Ratelimit cross-user key sharing requests. (#8957)

9 files changed, 67 insertions, 17 deletions

diff --git a/changelog.d/8957.feature b/changelog.d/8957.feature
new file mode 100644
index 0000000000..fa8961f840
--- /dev/null
+++ b/changelog.d/8957.feature
@@ -0,0 +1 @@
+Add rate limiters to cross-user key sharing requests.
diff --git a/synapse/api/constants.py b/synapse/api/constants.py
index af8d59cf87..691f8f9adf 100644
--- a/synapse/api/constants.py
+++ b/synapse/api/constants.py
@@ -98,11 +98,14 @@ class EventTypes:
 
     Retention = "m.room.retention"
 
-    Presence = "m.presence"
-
     Dummy = "org.matrix.dummy_event"
 
 
+class EduTypes:
+    Presence = "m.presence"
+    RoomKeyRequest = "m.room_key_request"
+
+
 class RejectedReason:
     AUTH_ERROR = "auth_error"
 
diff --git a/synapse/api/ratelimiting.py b/synapse/api/ratelimiting.py
index 5d9d5a228f..c3f07bc1a3 100644
--- a/synapse/api/ratelimiting.py
+++ b/synapse/api/ratelimiting.py
@@ -14,7 +14,7 @@
 # limitations under the License.
 
 from collections import OrderedDict
-from typing import Any, Optional, Tuple
+from typing import Hashable, Optional, Tuple
 
 from synapse.api.errors import LimitExceededError
 from synapse.types import Requester
@@ -42,7 +42,9 @@ class Ratelimiter:
         #   * How many times an action has occurred since a point in time
         #   * The point in time
         #   * The rate_hz of this particular entry. This can vary per request
-        self.actions = OrderedDict()  # type: OrderedDict[Any, Tuple[float, int, float]]
+        self.actions = (
+            OrderedDict()
+        )  # type: OrderedDict[Hashable, Tuple[float, int, float]]
 
     def can_requester_do_action(
         self,
@@ -82,7 +84,7 @@ class Ratelimiter:
 
     def can_do_action(
         self,
-        key: Any,
+        key: Hashable,
         rate_hz: Optional[float] = None,
         burst_count: Optional[int] = None,
         update: bool = True,
@@ -175,7 +177,7 @@ class Ratelimiter:
 
     def ratelimit(
         self,
-        key: Any,
+        key: Hashable,
         rate_hz: Optional[float] = None,
         burst_count: Optional[int] = None,
         update: bool = True,
diff --git a/synapse/config/ratelimiting.py b/synapse/config/ratelimiting.py
index def33a60ad..847d25122c 100644
--- a/synapse/config/ratelimiting.py
+++ b/synapse/config/ratelimiting.py
@@ -102,6 +102,16 @@ class RatelimitConfig(Config):
             defaults={"per_second": 0.01, "burst_count": 3},
         )
 
+        # Ratelimit cross-user key requests:
+        # * For local requests this is keyed by the sending device.
+        # * For requests received over federation this is keyed by the origin.
+        #
+        # Note that this isn't exposed in the configuration as it is obscure.
+        self.rc_key_requests = RateLimitConfig(
+            config.get("rc_key_requests", {}),
+            defaults={"per_second": 20, "burst_count": 100},
+        )
+
         self.rc_3pid_validation = RateLimitConfig(
             config.get("rc_3pid_validation") or {},
             defaults={"per_second": 0.003, "burst_count": 5},
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index 8d4bb621e7..2f832b47f6 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -34,7 +34,7 @@ from twisted.internet import defer
 from twisted.internet.abstract import isIPAddress
 from twisted.python import failure
 
-from synapse.api.constants import EventTypes, Membership
+from synapse.api.constants import EduTypes, EventTypes, Membership
 from synapse.api.errors import (
     AuthError,
     Codes,
@@ -44,6 +44,7 @@ from synapse.api.errors import (
     SynapseError,
     UnsupportedRoomVersionError,
 )
+from synapse.api.ratelimiting import Ratelimiter
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
 from synapse.events import EventBase
 from synapse.federation.federation_base import FederationBase, event_from_pdu_json
@@ -869,6 +870,13 @@ class FederationHandlerRegistry:
         # EDU received.
         self._edu_type_to_instance = {}  # type: Dict[str, List[str]]
 
+        # A rate limiter for incoming room key requests per origin.
+        self._room_key_request_rate_limiter = Ratelimiter(
+            clock=self.clock,
+            rate_hz=self.config.rc_key_requests.per_second,
+            burst_count=self.config.rc_key_requests.burst_count,
+        )
+
     def register_edu_handler(
         self, edu_type: str, handler: Callable[[str, JsonDict], Awaitable[None]]
     ):
@@ -917,7 +925,15 @@ class FederationHandlerRegistry:
         self._edu_type_to_instance[edu_type] = instance_names
 
     async def on_edu(self, edu_type: str, origin: str, content: dict):
-        if not self.config.use_presence and edu_type == "m.presence":
+        if not self.config.use_presence and edu_type == EduTypes.Presence:
+            return
+
+        # If the incoming room key requests from a particular origin are over
+        # the limit, drop them.
+        if (
+            edu_type == EduTypes.RoomKeyRequest
+            and not self._room_key_request_rate_limiter.can_do_action(origin)
+        ):
             return
 
