, EduTypes, Membership, PresenceState
-from synapse.api.errors import Codes, StoreError, SynapseError
+from synapse.api.errors import Codes, LimitExceededError, StoreError, SynapseError
 from synapse.api.filtering import FilterCollection
 from synapse.api.presence import UserPresenceState
+from synapse.api.ratelimiting import Ratelimiter
 from synapse.events.utils import (
     SerializeEventConfig,
     format_event_for_client_v2_without_room_id,
@@ -126,6 +127,13 @@ def __init__(self, hs: "HomeServer"):
             cache_name="sync_valid_filter",
         )
 
+        # Ratelimiter for set_presence updates, keyed by requester.
+        self._set_presence_per_user_limiter = Ratelimiter(
+            store=self.store,
+            clock=self.clock,
+            cfg=hs.config.ratelimiting.rc_set_presence_per_user,
+        )
+
     async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
         # This will always be set by the time Twisted calls us.
         assert request.args is not None
@@ -239,7 +247,14 @@ async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
         # send any outstanding server notices to the user.
         await self._server_notices_sender.on_user_syncing(user.to_string())
 
-        affect_presence = set_presence != PresenceState.OFFLINE
+        # ignore the presence update if the ratelimit is exceeded
+        try:
+            await self._set_presence_per_user_limiter.ratelimit(requester)
+        except LimitExceededError:
+            affect_presence = False
+            logger.debug("User set_presence ratelimit exceeded; ignoring it.")
+        else:
+            affect_presence = set_presence != PresenceState.OFFLINE
 
         context = await self.presence_handler.user_syncing(
             user.to_string(),