add default_power_level_content_override config option. (#12618)

Co-authored-by: Matthew Hodgson <matthew@matrix.org>

6 files changed, 381 insertions, 1 deletions

diff --git a/changelog.d/12618.feature b/changelog.d/12618.feature
new file mode 100644
index 0000000000..37fa03b3cb
--- /dev/null
+++ b/changelog.d/12618.feature
@@ -0,0 +1 @@
+Add a `default_power_level_content_override` config option to set default room power levels per room preset.
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index e7b57f5a0b..03a0f6314c 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -2468,6 +2468,40 @@ push:
 #
 #encryption_enabled_by_default_for_room_type: invite
 
+# Override the default power levels for rooms created on this server, per
+# room creation preset.
+#
+# The appropriate dictionary for the room preset will be applied on top
+# of the existing power levels content.
+#
+# Useful if you know that your users need special permissions in rooms
+# that they create (e.g. to send particular types of state events without
+# needing an elevated power level).  This takes the same shape as the
+# `power_level_content_override` parameter in the /createRoom API, but
+# is applied before that parameter.
+#
+# Valid keys are some or all of `private_chat`, `trusted_private_chat`
+# and `public_chat`. Inside each of those should be any of the
+# properties allowed in `power_level_content_override` in the
+# /createRoom API. If any property is missing, its default value will
+# continue to be used. If any property is present, it will overwrite
+# the existing default completely (so if the `events` property exists,
+# the default event power levels will be ignored).
+#
+#default_power_level_content_override:
+#    private_chat:
+#        "events":
+#            "com.example.myeventtype" : 0
+#            "m.room.avatar": 50
+#            "m.room.canonical_alias": 50
+#            "m.room.encryption": 100
+#            "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": 1
+
 
 # Uncomment to allow non-server-admin users to create groups on this server
 #
diff --git a/docs/usage/configuration/config_documentation.md b/docs/usage/configuration/config_documentation.md
index f292b94fb0..2af1f284b1 100644
--- a/docs/usage/configuration/config_documentation.md
+++ b/docs/usage/configuration/config_documentation.md
@@ -3315,6 +3315,32 @@ room_list_publication_rules:
     room_id: "*"
     action: allow
 ```
+
+---
+Config option: `default_power_level_content_override`
+
+The `default_power_level_content_override` option controls the default power
+levels for rooms.
+
+Useful if you know that your users need special permissions in rooms
+that they create (e.g. to send particular types of state events without
+needing an elevated power level).  This takes the same shape as the
+`power_level_content_override` parameter in the /createRoom API, but
+is applied before that parameter.
+
+Note that each key provided inside a preset (for example `events` in the example
+below) will overwrite all existing defaults inside that key. So in the example
+below, newly-created private_chat rooms will have no rules for any event types
+except `com.example.foo`.
+
+Example configuration:
+```yaml
+default_power_level_content_override:
+   private_chat: { "events": { "com.example.foo" : 0 } }
+   trusted_private_chat: null
+   public_chat: null
+```
+
 ---
 ## Opentracing ##
 Configuration options related to Opentracing support.
diff --git a/synapse/config/room.py b/synapse/config/room.py
index e18a87ea37..462d85ac1d 100644
--- a/synapse/config/room.py
+++ b/synapse/config/room.py
@@ -63,6 +63,19 @@ class RoomConfig(Config):
                 "Invalid value for encryption_enabled_by_default_for_room_type"
             )
 
+        self.default_power_level_content_override = config.get(
+            "default_power_level_content_override",
+            None,
+        )
+        if self.default_power_level_content_override is not None:
+            for preset in self.default_power_level_content_override:
+                if preset not in vars(RoomCreationPreset).values():
+                    raise ConfigError(
+                        "Unrecognised room preset %s in default_power_level_content_override"
+                        % preset
+                    )
+                # We validate the actual overrides when we try to apply them.
