# Restricting room membership based on space membership

Draft join rule changes for [spaces](https://github.com/matrix-org/matrix-doc/pull/1772),
this is meant to replaces the second half of [MSC2962](https://github.com/matrix-org/matrix-doc/pull/2962/).

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, for example:

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

We could represent the allowed spaces with a new `join_rule` - `restricted` - to
reflect the fact that what we have is a cross between `invite` and `public`. This
would have additional content of the rooms to trust for membership. For example:

{
    "type": "m.room.join_rules",
    "state_key": "",
    "content": {
        "join_rule": "restricted",
        "allow": [
            {
                "space": "!mods:example.org",
                "via": ["example.org"]
            },
            {
                "space": "!users:example.org",
                "via": ["example.org"]
            }
        ]
    }
}

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

* `space`: The room ID of the space to check the membership of.
* `via`: A list of servers which may be used to peek for membership of the space.

Any entries in the list which do not match the expected format are ignored.

When a server receives a `/join` request from a client or a `/make_join` / `/send_join`
request from a server, the request should only be permitted if the user has a valid
invite or is in one of the listed spaces (established by peeking if the server is not
already in the space, see [MSC2444](https://github.com/matrix-org/matrix-doc/pull/2444)).

Unlike the `invite` join rule, confirmation that the `allow` rules were properly
checked cannot be enforced over federation by event authorization, so servers in
the room are trusted not to allow invalid users to join.<sup id="a3">[3](#f3)</sup>
However, user IDs listed as strings can be properly checked over federation.

## 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, but the summary below should 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, 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 and not implemented.
* `restricted`: the same as `public` from the perspective of the auth rules, but
with the additional caveat that servers are expected to check the `allow` rules
before generating a `join` event (whether for a local or a remote user).

## Security considerations

The `allow` feature for `join_rules` places increased trust in the servers in the
room. We consider this acceptable: if you don't want evil servers randomly
joining spurious users into your rooms, then:

1. Don't let evil servers in your room in the first place
2. Don't use `allow` lists, given the expansion increases the attack surface anyway by letting members in other rooms dictate who's allowed into your room.

The peek server also has significant power. For example, a poorly chosen peek
server could lie about the space membership and add an `@evil_user:example.org`.

## Unstable prefix

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

During development it is expected that an unstable room version of
`org.matrix.mscXXXX` is used. Since the room version namespaces the behaviour,
the `allow` key and the `restricted` value do not need unstable prefixes.

## History / Rationale

It may seem that just having the `allow` key with `public` join rules is enough,
as suggested in [MSC2962](https://github.com/matrix-org/matrix-doc/pull/2962/),
but there are concerns that having a `public` join rule that is restricted may
cause issues if an implementation does not understand the semantics of the `allow`
keyword. Using an `allow` key with `invite` join rules also does not make sense as
from the perspective of the auth rules, this is akin to `public` (since the checking
of whether a member is in the space is done during the call to `/join`
or `/make_join` / `/send_join`).

The above concerns about an implementation not understanding the semantics of `allow`
could be solved by introducing a new room version, but if this is done it seems clearer
to just introduce a a new join rule - `restricted` - as described above.

## Future extensions

Potential future extensions which should not be designed out
include, but are not included in this MSC.

### Checking space membership over federation

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

In the above example, suppose `@bob:server.example` leaves `!users:example.org`:
they should be removed from the room. 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 space.

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

### Inheriting join rules

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

## 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 space could simply leave it at any time
2. This functionality is already somewhat 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 is a marginal decrease in security from the current
situation. Currently, a misbehaving server can allow unauthorized users to join
any room by first issuing an invite to that user. In theory that can be
prevented by raising the PL required to send an invite, but in practice that is
rarely done. [↩](#a2)