Understanding Tacker Policies

Tacker supports RBAC policy system to control the access permission of APIs. Tacker RBAC policy rule have the default value which can be overridden by operators via policy.yaml file.

We try to make the policy default value as close to operators requirements. In the Tacker 11.0.0 (OpenStack 2024.1 caracal) release, further work was undertaken to address some issues that had been identified:

1. 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. To solve this, we should have reader role in the policy.

2. The admin_or_owner rule did not work as expected. It has check_str "is_admin:True or project_id:%(project_id)s", which means it only check project_id for owner and not any role. This means admin_or_owner` rule will allow user with any role in project. For example, user with role:foo in project will behaves as the owner of the project resources. To solve this we should also check the member role in admin_or_owner rule.

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

In the Tacker 11.0.0 (OpenStack 2024.1 caracal), Tacker policies implemented the new 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. Along with new defaults, Tacker policy define scope_type which is hardcoded to project so that if system scope token try to access the Tacker APIs, they can get better error message.

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

Scope

OpenStack Keystone supports different scopes in tokens. 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.

Tacker 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 Tacker APIs with system scoped token they will get 403 permission denied error.

For example, consider the POST /vnflcm/v1/vnf_instances API.

# Creates a new VNF instance resource
# POST  /vnflcm/v1/vnf_instances
# Intended scope(s): project
#"os_nfv_orchestration_api:vnf_instances:create": "rule:project_member_or_admin"

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 in tacker.conf on controller node.

Note

[oslo_policy] enforce_scope=True

Roles

You can refer to keystone role documentation to know about all available defaults from Keystone.

Tacker policy defines new defaults for each policy.

reader

This provides read-only access to the resources. Tacker policies are defaulted to below rules:

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

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_nfv_orchestration_api:vnf_instances:show',
    check_str='role:admin or (' + 'role:reader and project_id:%(project_id)s)',
    description="Query an Individual VNF instance.",
    operations=[
        {
            'method': 'GET',
            'path': '/vnflcm/v1/vnf_instances/{vnfInstanceId}'
        }
    ],
    scope_types=['project'],
)

OR

policy.DocumentedRuleDefault(
    name='os_nfv_orchestration_api:vnf_instances:show',
    check_str='rule: admin or rule:project_reader',
    description="Query an Individual VNF instance.",
    operations=[
        {
            'method': 'GET',
            'path': '/vnflcm/v1/vnf_instances/{vnfInstanceId}'
        }
    ],
    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="admin_api",
    check_str="role:admin",
    description="Default rule for administrative APIs."
)

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_nfv_orchestration_api:vnf_instances:create',
    check_str='role:admin or (' + 'role:member and project_id:%(project_id)s)',
    description="Creates vnf instance.",
    operations=[
        {
            'method': 'POST',
            'path': '/vnflcm/v1/vnf_instances/{vnfInstanceId}'
        }
    ],
    scope_types=['project'],
)

OR

policy.DocumentedRuleDefault(
    name='os_nfv_orchestration_api:vnf_instances:create',
    check_str='rule: admin or rule:project_member',
    description="Query an Individual VNF instance.",
    operations=[
        {
            'method': 'POST',
            'path': '/vnflcm/v1/vnf_instances/{vnfInstanceId}'
        }
    ],
    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. Tacker policies are defaulted to below rules:

policy.DocumentedRuleDefault(
    name='Polciy name',
    check_str='role:admin',
    scope_types=['project']
)

Tacker supported scope & Roles

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

1. ADMIN: admin role on project scope. This is an administrator to perform the admin level operations.

2. PROJECT_MEMBER: member role on project scope. This is used to perform resource owner level operation within project. For example: create vnf instance.

3. PROJECT_READER: reader role on project scope. This is used to perform read-only operation within project. For example: Get vnf instance.

4. PROJECT_MEMBER_OR_ADMIN: admin or member role on project scope. Such policy rules are default to most of the owner level APIs and align with member role legacy admin can continue to access those APIs.

5. 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 Tacker 11.0.0 (OpenStack 2024.1 Caracal) is maintained by supporting the old defaults 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 Tacker 12.0.0 (OpenStack 2024.2 Dalmatian) release but we will keep the old default in deprecated defaults will be removed starting in the Tacker 15.0.0 (OpenStack 2026.1) release.

Migration Plan

To have a graceful migration, Tacker 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 project scoped token via below CLI:

   • Create Project Scoped Token.

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.

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.

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 Tacker APIs.

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 Tacker 13.0.0 (OpenStack 2025.1) release so that we can remove the support of old policies in future release.