+
     def generate_config_section(self, **kwargs: Any) -> str:
         return """\
         ## Rooms ##
@@ -83,4 +96,38 @@ class RoomConfig(Config):
         # will also not affect rooms created by other servers.
         #
         #encryption_enabled_by_default_for_room_type: invite
+
+        # Override the default power levels for rooms created on this server, per
+        # room creation preset.
+        #
+        # The appropriate dictionary for the room preset will be applied on top
+        # of the existing power levels content.
+        #
+        # Useful if you know that your users need special permissions in rooms
+        # that they create (e.g. to send particular types of state events without
+        # needing an elevated power level).  This takes the same shape as the
+        # `power_level_content_override` parameter in the /createRoom API, but
+        # is applied before that parameter.
+        #
+        # Valid keys are some or all of `private_chat`, `trusted_private_chat`
+        # and `public_chat`. Inside each of those should be any of the
+        # properties allowed in `power_level_content_override` in the
+        # /createRoom API. If any property is missing, its default value will
+        # continue to be used. If any property is present, it will overwrite
+        # the existing default completely (so if the `events` property exists,
+        # the default event power levels will be ignored).
+        #
+        #default_power_level_content_override:
+        #    private_chat:
+        #        "events":
+        #            "com.example.myeventtype" : 0
+        #            "m.room.avatar": 50
+        #            "m.room.canonical_alias": 50
+        #            "m.room.encryption": 100
+        #            "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": 1
         """
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 604eb6ec15..e71c78adad 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -149,6 +149,10 @@ class RoomCreationHandler:
             )
             preset_config["encrypted"] = encrypted
 
+        self._default_power_level_content_override = (
+            self.config.room.default_power_level_content_override
+        )
+
         self._replication = hs.get_replication_data_handler()
 
         # linearizer to stop two upgrades happening at once
@@ -1042,9 +1046,19 @@ class RoomCreationHandler:
                 for invitee in invite_list:
                     power_level_content["users"][invitee] = 100
 
-            # Power levels overrides are defined per chat preset
+            # If the user supplied a preset name e.g. "private_chat",
+            # we apply that preset
             power_level_content.update(config["power_level_content_override"])
 
+            # If the server config contains default_power_level_content_override,
+            # and that contains information for this room preset, apply it.
+            if self._default_power_level_content_override:
+                override = self._default_power_level_content_override.get(preset_config)
+                if override is not None:
+                    power_level_content.update(override)
+
+            # Finally, if the user supplied specific permissions for this room,
+            # apply those.
             if power_level_content_override:
                 power_level_content.update(power_level_content_override)
 
diff --git a/tests/rest/client/test_rooms.py b/tests/rest/client/test_rooms.py
index 9443daa056..ad416e2fd8 100644
--- a/tests/rest/client/test_rooms.py
+++ b/tests/rest/client/test_rooms.py
@@ -1116,6 +1116,264 @@ class RoomMessagesTestCase(RoomBase):
         self.assertEqual(200, channel.code, msg=channel.result["body"])
 
 
+class RoomPowerLevelOverridesTestCase(RoomBase):
+    """Tests that the power levels can be overridden with server config."""
+
+    user_id = "@sid1:red"
+
+    servlets = [
+        admin.register_servlets,
+        room.register_servlets,
+        login.register_servlets,
+    ]
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.admin_user_id = self.register_user("admin", "pass")
+        self.admin_access_token = self.login("admin", "pass")
+
+    def power_levels(self, room_id: str) -> Dict[str, Any]:
+        return self.helper.get_state(
+            room_id, "m.room.power_levels", self.admin_access_token
+        )
+
+    def test_default_power_levels_with_room_override(self) -> None:
+        """
+        Create a room, providing power level overrides.
+        Confirm that the room's power levels reflect the overrides.
+
+        See https://github.com/matrix-org/matrix-spec/issues/492
+        - currently we overwrite each key of power_level_content_override
+        completely.
