Support MSC3757: Restricting who can overwrite a state event (#17513)

Link to the
MSC: https://github.com/matrix-org/matrix-spec-proposals/pull/3757

---------

Co-authored-by: Quentin Gliech <quenting@element.io>

1 files changed, 31 insertions, 6 deletions

diff --git a/synapse/event_auth.py b/synapse/event_auth.py

index b834547d11..c208b900c5 100644
--- a/synapse/event_auth.py
+++ b/synapse/event_auth.py
@@ -388,6 +388,7 @@ LENIENT_EVENT_BYTE_LIMITS_ROOM_VERSIONS = {
     RoomVersions.V9,
     RoomVersions.V10,
     RoomVersions.MSC1767v10,
+    RoomVersions.MSC3757v10,
 }
 
 
@@ -790,9 +791,10 @@ def get_send_level(
 
 
 def _can_send_event(event: "EventBase", auth_events: StateMap["EventBase"]) -> bool:
+    state_key = event.get_state_key()
     power_levels_event = get_power_level_event(auth_events)
 
-    send_level = get_send_level(event.type, event.get("state_key"), power_levels_event)
+    send_level = get_send_level(event.type, state_key, power_levels_event)
     user_level = get_user_power_level(event.user_id, auth_events)
 
     if user_level < send_level:
@@ -803,11 +805,34 @@ def _can_send_event(event: "EventBase", auth_events: StateMap["EventBase"]) -> b
             errcode=Codes.INSUFFICIENT_POWER,
         )
 
-    # Check state_key
-    if hasattr(event, "state_key"):
-        if event.state_key.startswith("@"):
-            if event.state_key != event.user_id:
-                raise AuthError(403, "You are not allowed to set others state")
+    if (
+        state_key is not None
+        and state_key.startswith("@")
+        and state_key != event.user_id
+    ):
+        if event.room_version.msc3757_enabled:
+            try:
+                colon_idx = state_key.index(":", 1)
+                suffix_idx = state_key.find("_", colon_idx + 1)
+                state_key_user_id = (
+                    state_key[:suffix_idx] if suffix_idx != -1 else state_key
+                )
+                if not UserID.is_valid(state_key_user_id):
+                    raise ValueError
+            except ValueError:
+                raise SynapseError(
+                    400,
+                    "State key neither equals a valid user ID, nor starts with one plus an underscore",
+                    errcode=Codes.BAD_JSON,
+                )
+            if (
+                # sender is owner of the state key
+                state_key_user_id == event.user_id
+                # sender has higher PL than the owner of the state key
+                or user_level > get_user_power_level(state_key_user_id, auth_events)
+            ):
+                return True
+        raise AuthError(403, "You are not allowed to set others state")
 
     return True