Correctly ratelimit invites when creating a room (#9968)

* Correctly ratelimit invites when creating a room

Also allow ratelimiting for more than one action at a time.

6 files changed, 157 insertions, 12 deletions

diff --git a/changelog.d/9968.bugfix b/changelog.d/9968.bugfix
new file mode 100644
index 0000000000..39e75f9956
--- /dev/null
+++ b/changelog.d/9968.bugfix
@@ -0,0 +1 @@
+Fix a bug introduced in v1.27.0 preventing users and appservices exempt from ratelimiting from creating rooms with many invitees.
diff --git a/synapse/api/ratelimiting.py b/synapse/api/ratelimiting.py
index 2244b8a340..b9a10283f4 100644
--- a/synapse/api/ratelimiting.py
+++ b/synapse/api/ratelimiting.py
@@ -57,6 +57,7 @@ class Ratelimiter:
         rate_hz: Optional[float] = None,
         burst_count: Optional[int] = None,
         update: bool = True,
+        n_actions: int = 1,
         _time_now_s: Optional[int] = None,
     ) -> Tuple[bool, float]:
         """Can the entity (e.g. user or IP address) perform the action?
@@ -76,6 +77,9 @@ class Ratelimiter:
             burst_count: How many actions that can be performed before being limited.
                 Overrides the value set during instantiation if set.
             update: Whether to count this check as performing the action
+            n_actions: The number of times the user wants to do this action. If the user
+                cannot do all of the actions, the user's action count is not incremented
+                at all.
             _time_now_s: The current time. Optional, defaults to the current time according
                 to self.clock. Only used by tests.
 
@@ -124,17 +128,20 @@ class Ratelimiter:
         time_delta = time_now_s - time_start
         performed_count = action_count - time_delta * rate_hz
         if performed_count < 0:
-            # Allow, reset back to count 1
-            allowed = True
+            performed_count = 0
             time_start = time_now_s
-            action_count = 1.0
-        elif performed_count > burst_count - 1.0:
+
+        # This check would be easier read as performed_count + n_actions > burst_count,
+        # but performed_count might be a very precise float (with lots of numbers
+        # following the point) in which case Python might round it up when adding it to
+        # n_actions. Writing it this way ensures it doesn't happen.
+        if performed_count > burst_count - n_actions:
             # Deny, we have exceeded our burst count
             allowed = False
         else:
             # We haven't reached our limit yet
             allowed = True
-            action_count += 1.0
+            action_count = performed_count + n_actions
 
         if update:
             self.actions[key] = (action_count, time_start, rate_hz)
@@ -182,6 +189,7 @@ class Ratelimiter:
         rate_hz: Optional[float] = None,
         burst_count: Optional[int] = None,
         update: bool = True,
+        n_actions: int = 1,
         _time_now_s: Optional[int] = None,
     ):
         """Checks if an action can be performed. If not, raises a LimitExceededError
@@ -201,6 +209,9 @@ class Ratelimiter:
             burst_count: How many actions that can be performed before being limited.
                 Overrides the value set during instantiation if set.
             update: Whether to count this check as performing the action
+            n_actions: The number of times the user wants to do this action. If the user
+                cannot do all of the actions, the user's action count is not incremented
+                at all.
             _time_now_s: The current time. Optional, defaults to the current time according
                 to self.clock. Only used by tests.
 
@@ -216,6 +227,7 @@ class Ratelimiter:
             rate_hz=rate_hz,
             burst_count=burst_count,
             update=update,
+            n_actions=n_actions,
             _time_now_s=time_now_s,
         )
 
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index fb4823a5cc..835d874cee 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -32,7 +32,14 @@ from synapse.api.constants import (
     RoomCreationPreset,
     RoomEncryptionAlgorithms,
 )
-from synapse.api.errors import AuthError, Codes, NotFoundError, StoreError, SynapseError
+from synapse.api.errors import (
+    AuthError,
+    Codes,
+    LimitExceededError,
+    NotFoundError,
+    StoreError,
+    SynapseError,
+)
 from synapse.api.filtering import Filter
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion
 from synapse.events import EventBase
@@ -126,10 +133,6 @@ class RoomCreationHandler(BaseHandler):
 
         self.third_party_event_rules = hs.get_third_party_event_rules()
 
-        self._invite_burst_count = (
-            hs.config.ratelimiting.rc_invites_per_room.burst_count
-        )
-
     async def upgrade_room(
         self, requester: Requester, old_room_id: str, new_version: RoomVersion
     ) -> str:
@@ -676,8 +679,18 @@ class RoomCreationHandler(BaseHandler):
             invite_3pid_list = []
             invite_list = []
 
-        if len(invite_list) + len(invite_3pid_list) > self._invite_burst_count:
-            raise SynapseError(400, "Cannot invite so many users at once")
+        if invite_list or invite_3pid_list:
+            try:
+                # If there are invites in the request, see if the ratelimiting settings
+                # allow that number of invites to be sent from the current user.
+                await self.room_member_handler.ratelimit_multiple_invites(
+                    requester,
+                    room_id=None,
+                    n_invites=len(invite_list) + len(invite_3pid_list),
+                    update=False,
+                )
+            except LimitExceededError:
+                raise SynapseError(400, "Cannot invite so many users at once")
 
         await self.event_creation_handler.assert_accepted_privacy_policy(requester)
 
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 20700fc5a8..9a092da715 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -163,6 +163,31 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
     async def forget(self, user: UserID, room_id: str) -> None:
         raise NotImplementedError()
 
