MSC4150: m.allow recommendation for moderation policy lists

Signed-off-by: Johannes Marbach <[email protected]>

proposals/4150-m-allow-recommendation

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+# MSC4150: `m.allow` recommendation for moderation policy lists
+
+The [moderation policy lists module] provides data structures to build rule-based moderation features
+for Matrix. Each rule in a policy list has a so called recommendation which describes an action to be
+taken when the rule matches against a certain subject.
+
+At the time of writing, the only supported recommendation is [`m.ban`]. This is sufficient for building
+block-list moderation tools where every entity is permitted by default and only those entities that match
+to rules are denied.
+
+With only a single recommendation, however, it's not possible to _also_ support allow-list systems where
+every entity is denied by default and only those entities that match to rules are permitted. This is
+prominently evidenced by the fact that Mjolnir defines a custom [`org.matrix.mjolnir.allow`] recommendation
+to provide both block-list and allow-list semantics[^1]. 
+
+The proposal at hand attempts to close this gap by standardizing an `m.allow` recommendation.
+
+
+## Proposal
+
+A new recommendation `m.allow` is introduced. When this recommendation is used, the entities affected by
+the rule should be allowed for participation where possible. As with `m.ban`, the enforcement is deliberately
+left as an implementation detail. The existing [suggestions] for `m.ban` could simply be inverted, however.
+
+
+## Potential issues
+
+When both `m.ban` and `m.allow` recommendations are used in the same policy list, confusion could arise if
+two rules with opposing recommendations match against an entity. Given that the spec doesn't currently
+impose _any_ logic for how policy lists are to be interpreted, it could well leave this question as an
+implementation detail, too.
+
+However, if a resolution is indeed desired, the spec could mandate that `m.ban` rules override `m.allow`
+rules but not the other way around. This matches both [Mjolnir's implementation] and the way
+[server access control lists] are evaluated.
+
+
+## Alternatives
+
+None that the author could think of.
+
+
+## Security considerations
+
+None on top of what's already [listed] in the spec.
+
+
+## Unstable prefix
+
+Until this proposal is accepted into the spec, implementations should refer to `m.allow` as
+`org.matrix.msc4150.allow`.
+
+
+## Dependencies
+
+None.
+
+
+[^1]: A concrete use case that requires both block-list and allow-list semantics can be found in the
+      Gematik messenger specification for the healthcare sector in Germany. The latter currently depends
+      on a custom state event to control invite permissions:
+      https://github.com/gematik/api-ti-messenger/blob/9b9f21b87949e778de85dbbc19e25f53495871e2/src/schema/permissionConfig.json
+
+[`m.ban`]: https://spec.matrix.org/v1.10/client-server-api/#mban-recommendation
+[`org.matrix.mjolnir.allow`]: https://github.com/matrix-org/mjolnir/blob/5e35efd1dbc0097a7c19bed2831a1308a29d7be7/src/models/ListRule.ts#L63
+[listed]: https://spec.matrix.org/v1.10/client-server-api/#security-considerations-16
+[Mjolnir's implementation]: https://github.com/matrix-org/mjolnir/blob/5e35efd1dbc0097a7c19bed2831a1308a29d7be7/src/models/AccessControlUnit.ts#L266
+[moderation policy lists module]: https://spec.matrix.org/v1.10/client-server-api/#moderation-policy-lists
+[server access control lists]: https://spec.matrix.org/v1.10/client-server-api/#mroomserver_acl
+[suggestions]: https://spec.matrix.org/v1.10/client-server-api/#mban-recommendation