Add a spamchecker method to allow or deny 3pid invites (#10894)

This is in the context of creating new module callbacks that modules in https://github.com/matrix-org/synapse-dinsic can use, in an effort to reconcile the spam checker API in synapse-dinsic with the one in mainline.

Note that a module callback already exists for 3pid invites (https://matrix-org.github.io/synapse/develop/modules/third_party_rules_callbacks.html#check_threepid_can_be_invited) but it doesn't check whether the sender of the invite is allowed to send it.

2 files changed, 47 insertions, 0 deletions

diff --git a/synapse/events/spamcheck.py b/synapse/events/spamcheck.py
index ec8863e397..ae4c8ab257 100644
--- a/synapse/events/spamcheck.py
+++ b/synapse/events/spamcheck.py
@@ -46,6 +46,7 @@ CHECK_EVENT_FOR_SPAM_CALLBACK = Callable[
 ]
 USER_MAY_JOIN_ROOM_CALLBACK = Callable[[str, str, bool], Awaitable[bool]]
 USER_MAY_INVITE_CALLBACK = Callable[[str, str, str], Awaitable[bool]]
+USER_MAY_SEND_3PID_INVITE_CALLBACK = Callable[[str, str, str, str], Awaitable[bool]]
 USER_MAY_CREATE_ROOM_CALLBACK = Callable[[str], Awaitable[bool]]
 USER_MAY_CREATE_ROOM_WITH_INVITES_CALLBACK = Callable[
     [str, List[str], List[Dict[str, str]]], Awaitable[bool]
@@ -168,6 +169,9 @@ class SpamChecker:
         self._check_event_for_spam_callbacks: List[CHECK_EVENT_FOR_SPAM_CALLBACK] = []
         self._user_may_join_room_callbacks: List[USER_MAY_JOIN_ROOM_CALLBACK] = []
         self._user_may_invite_callbacks: List[USER_MAY_INVITE_CALLBACK] = []
+        self._user_may_send_3pid_invite_callbacks: List[
+            USER_MAY_SEND_3PID_INVITE_CALLBACK
+        ] = []
         self._user_may_create_room_callbacks: List[USER_MAY_CREATE_ROOM_CALLBACK] = []
         self._user_may_create_room_with_invites_callbacks: List[
             USER_MAY_CREATE_ROOM_WITH_INVITES_CALLBACK
@@ -191,6 +195,7 @@ class SpamChecker:
         check_event_for_spam: Optional[CHECK_EVENT_FOR_SPAM_CALLBACK] = None,
         user_may_join_room: Optional[USER_MAY_JOIN_ROOM_CALLBACK] = None,
         user_may_invite: Optional[USER_MAY_INVITE_CALLBACK] = None,
+        user_may_send_3pid_invite: Optional[USER_MAY_SEND_3PID_INVITE_CALLBACK] = None,
         user_may_create_room: Optional[USER_MAY_CREATE_ROOM_CALLBACK] = None,
         user_may_create_room_with_invites: Optional[
             USER_MAY_CREATE_ROOM_WITH_INVITES_CALLBACK
@@ -215,6 +220,11 @@ class SpamChecker:
         if user_may_invite is not None:
             self._user_may_invite_callbacks.append(user_may_invite)
 
+        if user_may_send_3pid_invite is not None:
+            self._user_may_send_3pid_invite_callbacks.append(
+                user_may_send_3pid_invite,
+            )
+
         if user_may_create_room is not None:
             self._user_may_create_room_callbacks.append(user_may_create_room)
 
@@ -304,6 +314,31 @@ class SpamChecker:
 
         return True
 
+    async def user_may_send_3pid_invite(
+        self, inviter_userid: str, medium: str, address: str, room_id: str
+    ) -> bool:
+        """Checks if a given user may invite a given threepid into the room
+
+        If this method returns false, the threepid invite will be rejected.
+
+        Note that if the threepid is already associated with a Matrix user ID, Synapse
+        will call user_may_invite with said user ID instead.
+
+        Args:
+            inviter_userid: The user ID of the sender of the invitation
+            medium: The 3PID's medium (e.g. "email")
+            address: The 3PID's address (e.g. "alice@example.com")
+            room_id: The room ID
+
+        Returns:
+            True if the user may send the invite, otherwise False
+        """
+        for callback in self._user_may_send_3pid_invite_callbacks:
+            if await callback(inviter_userid, medium, address, room_id) is False:
+                return False
+
+        return True
+
     async def user_may_create_room(self, userid: str) -> bool:
         """Checks if a given user may create a room
 
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index c05461bf2a..eef337feeb 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -1299,10 +1299,22 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         if invitee:
             # Note that update_membership with an action of "invite" can raise
             # a ShadowBanError, but this was done above already.
+            # We don't check the invite against the spamchecker(s) here (through
+            # user_may_invite) because we'll do it further down the line anyway (in
+            # update_membership_locked).
             _, stream_id = await self.update_membership(
                 requester, UserID.from_string(invitee), room_id, "invite", txn_id=txn_id
             )
         else:
+            # Check if the spamchecker(s) allow this invite to go through.
+            if not await self.spam_checker.user_may_send_3pid_invite(
+                inviter_userid=requester.user.to_string(),
+                medium=medium,
+                address=address,
+                room_id=room_id,
+            ):
+                raise SynapseError(403, "Cannot send threepid invite")
+
             stream_id = await self._make_and_store_3pid_invite(
                 requester,
                 id_server,