# Restricting room membership based on space membership

A desirable feature is to give room admins the power to restrict membership of
their room based on the membership of one or more spaces from
[MSC1772](https://github.com/matrix-org/matrix-doc/pull/1772), for example:

> members of the #doglovers space can join this room without an invitation<sup id="a1">[1](#f1)</sup>

## Proposal

In a future room version a new `join_rule` (`restricted`) will be used to reflect
a cross between `invite` and `public` join rules. The content of the join rules
would include the rooms to trust for membership. For example:

```json
{
    "type": "m.room.join_rules",
    "state_key": "",
    "content": {
        "join_rule": "restricted",
        "allow": [
            {
                "type": "m.room_membership",
                "room_id": "!mods:example.org"
            },
            {
                "type": "m.room_membership",
                "room_id": "!users:example.org"
            }
        ]
    }
}
```

This means that a user must be a member of the `!mods:example.org` room or
`!users:example.org` room in order to join without an invite<sup id="a2">[2](#f2)</sup>.
Membership in a single allowed room is enough.

If the `allow` key is an empty list (or not a list at all), then no users are
allowed to join without an invite. Each entry is expected to be an object with the
following keys:

* `type`: `"m.room_membership"` to describe that we are allowing access via room
  membership. Future MSCs may define other types.
* `room_id`: The room ID to check the membership of.

Any entries in the list which do not match the expected format are ignored. Thus,
if all entries are invalid, the list behaves as if empty and all users without
an invite are rejected.

When an homeserver receives a `/join` request from a client or a `/make_join` /
`/send_join` request from another homeserver, the request should only be permitted
if the user has a valid invite or is in one of the listed rooms. If the user is
not a member of at least one of the rooms, the homeserver should return an error
response with HTTP status code of 403 and an `errcode` of `M_FORBIDDEN`.

It is possible for a homeserver receiving a `/make_join` / `/send_join` request
to not know if the user is in a particular room (due to not participating in any
of the necessary rooms). In this case the homeserver should reject the join,
the requesting server may wish to attempt to join via another homeserver. If no
servers are in an allowed room its membership cannot be checked (and this is a
misconfiguration).

From the perspective of the [auth rules](https://spec.matrix.org/unstable/rooms/v1/#authorization-rules),
the `restricted` join rule has the same behavior as `public`, with the additional
caveat that servers must ensure that:

* The user's previous membership was `invite` or `join`, or
* The `m.room.member` event has a valid signature from a homeserver whose users
  have the power to issue invites.

The above check must also be performed against the current room state to potentially
soft-fail the event. This is the primary mechanism for guarding against state
changes when old events are referenced. (E.g. if the power levels change, a
server should not be able to issue new membership events by referencing an old
event in the room.)

Note that the homeservers whose users can issue invites are trusted to confirm
that the `allow` rules were properly checked (since this cannot easily be
enforced over federation by event authorisation).<sup id="a3">[3](#f3)</sup>
(The rationale for trusting these homeservers is that they could easily
side-step the restriction by issuing an invite first.)

## Summary of the behaviour of join rules

See the [join rules](https://matrix.org/docs/spec/client_server/r0.6.1#m-room-join-rules)
specification for full details, the summary below is meant to highlight the differences
between `public`, `invite`, and `restricted`.

* `public`: anyone can join, subject to `ban` and `server_acls`, as today.
* `invite`: only people with membership `invite` can join, subject to `ban` and
  `server_acls`, as today.
* `knock`: the same as `invite`, except anyone can knock, subject to `ban` and
  `server_acls`. See [MSC2403](https://github.com/matrix-org/matrix-doc/pull/2403).
* `private`: This is reserved, but unspecified.
* `restricted`: the same as `public`, with the additional caveat that servers must
  verify the `m.room.member` event is signed by a homeserver whose users may issue
  invites if the joining member was not previously invited or joined into the room.

## Security considerations

Although increased trust to enforce the join rules during `/join` / `/make_join`
/ `/send_join` is placed in the homeservers whose users can issue invites, this
is considered only a miniscule change in room security.

This MSC limits the homeservers who can issue join events (via calls to `/join`,
`/make_join`, and `/send_join`) and trusts those servers to enforce the additional
allow rules. Although other homeservers may not be able to verify that a join
event was issued in good faith, there is no benefit for a homeserver to do this
since they could have issued an invite anyway.