         # Check if we have a handler on this instance
diff --git a/synapse/handlers/devicemessage.py b/synapse/handlers/devicemessage.py
index 1aa7d803b5..7db4f48965 100644
--- a/synapse/handlers/devicemessage.py
+++ b/synapse/handlers/devicemessage.py
@@ -16,7 +16,9 @@
 import logging
 from typing import TYPE_CHECKING, Any, Dict
 
+from synapse.api.constants import EduTypes
 from synapse.api.errors import SynapseError
+from synapse.api.ratelimiting import Ratelimiter
 from synapse.logging.context import run_in_background
 from synapse.logging.opentracing import (
     get_active_span_text_map,
@@ -25,7 +27,7 @@ from synapse.logging.opentracing import (
     start_active_span,
 )
 from synapse.replication.http.devices import ReplicationUserDevicesResyncRestServlet
-from synapse.types import JsonDict, UserID, get_domain_from_id
+from synapse.types import JsonDict, Requester, UserID, get_domain_from_id
 from synapse.util import json_encoder
 from synapse.util.stringutils import random_string
 
@@ -78,6 +80,12 @@ class DeviceMessageHandler:
                 ReplicationUserDevicesResyncRestServlet.make_client(hs)
             )
 
+        self._ratelimiter = Ratelimiter(
+            clock=hs.get_clock(),
+            rate_hz=hs.config.rc_key_requests.per_second,
+            burst_count=hs.config.rc_key_requests.burst_count,
+        )
+
     async def on_direct_to_device_edu(self, origin: str, content: JsonDict) -> None:
         local_messages = {}
         sender_user_id = content["sender"]
@@ -168,15 +176,27 @@ class DeviceMessageHandler:
 
     async def send_device_message(
         self,
-        sender_user_id: str,
+        requester: Requester,
         message_type: str,
         messages: Dict[str, Dict[str, JsonDict]],
     ) -> None:
+        sender_user_id = requester.user.to_string()
+
         set_tag("number_of_messages", len(messages))
         set_tag("sender", sender_user_id)
         local_messages = {}
         remote_messages = {}  # type: Dict[str, Dict[str, Dict[str, JsonDict]]]
         for user_id, by_device in messages.items():
+            # Ratelimit local cross-user key requests by the sending device.
+            if (
+                message_type == EduTypes.RoomKeyRequest
+                and user_id != sender_user_id
+                and self._ratelimiter.can_do_action(
+                    (sender_user_id, requester.device_id)
+                )
+            ):
+                continue
+
             # we use UserID.from_string to catch invalid user ids
             if self.is_mine(UserID.from_string(user_id)):
                 messages_by_device = {
diff --git a/synapse/handlers/events.py b/synapse/handlers/events.py
index 3e23f82cf7..f46cab7325 100644
--- a/synapse/handlers/events.py
+++ b/synapse/handlers/events.py
@@ -17,7 +17,7 @@ import logging
 import random
 from typing import TYPE_CHECKING, Iterable, List, Optional
 
-from synapse.api.constants import EventTypes, Membership
+from synapse.api.constants import EduTypes, EventTypes, Membership
 from synapse.api.errors import AuthError, SynapseError
 from synapse.events import EventBase
 from synapse.handlers.presence import format_user_presence_state
@@ -113,7 +113,7 @@ class EventStreamHandler(BaseHandler):
                     states = await presence_handler.get_states(users)
                     to_add.extend(
                         {
-                            "type": EventTypes.Presence,
+                            "type": EduTypes.Presence,
                             "content": format_user_presence_state(state, time_now),
                         }
                         for state in states
diff --git a/synapse/handlers/initial_sync.py b/synapse/handlers/initial_sync.py
index 78c3e5a10b..71a5076672 100644
--- a/synapse/handlers/initial_sync.py
+++ b/synapse/handlers/initial_sync.py
@@ -18,7 +18,7 @@ from typing import TYPE_CHECKING, Optional, Tuple
 
 from twisted.internet import defer
 
-from synapse.api.constants import EventTypes, Membership
+from synapse.api.constants import EduTypes, EventTypes, Membership
 from synapse.api.errors import SynapseError
 from synapse.events.validator import EventValidator
 from synapse.handlers.presence import format_user_presence_state
@@ -412,7 +412,7 @@ class InitialSyncHandler(BaseHandler):
 
             return [
                 {
-                    "type": EventTypes.Presence,
+                    "type": EduTypes.Presence,
                     "content": format_user_presence_state(s, time_now),
                 }
                 for s in states
diff --git a/synapse/rest/client/v2_alpha/sendtodevice.py b/synapse/rest/client/v2_alpha/sendtodevice.py
index a3dee14ed4..79c1b526ee 100644
--- a/synapse/rest/client/v2_alpha/sendtodevice.py
+++ b/synapse/rest/client/v2_alpha/sendtodevice.py
@@ -56,10 +56,8 @@ class SendToDeviceRestServlet(servlet.RestServlet):
         content = parse_json_object_from_request(request)
         assert_params_in_dict(content, ("messages",))
 
-        sender_user_id = requester.user.to_string()
-
         await self.device_message_handler.send_device_message(
-            sender_user_id, message_type, content["messages"]
+            requester, message_type, content["messages"]
         )
 
         response = (200, {})  # type: Tuple[int, dict]