Role-Based Access Control Overview¶

Introduction¶

Role-Based Access Control (RBAC) is used by most OpenStack services to control user access to resources. Authorization is granted if a user has the necessary role to perform an action. Patrole is concerned with validating that each of these resources can be accessed by authorized users and cannot be accessed by unauthorized users.

OpenStack services use oslo.policy as the library for RBAC authorization. Patrole relies on the same library for deriving expected test results.

Policy in Code¶

Services publish their policy-to-API mapping via policy in code documentation. This mapping includes the list of APIs that authorize a policy, for each policy declared within a service.

For example, Nova’s policy in code documentation is located in the Nova repository under nova/policies. Likewise, Keystone’s policy in code documentation is located in the Keystone repository under keystone/common/policies. The other OpenStack services follow the same directory layout pattern with respect to policy in code.

The policy in code governance goal enumerates many advantages with following this RBAC design approach. A so-called library of in-code policies offers the following advantages, with respect to facilitating validation:

includes every policy enforced by an OpenStack service, enabling the possibility of complete Patrole test coverage for that service (otherwise one has to read the source code to discover all the policies)
provides the policy-to-API mapping for each policy which can be used to write correct Patrole tests (otherwise reading source code and experimentation are required to derive this mapping)
by extension, the policy-to-API mapping facilitates writing multi-policy Patrole tests (otherwise even more experimentation and code reading is required to arrive at all the policies enforced by an API)
policy in code documentation includes additional information, like descriptions and (in the case of some services, like Keystone) scope types, which help with understanding how to correctly write Patrole tests
by extension, such information helps to determine whether a Patrole test should assume hard authorization or soft authorization

Policy in Code (Default) Validation¶

By default, Patrole validates default OpenStack policies. This is so that the out-of-the-box defaults are sanity-checked, to ensure that OpenStack services are secure, from an RBAC perspective, for each release.

Patrole strives to validate RBAC by using the policy in code documentation, wherever possible. See Validation Workflow Overview for more details.

Custom Policies¶

Operators can override policy in code defaults using policy.yaml. While this allows operators to offer more fine-grained RBAC control to their tenants, it opens the door to misconfiguration and bugs. Patrole can be used to validate that custom policy overrides don’t break anything and work as expected.

Custom Policy Validation¶

While testing default policy behavior is a valid use case, oftentimes default policies are modified with custom overrides in production. OpenStack’s policy.yaml documentation claims that “modifying policy can have unexpected side effects”, which is why Patrole was created: to ensure that custom overrides allow the principle of least privilege to be tailor-made to exact specifications via policy overrides, without:

causing unintended side effects (breaking API endpoints, breaking cross-service workflows, breaking the policy file itself); or
resulting in poor RBAC configuration, promoting security vulnerabilities

This has implications on Patrole’s Design Principles: validating custom overrides requires the ability to handle arbitrary roles, which requires logic capable of dynamically determining expected test behavior.

Note that support for custom policies is limited. This is because custom policies can be arbitrarily complex, requiring that tests be very robust in order to handle all edge cases.

Multiple Policies¶

Behind the scenes, many APIs enforce multiple policies, for many reasons, including:

to control complex cross-service workflows;
to control whether a server is booted from an image or booted from a volume (for example);
to control whether a response body should contain additional information conditioned upon successful policy authorization.

This makes policy in code especially important for policy validation: it is difficult to keep track of all the policies being enforced across all the individual APIs, without policy in code documentation.

Multi-Policy Validation¶

Patrole offers support for validating APIs that enforce multiple policies. Perhaps in an ideal world each API endpoint would enforce only one policy, but in reality some API endpoints enforce multiple policies. Thus, to offer accurate validation, Patrole handles multiple policies:

for services with policy in code documentation: this documentation indicates that a single API endpoint enforces multiple policy actions.
for services without policy in code documentation: the API code clearly shows multiple policy actions being validated. Note that in this case some degree of log tracing is required by developers to confirm that the expected policies are getting enforced, prior to the tests getting merged.

For more information, see Multi-policy Validation.

Error Codes¶

Most OpenStack services raise a 403 Forbidden following failed hard authorization. Neutron, however, can raise a 404 NotFound as well. See Neutron’s authorization policy enforcement documentation for more details.

Admin Context Policy¶

The so-called “admin context” policy refers to the following policy definition (using the legacy policy file syntax):

{
  "context_is_admin": "role:admin"
  ...
}

Which is unfortunately used to bypass oslo.policy authorization checks, for example:

# This function is responsible for calling oslo.policy to check whether
# requests are authorized to perform an API action.
def enforce(context, action, target, [...]):
  # Here this condition, if True, skips over the enforce call below which
  # is what calls oslo.policy.
  if context.is_admin:
      return True
  _ENFORCER.enforce([...])  # This is what can be skipped over.
  [...]

This type of behavior is currently present in many services. Unless such logic is removed in the future for services that implement it, Patrole won’t really be able to validate that admin role works from an oslo.policy perspective.

Glossary¶

The following nomenclature is used throughout Patrole documentation so it is important to understand what each term means in order to understand concepts related to RBAC in Patrole.

authorize¶

The act of oslo.policy determining whether a user can perform a policy given his or her role.

enforce¶

See authorize.

hard authorization¶

The do_raise flag controls whether policy authorization should result in an exception getting raised or a boolean value getting returned. Hard authorization results in an exception getting raised. Usually, this results in a 403 Forbidden getting returned for unauthorized requests. (See Error Codes for further details.)

Related term: soft authorization.

oslo.policy¶

The OpenStack library providing support for RBAC policy enforcement across all OpenStack services. See the official documentation for more information.

policy¶

Defines an RBAC rule. Each policy is defined by a one-line statement in the form “<target>” : “<rule>”. For more information, reference OpenStack’s policy documentation.

policy action¶

See policy target.

policy file¶

Prior to governance goal used by all OpenStack services to define policy defaults. Still used by some services, which is why Patrole needs to read the policy files to derive policy information for testing.

policy in code¶

Registers default OpenStack policies for a service in the service’s code base.

Beginning with the Queens release, policy in code became a governance goal.

policy rule¶

The policy rule determines under which circumstances the API call is permitted.

policy target¶

The name of a policy.

requirements file¶

Requirements-driven approach to declaring the expected RBAC test results referenced by Patrole. Uses a high-level YAML syntax to crystallize policy requirements concisely and unambiguously. See Requirements Authority Module for more information.

role¶

A designation for the set of actions that describe what a user can do in the system. Roles are managed through the Keystone Roles API.

Role-Based Access Control (RBAC)¶

May be formally defined as “an approach to restricting system access to authorized users.”

rule¶

See policy rule. Note that currently the Patrole code base conflates “rule” with policy target in some places.

soft authorization¶

The do_raise flag controls whether policy authorization should result in an exception getting raised or a boolean value getting returned. Soft authorization results in a boolean value getting returned. When policy authorization evaluates to true, additional operations are performed as a part of the API request or additional information is included in the response body (see response filtering for an example).

Related term: hard authorization.

Role-Based Access Control Overview