Document IaaS roles (#240)

Signed-off-by: Markus Hentsch <[email protected]>

Author: markus-hentsch

docs/05-iam/iaas-roles.md

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+# IaaS Roles
+
+An SCS-compliant cloud offers the following roles on the Infrastructure as a Service (IaaS) layer:
+
+| Role | Scope | Description |
+|---|---|---|
+| reader | All OpenStack APIs | Role with read-only access within authentication scope. |
+| member | All OpenStack APIs | Role with read-write access within authentication scope. |
+| manager | Identity API | Role with self-service capabilites for the Identity API in domain scope. |
+| admin | All OpenStack APIs | Most privileged role, reserved for CSP personnel, to administer the OpenStack cloud |
+
+:::info Role Inheritance
+
+The OpenStack Identity API implements automatic role inheritance between admin, manager, member and reader roles where a role with higher privileges inherits all lesser roles automatically.
+
+This means:
+
+- `admin` inherits `manager`, `member` and `reader`
+- `manager` inherits `member` and `reader`
+- `member` inherits `reader`
+
+:::
+
+## Role Assignment
+
+### Scopes & Targets
+
+Roles can be assigned in different scopes and either to users or groups.
+This leads to the following role assignment matrix:
+
+| | User | Group |
+|---|---|---|
+| Project | single user, permissions in project | all users of group, permissions in project |
+| Domain | single user, permissions in domain | all users of group, permissions in domain |
+| System | single user, permissions in whole cloud | all users of group, permissions in whole cloud |
+
+(rows are the scopes and columns are the role assignment target entities)
+
+### Assignment Procedure
+
+Roles are assigned to users or groups by administrative personnel.
+In most cases, this is either:
+
+- a cloud operator with the admin role, belonging to the provider
+- a domain manager with the manager role in domain scope, belonging to the provider or a tenant
+
+A domain manager can only assign and revoke roles within their own domain and is restricted to a specific subset of available roles to prevent privilege escalation.