+        """
+
+        room_id = self.helper.create_room_as(
+            self.user_id,
+            extra_content={
+                "power_level_content_override": {"events": {"custom.event": 0}}
+            },
+        )
+        self.assertEqual(
+            {
+                "custom.event": 0,
+            },
+            self.power_levels(room_id)["events"],
+        )
+
+    @unittest.override_config(
+        {
+            "default_power_level_content_override": {
+                "public_chat": {"events": {"custom.event": 0}},
+            }
+        },
+    )
+    def test_power_levels_with_server_override(self) -> None:
+        """
+        With a server configured to modify the room-level defaults,
+        Create a room, without providing any extra power level overrides.
+        Confirm that the room's power levels reflect the server-level overrides.
+
+        Similar to https://github.com/matrix-org/matrix-spec/issues/492,
+        we overwrite each key of power_level_content_override completely.
+        """
+
+        room_id = self.helper.create_room_as(self.user_id)
+        self.assertEqual(
+            {
+                "custom.event": 0,
+            },
+            self.power_levels(room_id)["events"],
+        )
+
+    @unittest.override_config(
+        {
+            "default_power_level_content_override": {
+                "public_chat": {
+                    "events": {"server.event": 0},
+                    "ban": 13,
+                },
+            }
+        },
+    )
+    def test_power_levels_with_server_and_room_overrides(self) -> None:
+        """
+        With a server configured to modify the room-level defaults,
+        create a room, providing different overrides.
+        Confirm that the room's power levels reflect both overrides, and
+        choose the room overrides where they clash.
+        """
+
+        room_id = self.helper.create_room_as(
+            self.user_id,
+            extra_content={
+                "power_level_content_override": {"events": {"room.event": 0}}
+            },
+        )
+
+        # Room override wins over server config
+        self.assertEqual(
+            {"room.event": 0},
+            self.power_levels(room_id)["events"],
+        )
+
+        # But where there is no room override, server config wins
+        self.assertEqual(13, self.power_levels(room_id)["ban"])
+
+
+class RoomPowerLevelOverridesInPracticeTestCase(RoomBase):
+    """
+    Tests that we can really do various otherwise-prohibited actions
+    based on overriding the power levels in config.
+    """
+
+    user_id = "@sid1:red"
+
+    def test_creator_can_post_state_event(self) -> None:
+        # Given I am the creator of a room
+        room_id = self.helper.create_room_as(self.user_id)
+
+        # When I send a state event
+        path = "/rooms/{room_id}/state/custom.event/my_state_key".format(
+            room_id=urlparse.quote(room_id),
+        )
+        channel = self.make_request("PUT", path, "{}")
+
+        # Then I am allowed
+        self.assertEqual(200, channel.code, msg=channel.result["body"])
+
+    def test_normal_user_can_not_post_state_event(self) -> None:
+        # Given I am a normal member of a room
+        room_id = self.helper.create_room_as("@some_other_guy:red")
+        self.helper.join(room=room_id, user=self.user_id)
+
+        # When I send a state event
+        path = "/rooms/{room_id}/state/custom.event/my_state_key".format(
+            room_id=urlparse.quote(room_id),
+        )
+        channel = self.make_request("PUT", path, "{}")
+
+        # Then I am not allowed because state events require PL>=50
+        self.assertEqual(403, channel.code, msg=channel.result["body"])
+        self.assertEqual(
+            "You don't have permission to post that to the room. "
+            "user_level (0) < send_level (50)",
+            channel.json_body["error"],
+        )
+
+    @unittest.override_config(
+        {
+            "default_power_level_content_override": {
+                "public_chat": {"events": {"custom.event": 0}},
+            }
+        },
+    )
+    def test_with_config_override_normal_user_can_post_state_event(self) -> None:
+        # Given the server has config allowing normal users to post my event type,
