Add a spamchecker callback to allow or deny room joins (#10910)

Co-authored-by: Erik Johnston <erik@matrix.org>

3 files changed, 57 insertions, 0 deletions

diff --git a/synapse/events/spamcheck.py b/synapse/events/spamcheck.py
index c389f70b8d..ec8863e397 100644
--- a/synapse/events/spamcheck.py
+++ b/synapse/events/spamcheck.py
@@ -44,6 +44,7 @@ CHECK_EVENT_FOR_SPAM_CALLBACK = Callable[
     ["synapse.events.EventBase"],
     Awaitable[Union[bool, str]],
 ]
+USER_MAY_JOIN_ROOM_CALLBACK = Callable[[str, str, bool], Awaitable[bool]]
 USER_MAY_INVITE_CALLBACK = Callable[[str, str, str], Awaitable[bool]]
 USER_MAY_CREATE_ROOM_CALLBACK = Callable[[str], Awaitable[bool]]
 USER_MAY_CREATE_ROOM_WITH_INVITES_CALLBACK = Callable[
@@ -165,6 +166,7 @@ def load_legacy_spam_checkers(hs: "synapse.server.HomeServer"):
 class SpamChecker:
     def __init__(self):
         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_create_room_callbacks: List[USER_MAY_CREATE_ROOM_CALLBACK] = []
         self._user_may_create_room_with_invites_callbacks: List[
@@ -187,6 +189,7 @@ class SpamChecker:
     def register_callbacks(
         self,
         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_create_room: Optional[USER_MAY_CREATE_ROOM_CALLBACK] = None,
         user_may_create_room_with_invites: Optional[
@@ -206,6 +209,9 @@ class SpamChecker:
         if check_event_for_spam is not None:
             self._check_event_for_spam_callbacks.append(check_event_for_spam)
 
+        if user_may_join_room is not None:
+            self._user_may_join_room_callbacks.append(user_may_join_room)
+
         if user_may_invite is not None:
             self._user_may_invite_callbacks.append(user_may_invite)
 
@@ -259,6 +265,24 @@ class SpamChecker:
 
         return False
 
+    async def user_may_join_room(self, user_id: str, room_id: str, is_invited: bool):
+        """Checks if a given users is allowed to join a room.
+        Not called when a user creates a room.
+
+        Args:
+            userid: The ID of the user wanting to join the room
+            room_id: The ID of the room the user wants to join
+            is_invited: Whether the user is invited into the room
+
+        Returns:
+            bool: Whether the user may join the room
+        """
+        for callback in self._user_may_join_room_callbacks:
+            if await callback(user_id, room_id, is_invited) is False:
+                return False
+
+        return True
+
     async def user_may_invite(
         self, inviter_userid: str, invitee_userid: str, room_id: str
     ) -> bool:
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 873e08258e..d40dbd761d 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -860,6 +860,7 @@ class RoomCreationHandler(BaseHandler):
                     "invite",
                     ratelimit=False,
                     content=content,
+                    new_room=True,
                 )
 
         for invite_3pid in invite_3pid_list:
@@ -962,6 +963,7 @@ class RoomCreationHandler(BaseHandler):
             "join",
             ratelimit=ratelimit,
             content=creator_join_profile,
+            new_room=True,
         )
 
         # We treat the power levels override specially as this needs to be one
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index c8fb24a20c..0b79dbcf8d 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -434,6 +434,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         third_party_signed: Optional[dict] = None,
         ratelimit: bool = True,
         content: Optional[dict] = None,
+        new_room: bool = False,
         require_consent: bool = True,
         outlier: bool = False,
         prev_event_ids: Optional[List[str]] = None,
@@ -451,6 +452,8 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
             third_party_signed: Information from a 3PID invite.
             ratelimit: Whether to rate limit the request.
             content: The content of the created event.
+            new_room: Whether the membership update is happening in the context of a room
+                creation.
             require_consent: Whether consent is required.
             outlier: Indicates whether the event is an `outlier`, i.e. if
                 it's from an arbitrary point and floating in the DAG as
@@ -485,6 +488,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
                 third_party_signed=third_party_signed,
                 ratelimit=ratelimit,
                 content=content,
+                new_room=new_room,
                 require_consent=require_consent,
                 outlier=outlier,
                 prev_event_ids=prev_event_ids,
@@ -504,6 +508,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         third_party_signed: Optional[dict] = None,
         ratelimit: bool = True,
         content: Optional[dict] = None,
+        new_room: bool = False,
         require_consent: bool = True,
         outlier: bool = False,
         prev_event_ids: Optional[List[str]] = None,
@@ -523,6 +528,8 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
             third_party_signed:
             ratelimit:
             content:
+            new_room: Whether the membership update is happening in the context of a room
+                creation.
             require_consent:
             outlier: Indicates whether the event is an `outlier`, i.e. if
                 it's from an arbitrary point and floating in the DAG as
@@ -726,6 +733,30 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
                     # so don't really fit into the general auth process.
                     raise AuthError(403, "Guest access not allowed")
 
+            # Figure out whether the user is a server admin to determine whether they
+            # should be able to bypass the spam checker.
+            if (
+                self._server_notices_mxid is not None
+                and requester.user.to_string() == self._server_notices_mxid
+            ):
+                # allow the server notices mxid to join rooms
+                bypass_spam_checker = True
+
+            else:
+                bypass_spam_checker = await self.auth.is_server_admin(requester.user)
+
+            inviter = await self._get_inviter(target.to_string(), room_id)
+            if (
+                not bypass_spam_checker
+                # We assume that if the spam checker allowed the user to create
+                # a room then they're allowed to join it.
+                and not new_room
+                and not await self.spam_checker.user_may_join_room(
+                    target.to_string(), room_id, is_invited=inviter is not None
+                )
+            ):
+                raise SynapseError(403, "Not allowed to join this room")
+
             # Check if a remote join should be performed.
             remote_join, remote_room_hosts = await self._should_perform_remote_join(
                 target.to_string(), room_id, remote_room_hosts, content, is_host_in_room