Reject mentions on the C-S API which are invalid.

changelog.d/15311.misc

@@ -0,0 +1 @@
+Reject events with an invalid "mentions" property pert [MSC3952](https://github.com/matrix-org/matrix-spec-proposals/pull/3952).

synapse/events/validator.py

@@ -16,7 +16,12 @@
 
 import jsonschema
 
-from synapse.api.constants import MAX_ALIAS_LENGTH, EventTypes, Membership
+from synapse.api.constants import (
+    MAX_ALIAS_LENGTH,
+    EventTypes,
+    Membership,
+    EventContentFields,
+)
 from synapse.api.errors import Codes, SynapseError
 from synapse.api.room_versions import EventFormatVersions
 from synapse.config.homeserver import HomeServerConfig
@@ -88,27 +93,27 @@ def validate_new(self, event: EventBase, config: HomeServerConfig) -> None:
                             Codes.INVALID_PARAM,
                         )
 
-        if event.type == EventTypes.Retention:
+        elif event.type == EventTypes.Retention:
             self._validate_retention(event)
 
-        if event.type == EventTypes.ServerACL:
+        elif event.type == EventTypes.ServerACL:
             if not server_matches_acl_event(config.server.server_name, event):
                 raise SynapseError(
                     400, "Can't create an ACL event that denies the local server"
                 )
 
-        if event.type == EventTypes.PowerLevels:
+        elif event.type == EventTypes.PowerLevels:
             try:
                 jsonschema.validate(
                     instance=event.content,
                     schema=POWER_LEVELS_SCHEMA,
-                    cls=plValidator,
+                    cls=POWER_LEVELS_VALIDATOR,
                 )
             except jsonschema.ValidationError as e:
                 if e.path:
                     # example: "users_default": '0' is not of type 'integer'
                     # cast safety: path entries can be integers, if we fail to validate
-                    # items in an array. However the POWER_LEVELS_SCHEMA doesn't expect
+                    # items in an array. However, the POWER_LEVELS_SCHEMA doesn't expect
                     # to see any arrays.
                     message = (
                         '"' + cast(str, e.path[-1]) + '": ' + e.message  # noqa: B306
@@ -125,6 +130,19 @@ def validate_new(self, event: EventBase, config: HomeServerConfig) -> None:
                     errcode=Codes.BAD_JSON,
                 )
 
+        # If the event contains a mentions key, validate it.
+        if (
+            EventContentFields.MSC3952_MENTIONS in event.content
+            and config.experimental.msc3952_intentional_mentions
+        ):
+            mentions = event.content[EventContentFields.MSC3952_MENTIONS]
+            try:
+                jsonschema.validate(
+                    instance=mentions, schema=MENTIONS_SCHEMA, cls=MENTIONS_VALIDATOR
+                )
+            except jsonschema.ValidationError as e:
+                raise SynapseError(400, e.message)
+
     def _validate_retention(self, event: EventBase) -> None:
         """Checks that an event that defines the retention policy for a room respects the
         format enforced by the spec.
@@ -252,11 +270,26 @@ def _ensure_state_event(self, event: Union[EventBase, EventBuilder]) -> None:
     },
 }
 
+MENTIONS_SCHEMA = {
+    "type": "object",
+    "properties": {
+        "user_ids": {
+            "type": "array",
+            "items": {
+                "type": "string",
+            },
+        },
+        "room": {
+            "type": "boolean",
+        },
+    },
+}
+
 
 # This could return something newer than Draft 7, but that's the current "latest"
 # validator.
-def _create_power_level_validator() -> Type[jsonschema.Draft7Validator]:
-    validator = jsonschema.validators.validator_for(POWER_LEVELS_SCHEMA)
+def _create_validator(schema: JsonDict) -> Type[jsonschema.Draft7Validator]:
+    validator = jsonschema.validators.validator_for(schema)
 
     # by default jsonschema does not consider a frozendict to be an object so
     # we need to use a custom type checker
@@ -268,4 +301,5 @@ def _create_power_level_validator() -> Type[jsonschema.Draft7Validator]:
     return jsonschema.validators.extend(validator, type_checker=type_checker)
 
 
-plValidator = _create_power_level_validator()
+POWER_LEVELS_VALIDATOR = _create_validator(POWER_LEVELS_SCHEMA)
+MENTIONS_VALIDATOR = _create_validator(MENTIONS_SCHEMA)