Understanding Nova Policies

Nova supports a rich policy system that has evolved significantly over its lifetime. Initially, this took the form of a large, mostly hand-written policy.json file but, starting in the Newton (14.0.0) release, policy defaults have been defined in the codebase, requiring the policy.json file only to override these defaults.

In the Ussuri (21.0.0) release, further work was undertaken to address some issues that had been identified:

  1. No global vs project admin. The admin_only role is used for the global admin that is able to make almost any change to Nova, and see all details of the Nova system. The rule passes for any user with an admin role, it doesn’t matter which project is used.

  2. No read-only roles. Since several APIs tend to share a single policy rule for read and write actions, they did not provide the granularity necessary for read-only access roles.

  3. The admin_or_owner role did not work as expected. For most APIs with admin_or_owner, the project authentication happened in a separate component than API in Nova that did not honor changes to policy. As a result, policy could not override hard-coded in-project checks.

Keystone comes with admin, member and reader roles by default. Please refer to this document for more information about these new defaults. In addition, keystone supports a new “system scope” concept that makes it easier to protect deployment level resources from project or system level resources. Please refer to this document and system scope specification to understand the scope concept.

In the Nova 21.0.0 (Ussuri) release, Nova policies implemented the scope concept and default roles provided by keystone (admin, member, and reader). Using common roles from keystone reduces the likelihood of similar, but different, roles implemented across projects or deployments (e.g., a role called observer versus reader versus auditor). With the help of the new defaults it is easier to understand who can do what across projects, reduces divergence, and increases interoperability.

The below sections explain how these new defaults in the Nova can solve the first two issues mentioned above and extend more functionality to end users in a safe and secure way.

More information is provided in the nova specification.

Scope

OpenStack Keystone supports different scopes in tokens. These are described here. Token scopes represent the layer of authorization. Policy scope_types represent the layer of authorization required to access an API.

Note

The scope_type of each policy is hardcoded and is not overridable via the policy file.

Nova policies have implemented the scope concept by defining the scope_type in policies. To know each policy’s scope_type, please refer to the Policy Reference and look for Scope Types or Intended scope(s) in Policy Sample File as shown in below examples.

system scope

Policies with a scope_type of system means a user with a system-scoped token has permission to access the resource. This can be seen as a global role. All the system-level operation’s policies have defaulted to scope_type of ['system'].

For example, consider the GET /os-hypervisors API.

# List all hypervisors.
# GET  /os-hypervisors
# Intended scope(s): system
#"os_compute_api:os-hypervisors:list": "rule:system_reader_api"

project scope

Policies with a scope_type of project means a user with a project-scoped token has permission to access the resource. Project-level only operation’s policies are defaulted to scope_type of ['project'].

For example, consider the POST /os-server-groups API.

# Create a new server group
# POST  /os-server-groups
# Intended scope(s): project
#"os_compute_api:os-server-groups:create": "rule:project_member_api"

system and project scope

Policies with a scope_type of system and project means a user with a system-scoped or project-scoped token has permission to access the resource. All the system and project level operation’s policies have defaulted to scope_type of ['system', 'project'].

For example, consider the POST /servers/{server_id}/action (os-migrateLive) API.

# Live migrate a server to a new host without a reboot
# POST  /servers/{server_id}/action (os-migrateLive)
# Intended scope(s): system, project
#"os_compute_api:os-migrate-server:migrate_live": "rule:system_admin_api"

These scope types provide a way to differentiate between system-level and project-level access roles. You can control the information with scope of the users. This means you can control that none of the project level role can get the hypervisor information.

Policy scope is disabled by default to allow operators to migrate from the old policy enforcement system in a graceful way. This can be enabled by configuring the oslo_policy.enforce_scope option to True.

Note

[oslo_policy] enforce_scope=True

Roles

You can refer to this document to know about all available defaults from Keystone.

Along with the scope_type feature, Nova policy defines new defaults for each policy.

reader

This provides read-only access to the resources within the system or project. Nova policies are defaulted to below rules:

system_reader_api
   Default
      role:reader and system_scope:all

system_or_project_reader
   Default
      (rule:system_reader_api) or (role:reader and project_id:%(project_id)s)

member

This role is to perform the project level write operation with combination to the system admin. Nova policies are defaulted to below rules:

project_member_api
   Default
      role:member and project_id:%(project_id)s

system_admin_or_owner
   Default
      (role:admin and system_scope:all) or (role:member and project_id:%(project_id)s)

admin

This role is to perform the admin level write operation at system as well as at project-level operations. Nova policies are defaulted to below rules:

system_admin_api
   Default
      role:admin and system_scope:all

project_admin_api
   Default
      role:admin and project_id:%(project_id)s

system_admin_or_owner
   Default
      (role:admin and system_scope:all) or (role:member and project_id:%(project_id)s)

With these new defaults, you can solve the problem of:

  1. Providing the read-only access to the user. Polices are made more granular and defaulted to reader rules. For exmaple: If you need to let someone audit your deployment for security purposes.

  2. Customize the policy in better way. For example, you will be able to provide access to project level user to perform live migration for their server or any other project with their token.

Backward Compatibility

Backward compatibility with versions prior to 21.0.0 (Ussuri) is maintained by supporting the old defaults and disabling the scope_type feature by default. This means the old defaults and deployments that use them will keep working as-is. However, we encourage every deployment to switch to new policy. scope_type will be enabled by default and the old defaults will be removed starting in the 23.0.0 (W) release.

To implement the new default reader roles, some policies needed to become granular. They have been renamed, with the old names still supported for backwards compatibility.

Migration Plan

To have a graceful migration, Nova provides two flags to switch to the new policy completely. You do not need to overwrite the policy file to adopt the new policy defaults.

Here is step wise guide for migration:

  1. Create scoped token:

    You need to create the new token with scope knowledge via below CLI:

  2. Create new default roles in keystone if not done:

    If you do not have new defaults in Keystone then you can create and re-run the Keystone Bootstrap. Keystone added this support in 14.0.0 (Rocky) release.

  3. Enable Scope Checks

    The oslo_policy.enforce_scope flag is to enable the scope_type features. The scope of the token used in the request is always compared to the scope_type of the policy. If the scopes do not match, one of two things can happen. If oslo_policy.enforce_scope is True, the request will be rejected. If oslo_policy.enforce_scope is False, an warning will be logged, but the request will be accepted (assuming the rest of the policy passes). The default value of this flag is False.

    Note

    Before you enable this flag, you need to audit your users and make sure everyone who needs system-level access has a system role assignment in keystone.

  4. Enable new defaults

    The oslo_policy.enforce_new_defaults flag switches the policy to new defaults-only. This flag controls whether or not to use old deprecated defaults when evaluating policies. If True, the old deprecated defaults are not evaluated. This means if any existing token is allowed for old defaults but is disallowed for new defaults, it will be rejected. The default value of this flag is False.

    Note

    Before you enable this flag, you need to educate users about the different roles they need to use to continue using Nova APIs.

  5. Check for deprecated policies

    A few policies were made more granular to implement the reader roles. New policy names are available to use. If old policy names which are renamed are overwritten in policy file, then warning will be logged. Please migrate those policies to new policy names.

We expect all deployments to migrate to new policy by 23.0.0 release so that we can remove the support of old policies.