Option to allow server admins to join complex rooms (#7902)

Fixes #7901.

Signed-off-by: Niklas Tittjung <nik_t.01@web.de>

5 files changed, 127 insertions, 2 deletions

diff --git a/changelog.d/7902.feature b/changelog.d/7902.feature
new file mode 100644
index 0000000000..4feae8cc29
--- /dev/null
+++ b/changelog.d/7902.feature
@@ -0,0 +1 @@
+Add option to allow server admins to join rooms which fail complexity checks. Contributed by @lugino-emeritus.
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index 3227294e0b..09a7299871 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -314,6 +314,10 @@ limit_remote_rooms:
   #
   #complexity_error: "This room is too complex."
 
+  # allow server admins to join complex rooms. Default is false.
+  #
+  #admins_can_join: true
+
 # Whether to require a user to be in the room to add an alias to it.
 # Defaults to 'true'.
 #
diff --git a/synapse/config/server.py b/synapse/config/server.py
index 3747a01ca7..848587d232 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -439,6 +439,9 @@ class ServerConfig(Config):
                 validator=attr.validators.instance_of(str),
                 default=ROOM_COMPLEXITY_TOO_GREAT,
             )
+            admins_can_join = attr.ib(
+                validator=attr.validators.instance_of(bool), default=False
+            )
 
         self.limit_remote_rooms = LimitRemoteRoomsConfig(
             **(config.get("limit_remote_rooms") or {})
@@ -893,6 +896,10 @@ class ServerConfig(Config):
           #
           #complexity_error: "This room is too complex."
 
+          # allow server admins to join complex rooms. Default is false.
+          #
+          #admins_can_join: true
+
         # Whether to require a user to be in the room to add an alias to it.
         # Defaults to 'true'.
         #
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index a1a8fa1d3b..5a40e8c144 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -952,7 +952,11 @@ class RoomMemberMasterHandler(RoomMemberHandler):
         if len(remote_room_hosts) == 0:
             raise SynapseError(404, "No known servers")
 
-        if self.hs.config.limit_remote_rooms.enabled:
+        check_complexity = self.hs.config.limit_remote_rooms.enabled
+        if check_complexity and self.hs.config.limit_remote_rooms.admins_can_join:
+            check_complexity = not await self.hs.auth.is_server_admin(user)
+
+        if check_complexity:
             # Fetch the room complexity
             too_complex = await self._is_remote_room_too_complex(
                 room_id, remote_room_hosts
@@ -975,7 +979,7 @@ class RoomMemberMasterHandler(RoomMemberHandler):
 
         # Check the room we just joined wasn't too large, if we didn't fetch the
         # complexity of it before.
-        if self.hs.config.limit_remote_rooms.enabled:
+        if check_complexity:
             if too_complex is False:
                 # We checked, and we're under the limit.
                 return event_id, stream_id
diff --git a/tests/federation/test_complexity.py b/tests/federation/test_complexity.py
index 0c9987be54..5cd0510f0d 100644
--- a/tests/federation/test_complexity.py
+++ b/tests/federation/test_complexity.py
@@ -99,6 +99,37 @@ class RoomComplexityTests(unittest.FederatingHomeserverTestCase):
         self.assertEqual(f.value.code, 400, f.value)
         self.assertEqual(f.value.errcode, Codes.RESOURCE_LIMIT_EXCEEDED)
 
+    def test_join_too_large_admin(self):
+        # Check whether an admin can join if option "admins_can_join" is undefined,
+        # this option defaults to false, so the join should fail.
+
+        u1 = self.register_user("u1", "pass", admin=True)
+
+        handler = self.hs.get_room_member_handler()
+        fed_transport = self.hs.get_federation_transport_client()
+
+        # Mock out some things, because we don't want to test the whole join
+        fed_transport.client.get_json = Mock(return_value=defer.succeed({"v1": 9999}))
+        handler.federation_handler.do_invite_join = Mock(
+            return_value=defer.succeed(("", 1))
+        )
+
+        d = handler._remote_join(
+            None,
+            ["other.example.com"],
+            "roomid",
+            UserID.from_string(u1),
+            {"membership": "join"},
+        )
+
+        self.pump()
+
+        # The request failed with a SynapseError saying the resource limit was
+        # exceeded.
+        f = self.get_failure(d, SynapseError)
+        self.assertEqual(f.value.code, 400, f.value)
+        self.assertEqual(f.value.errcode, Codes.RESOURCE_LIMIT_EXCEEDED)
+
     def test_join_too_large_once_joined(self):
 
         u1 = self.register_user("u1", "pass")
@@ -141,3 +172,81 @@ class RoomComplexityTests(unittest.FederatingHomeserverTestCase):
         f = self.get_failure(d, SynapseError)
         self.assertEqual(f.value.code, 400)
         self.assertEqual(f.value.errcode, Codes.RESOURCE_LIMIT_EXCEEDED)
+
+
+class RoomComplexityAdminTests(unittest.FederatingHomeserverTestCase):
+    # Test the behavior of joining rooms which exceed the complexity if option
+    # limit_remote_rooms.admins_can_join is True.
+
+    servlets = [
+        admin.register_servlets,
+        room.register_servlets,
+        login.register_servlets,
+    ]
+
+    def default_config(self):
+        config = super().default_config()
+        config["limit_remote_rooms"] = {
+            "enabled": True,
+            "complexity": 0.05,
+            "admins_can_join": True,
+        }
+        return config
+
+    def test_join_too_large_no_admin(self):
+        # A user which is not an admin should not be able to join a remote room
+        # which is too complex.
+
+        u1 = self.register_user("u1", "pass")
+
+        handler = self.hs.get_room_member_handler()
+        fed_transport = self.hs.get_federation_transport_client()
+
+        # Mock out some things, because we don't want to test the whole join
+        fed_transport.client.get_json = Mock(return_value=defer.succeed({"v1": 9999}))
+        handler.federation_handler.do_invite_join = Mock(
+            return_value=defer.succeed(("", 1))
+        )
+
+        d = handler._remote_join(
+            None,
+            ["other.example.com"],
+            "roomid",
+            UserID.from_string(u1),
+            {"membership": "join"},
+        )
+
+        self.pump()
+
+        # The request failed with a SynapseError saying the resource limit was
+        # exceeded.
+        f = self.get_failure(d, SynapseError)
+        self.assertEqual(f.value.code, 400, f.value)
+        self.assertEqual(f.value.errcode, Codes.RESOURCE_LIMIT_EXCEEDED)
+
+    def test_join_too_large_admin(self):
+        # An admin should be able to join rooms where a complexity check fails.
+
+        u1 = self.register_user("u1", "pass", admin=True)
+
+        handler = self.hs.get_room_member_handler()
+        fed_transport = self.hs.get_federation_transport_client()
+
+        # Mock out some things, because we don't want to test the whole join
+        fed_transport.client.get_json = Mock(return_value=defer.succeed({"v1": 9999}))
+        handler.federation_handler.do_invite_join = Mock(
+            return_value=defer.succeed(("", 1))
+        )
+
+        d = handler._remote_join(
+            None,
+            ["other.example.com"],
+            "roomid",
+            UserID.from_string(u1),
+            {"membership": "join"},
+        )
+
+        self.pump()
+
+        # The request success since the user is an admin
+        self.get_success(d)