## Unstable prefix

The `restricted` join rule will be included in a future room version to allow
servers and clients to opt-into the new functionality.

During development, an unstable room version of `org.matrix.msc3083` will be used.
Since the room version namespaces the behaviour, the `allow` key and value, as well
as the `restricted` join rule value do not need unstable prefixes.

## Alternatives

It may seem that just having the `allow` key with `public` join rules is enough
(as originally suggested in [MSC2962](https://github.com/matrix-org/matrix-doc/pull/2962)),
but there are concerns that changing the behaviour of a pre-existing a `public`
join rule may cause security issues in older implementations (that do not yet
understand the new behaviour). This could be solved by introducing a new room
version, thus it seems clearer to introduce a new join rule -- `restricted`.

Using an `allow` key with the `invite` join rules to broaden who can join was rejected
as an option since it requires weakening the [auth rules](https://spec.matrix.org/unstable/rooms/v1/#authorization-rules).
From the perspective of the auth rules, the `restricted` join rule is identical
to `public` with additional checks on the signature of the event.

## Future extensions

### Checking room membership over federation

If a homeserver is not in an allowed room (and thus doesn't know the
membership of it) then the server cannot enforce the membership checks while
generating a join event. Peeking over federation, as described in
[MSC2444](https://github.com/matrix-org/matrix-doc/pull/2444),
could be used to establish if the user is in any of the proper rooms.

This would then delegate power out to a (potentially) untrusted server, giving that
the peek server significant power. For example, a poorly chosen peek
server could lie about the room membership and add an `@evil_user:example.org`
to an allowed room to gain membership to a room.

As iterated above, this MSC recommends rejecting the join, potentially allowing
the requesting homeserver to retry via another homeserver.

### Kicking users out when they leave the allowed room

In the above example, suppose `@bob:server.example` leaves `!users:example.org`:
should they be removed from the room? Likely not, by analogy with what happens
when you switch the join rules from public to invite. Join rules currently govern
joins, not existing room membership.

It is left to a future MSC to consider this, but some potential thoughts are
given below.

If you assume that a user *should* be removed in this case, one option is to
leave the departure up to Bob's server `server.example`, but this places a
relatively high level of trust in that server. Additionally, if `server.example`
were offline, other users in the room would still see Bob in the room (and their
servers would attempt to send message traffic to it).

Another consideration is that users may have joined via a direct invite, not via
access through a room.

Fixing this is thorny. Some sort of annotation on the membership events might
help. but it's unclear what the desired semantics are:

* Assuming that users in an allowed room are *not* kicked when that room is
  removed from `allow`, are those users then given a pass to remain
  in the room indefinitely? What happens if the room is added back to
  `allow` and *then* the user leaves it?
* Suppose a user joins a room via an allowed room (RoomA). Later, RoomB is added
  to the `allow` list and RoomA is removed. What should happen when the
  user leaves RoomB? Are they exempt from the kick?

It is possible that completely different state should be kept, or a different
`m.room.member` state could be used in a more reasonable way to track this.

### Inheriting join rules

If an allowed room is a space and you make a parent space invite-only, should that
(optionally?) cascade into child rooms? This would have some of the same problems
as inheriting power levels, as discussed in [MSC2962](https://github.com/matrix-org/matrix-doc/pull/2962).

### Additional allow types

Future MSCs may wish to define additional values for the `type` argument, potentially
restricting access via:

* MXIDs or servers.
* A shared secret (room password).

These are just examples are not fully thought through for this MSC, but it should
be possible to add these behaviors in the future.

## Footnotes

<a id="f1"/>[1]: The converse restriction, "anybody can join, provided they are not members
of the '#catlovers' space" is less useful since:

1. Users in the banned room could simply leave it at any time
2. This functionality is already partially provided by
   [Moderation policy lists](https://matrix.org/docs/spec/client_server/r0.6.1#moderation-policy-lists). [↩](#a1)

<a id="f2"/>[2]: Note that there is nothing stopping users sending and
receiving invites in `public` rooms today, and they work as you might expect.
The only difference is that you are not *required* to hold an invite when
joining the room. [↩](#a2)

<a id="f3"/>[3]: This has the downside of increased centralisation, as some
homeservers that are already in the room may not issue a join event for another
user on that server. (It must go through the `/make_join` / `/send_join` flow of
a server whose users may issue invites.) This is considered a reasonable
trade-off. [↩](#a3)