Check required power levels earlier in createRoom handler. (#15695)

* Check required power levels earlier in createRoom handler.

- If a server was configured to reject the creation of rooms with E2EE
  enabled (by specifying an unattainably high power level for
  "m.room.encryption" in default_power_level_content_override), the 403
  error was not being triggered until after the room was created and
  before the "m.room.power_levels" was sent.  This allowed a user to
  access the partially-configured room and complete the setup of E2EE
  and power levels manually.

- This change causes the power level overrides to be checked earlier and
  the request to be rejected before the user gains access to the room.

- A new `_validate_room_config` method is added to contain checks that
  should be run before a room is created.

- The new test case confirms that a user request is rejected by the new
  validation method.

Signed-off-by: Grant McLean <grant@catalyst.net.nz>

* Add a changelog file.

* Formatting fix for black.

* Remove unneeded line from test.

---------

Signed-off-by: Grant McLean <grant@catalyst.net.nz>

1 files changed, 37 insertions, 0 deletions

diff --git a/tests/rest/client/test_rooms.py b/tests/rest/client/test_rooms.py

index 4d39c89f6f..f1b4e1ad2f 100644
--- a/tests/rest/client/test_rooms.py
+++ b/tests/rest/client/test_rooms.py
@@ -1941,6 +1941,43 @@ class RoomPowerLevelOverridesInPracticeTestCase(RoomBase):
             channel.json_body["error"],
         )
 
+    @unittest.override_config(
+        {
+            "default_power_level_content_override": {
+                "private_chat": {
+                    "events": {
+                        "m.room.avatar": 50,
+                        "m.room.canonical_alias": 50,
+                        "m.room.encryption": 999,
+                        "m.room.history_visibility": 100,
+                        "m.room.name": 50,
+                        "m.room.power_levels": 100,
+                        "m.room.server_acl": 100,
+                        "m.room.tombstone": 100,
+                    },
+                    "events_default": 0,
+                },
+            }
+        },
+    )
+    def test_config_override_blocks_encrypted_room(self) -> None:
+        # Given the server has config for private_chats,
+
+        # When I attempt to create an encrypted private_chat room
+        channel = self.make_request(
+            "POST",
+            "/createRoom",
+            '{"creation_content": {"m.federate": false},"name": "Secret Private Room","preset": "private_chat","initial_state": [{"type": "m.room.encryption","state_key": "","content": {"algorithm": "m.megolm.v1.aes-sha2"}}]}',
+        )
+
+        # Then I am not allowed because the required power level is unattainable
+        self.assertEqual(HTTPStatus.FORBIDDEN, channel.code, msg=channel.result["body"])
+        self.assertEqual(
+            "You cannot create an encrypted room. "
+            + "user_level (100) < send_level (999)",
+            channel.json_body["error"],
+        )
+
 
 class RoomInitialSyncTestCase(RoomBase):
     """Tests /rooms/$room_id/initialSync."""