+    async def ratelimit_multiple_invites(
+        self,
+        requester: Optional[Requester],
+        room_id: Optional[str],
+        n_invites: int,
+        update: bool = True,
+    ):
+        """Ratelimit more than one invite sent by the given requester in the given room.
+
+        Args:
+            requester: The requester sending the invites.
+            room_id: The room the invites are being sent in.
+            n_invites: The amount of invites to ratelimit for.
+            update: Whether to update the ratelimiter's cache.
+
+        Raises:
+            LimitExceededError: The requester can't send that many invites in the room.
+        """
+        await self._invites_per_room_limiter.ratelimit(
+            requester,
+            room_id,
+            update=update,
+            n_actions=n_invites,
+        )
+
     async def ratelimit_invite(
         self,
         requester: Optional[Requester],
diff --git a/tests/api/test_ratelimiting.py b/tests/api/test_ratelimiting.py
index fa96ba07a5..dcf0110c16 100644
--- a/tests/api/test_ratelimiting.py
+++ b/tests/api/test_ratelimiting.py
@@ -230,3 +230,60 @@ class TestRatelimiter(unittest.HomeserverTestCase):
         # Shouldn't raise
         for _ in range(20):
             self.get_success_or_raise(limiter.ratelimit(requester, _time_now_s=0))
+
+    def test_multiple_actions(self):
+        limiter = Ratelimiter(
+            store=self.hs.get_datastore(), clock=None, rate_hz=0.1, burst_count=3
+        )
+        # Test that 4 actions aren't allowed with a maximum burst of 3.
+        allowed, time_allowed = self.get_success_or_raise(
+            limiter.can_do_action(None, key="test_id", n_actions=4, _time_now_s=0)
+        )
+        self.assertFalse(allowed)
+
+        # Test that 3 actions are allowed with a maximum burst of 3.
+        allowed, time_allowed = self.get_success_or_raise(
+            limiter.can_do_action(None, key="test_id", n_actions=3, _time_now_s=0)
+        )
+        self.assertTrue(allowed)
+        self.assertEquals(10.0, time_allowed)
+
+        # Test that, after doing these 3 actions, we can't do any more action without
+        # waiting.
+        allowed, time_allowed = self.get_success_or_raise(
+            limiter.can_do_action(None, key="test_id", n_actions=1, _time_now_s=0)
+        )
+        self.assertFalse(allowed)
+        self.assertEquals(10.0, time_allowed)
+
+        # Test that after waiting we can do only 1 action.
+        allowed, time_allowed = self.get_success_or_raise(
+            limiter.can_do_action(
+                None,
+                key="test_id",
+                update=False,
+                n_actions=1,
+                _time_now_s=10,
+            )
+        )
+        self.assertTrue(allowed)
+        # The time allowed is the current time because we could still repeat the action
+        # once.
+        self.assertEquals(10.0, time_allowed)
+
+        allowed, time_allowed = self.get_success_or_raise(
+            limiter.can_do_action(None, key="test_id", n_actions=2, _time_now_s=10)
+        )
+        self.assertFalse(allowed)
+        # The time allowed doesn't change despite allowed being False because, while we
+        # don't allow 2 actions, we could still do 1.
+        self.assertEquals(10.0, time_allowed)
+
+        # Test that after waiting a bit more we can do 2 actions.
+        allowed, time_allowed = self.get_success_or_raise(
+            limiter.can_do_action(None, key="test_id", n_actions=2, _time_now_s=20)
+        )
+        self.assertTrue(allowed)
+        # The time allowed is the current time because we could still repeat the action
+        # once.
+        self.assertEquals(20.0, time_allowed)
diff --git a/tests/rest/client/v1/test_rooms.py b/tests/rest/client/v1/test_rooms.py
index a3694f3d02..7c4bdcdfdd 100644
--- a/tests/rest/client/v1/test_rooms.py
+++ b/tests/rest/client/v1/test_rooms.py
@@ -463,6 +463,43 @@ class RoomsCreateTestCase(RoomBase):
         )
         self.assertEquals(400, channel.code)
 
+    @unittest.override_config({"rc_invites": {"per_room": {"burst_count": 3}}})
+    def test_post_room_invitees_ratelimit(self):
+        """Test that invites sent when creating a room are ratelimited by a RateLimiter,
+        which ratelimits them correctly, including by not limiting when the requester is
+        exempt from ratelimiting.
+        """
+
+        # Build the request's content. We use local MXIDs because invites over federation
+        # are more difficult to mock.
+        content = json.dumps(
+            {
+                "invite": [
+                    "@alice1:red",
+                    "@alice2:red",
+                    "@alice3:red",
+                    "@alice4:red",
+                ]
+            }
+        ).encode("utf8")
+
+        # Test that the invites are correctly ratelimited.
+        channel = self.make_request("POST", "/createRoom", content)
+        self.assertEqual(400, channel.code)
+        self.assertEqual(
+            "Cannot invite so many users at once",
+            channel.json_body["error"],
+        )
+
+        # Add the current user to the ratelimit overrides, allowing them no ratelimiting.
+        self.get_success(
+            self.hs.get_datastore().set_ratelimit_for_user(self.user_id, 0, 0)
+        )
+
+        # Test that the invites aren't ratelimited anymore.
+        channel = self.make_request("POST", "/createRoom", content)
+        self.assertEqual(200, channel.code)
+
 
 class RoomTopicTestCase(RoomBase):
     """ Tests /rooms/$room_id/topic REST events. """