+        # and I am a normal member of a room
+        room_id = self.helper.create_room_as("@some_other_guy:red")
+        self.helper.join(room=room_id, user=self.user_id)
+
+        # When I send a state event
+        path = "/rooms/{room_id}/state/custom.event/my_state_key".format(
+            room_id=urlparse.quote(room_id),
+        )
+        channel = self.make_request("PUT", path, "{}")
+
+        # Then I am allowed
+        self.assertEqual(200, channel.code, msg=channel.result["body"])
+
+    @unittest.override_config(
+        {
+            "default_power_level_content_override": {
+                "public_chat": {"events": {"custom.event": 0}},
+            }
+        },
+    )
+    def test_any_room_override_defeats_config_override(self) -> None:
+        # Given the server has config allowing normal users to post my event type
+        # And I am a normal member of a room
+        # But the room was created with special permissions
+        extra_content: Dict[str, Any] = {
+            "power_level_content_override": {"events": {}},
+        }
+        room_id = self.helper.create_room_as(
+            "@some_other_guy:red", extra_content=extra_content
+        )
+        self.helper.join(room=room_id, user=self.user_id)
+
+        # When I send a state event
+        path = "/rooms/{room_id}/state/custom.event/my_state_key".format(
+            room_id=urlparse.quote(room_id),
+        )
+        channel = self.make_request("PUT", path, "{}")
+
+        # Then I am not allowed
+        self.assertEqual(403, channel.code, msg=channel.result["body"])
+
+    @unittest.override_config(
+        {
+            "default_power_level_content_override": {
+                "public_chat": {"events": {"custom.event": 0}},
+            }
+        },
+    )
+    def test_specific_room_override_defeats_config_override(self) -> None:
+        # Given the server has config allowing normal users to post my event type,
+        # and I am a normal member of a room,
+        # but the room was created with special permissions for this event type
+        extra_content = {
+            "power_level_content_override": {"events": {"custom.event": 1}},
+        }
+        room_id = self.helper.create_room_as(
+            "@some_other_guy:red", extra_content=extra_content
+        )
+        self.helper.join(room=room_id, user=self.user_id)
+
+        # When I send a state event
+        path = "/rooms/{room_id}/state/custom.event/my_state_key".format(
+            room_id=urlparse.quote(room_id),
+        )
+        channel = self.make_request("PUT", path, "{}")
+
+        # Then I am not allowed
+        self.assertEqual(403, channel.code, msg=channel.result["body"])
+        self.assertEqual(
+            "You don't have permission to post that to the room. "
+            + "user_level (0) < send_level (1)",
+            channel.json_body["error"],
+        )
+
+    @unittest.override_config(
+        {
+            "default_power_level_content_override": {
+                "public_chat": {"events": {"custom.event": 0}},
+                "private_chat": None,
+                "trusted_private_chat": None,
+            }
+        },
+    )
+    def test_config_override_applies_only_to_specific_preset(self) -> None:
+        # Given the server has config for public_chats,
+        # and I am a normal member of a private_chat room
+        room_id = self.helper.create_room_as("@some_other_guy:red", is_public=False)
+        self.helper.invite(room=room_id, src="@some_other_guy:red", targ=self.user_id)
+        self.helper.join(room=room_id, user=self.user_id)
+
+        # When I send a state event
+        path = "/rooms/{room_id}/state/custom.event/my_state_key".format(
+            room_id=urlparse.quote(room_id),
+        )
+        channel = self.make_request("PUT", path, "{}")
+
+        # Then I am not allowed because the public_chat config does not
+        # affect this room, because this room is a private_chat
+        self.assertEqual(403, channel.code, msg=channel.result["body"])
+        self.assertEqual(
+            "You don't have permission to post that to the room. "
+            + "user_level (0) < send_level (50)",
+            channel.json_body["error"],
+        )
+
+
 class RoomInitialSyncTestCase(RoomBase):
     """Tests /rooms/$room_id/initialSync."""