Understanding Nova Policies¶

Warning

JSON formatted policy file is deprecated since Nova 22.0.0(Victoria). Use YAML formatted file. Use oslopolicy-convert-json-to-yaml tool to convert the existing JSON to YAML formatted policy file in backward compatible way.

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.yaml file but, starting in the Newton (14.0.0) release, policy defaults have been defined in the codebase, requiring the policy.yaml 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:

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.
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.
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 25.0.0 (Yoga) 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 to project scoped and is not overridable via the policy file.

Nova policies have implemented the scope concept by defining the scope_type for all the policies to project scoped. It means if user tries to access nova APIs with system scoped token they will get 403 permission denied error.

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"

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. Nova policies are defaulted to below rules:

policy.RuleDefault(
    name="project_reader",
    check_str="role:reader and project_id:%(project_id)s",
    description="Default rule for Project level read only APIs."
)

Using it in policy rule (with admin + reader access): (because we want to keep legacy admin behavior the same we need to give access of reader APIs to admin role too.)

policy.DocumentedRuleDefault(
    name='os_compute_api:servers:show',
    check_str='role:admin or (' + 'role:reader and project_id:%(project_id)s)',
    description="Show a server",
    operations=[
        {
            'method': 'GET',
            'path': '/servers/{server_id}'
        }
    ],
    scope_types=['project'],
)

policy.RuleDefault(
    name="admin_api",
    check_str="role:admin",
    description="Default rule for administrative APIs."
)

policy.DocumentedRuleDefault(
    name='os_compute_api:servers:show',
    check_str='rule: admin or rule:project_reader',
    description='Show a server',
    operations=[
        {
            'method': 'GET',
            'path': '/servers/{server_id}'
        }
    ],
    scope_types=['project'],
)

member

project-member is denoted by someone with the member role on a project. It is intended to be used by end users who consume resources within a project which requires higher permission than reader role but less than admin role. It inherits all the permissions of a project-reader.

project-member persona in the policy check string:

policy.RuleDefault(
    name="project_member",
    check_str="role:member and project_id:%(project_id)s",
    description="Default rule for Project level non admin APIs."
)

Using it in policy rule (with admin + member access): (because we want to keep legacy admin behavior, admin role gets access to the project level member APIs.)

policy.DocumentedRuleDefault(
    name='os_compute_api:servers:create',
    check_str='role:admin or (' + 'role:member and project_id:%(project_id)s)',
    description='Create a server',
    operations=[
        {
            'method': 'POST',
            'path': '/servers'
        }
    ],
    scope_types=['project'],
)

policy.RuleDefault(
    name="admin_api",
    check_str="role:admin",
    description="Default rule for administrative APIs."
)

policy.DocumentedRuleDefault(
    name='os_compute_api:servers:create',
    check_str='rule_admin or rule:project_member',
    description='Create a server',
    operations=[
        {
            'method': 'POST',
            'path': '/servers'
        }
    ],
    scope_types=['project'],
)

‘project_id:%(project_id)s’ in the check_str is important to restrict the access within the requested project.

admin

This role is to perform the admin level write operations. Nova policies are defaulted to below rules:

policy.DocumentedRuleDefault(
    name='os_compute_api:os-hypervisors:list',
    check_str='role:admin',
    scope_types=['project']
)

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

Providing the read-only access to the user. Polices are made more granular and defaulted to reader rules. For example: If you need to let someone audit your deployment for security purposes.
Customize the policy in better way. For example, you will be able to provide access to project level user to perform operations within their project only.

Nova supported scope & Roles¶

Nova supports the below combination of scopes and roles where roles can be overridden in the policy.yaml file but scope is not override-able.

ADMIN: admin role on project scope. This is an administrator to perform the admin level operations. Example: enable/disable compute service, Live migrate server etc.
PROJECT_MEMBER: member role on project scope. This is used to perform resource owner level operation within project. For example: Pause a server.
PROJECT_READER: reader role on project scope. This is used to perform read-only operation within project. For example: Get server.
PROJECT_MEMBER_OR_ADMIN: admin or member role on project scope. Such policy rules are default to most of the owner level APIs and aling with member role legacy admin can continue to access those APIs.
PROJECT_READER_OR_ADMIN: admin or reader role on project scope. Such policy rules are default to most of the read only APIs so that legacy admin can continue to access those APIs.

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 the new policy. The new defaults will be enabled by default in OpenStack 2023.1 (Nova 27.0.0) release and old defaults will be removed starting in the OpenStack 2023.2 (Nova 28.0.0) 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:

Create scoped token:

You need to create the new token with scope knowledge via below CLI:
- Create Project Scoped Token.
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.
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.
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.
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.

NOTE:

We recommend to enable the both scope as well new defaults together
otherwise you may experience some late failures with unclear error
messages. For example, if you enable new defaults and disable scope
check then it will allow system users to access the APIs but fail
later due to the project check which can be difficult to debug.

Below table show how legacy rules are mapped to new rules:

Legacy Rule	New Rules	Operation	scope_type
RULE_ADMIN_API	-> ADMIN	Global resource Write & Read	[project]
RULE_ADMIN_OR_OWNER	-> ADMIN	Project admin level operation	[project]
	-> PROJECT_MEMBER_OR_ADMIN	Project resource Write	[project]
	-> PROJECT_READER_OR_ADMIN	Project resource Read	[project]

We expect all deployments to migrate to the new policy by OpenStack 2023.1 (Nova 27.0.0) release so that we can remove the support of old policies.

Understanding Nova Policies

Understanding Nova Policies¶

Scope¶

Roles¶

Nova supported scope & Roles¶

Backward Compatibility¶

Migration Plan¶

nova 27.2.1.dev33

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