API policy enforcement

API policy is implemented using the Open Policy Agent (OPA). It is a very powerful tool and allows implementing policies much more complex than what the oslo.policy would ever allow. The policy folder contain default policies. They can be overloaded by the deployment.

OPA can be integrated into Keystone in 2 ways:

• HTTP. This is a default and recommended way of integrating applications with the OPA. Usually the OPA process is started as a side car container to keep network latencies as low as possible. Policies themselves are bundled into the container which OPA process is capable of downloading and even periodically refreshing. It can be started as opa run -s --log-level debug tools/opa-config.yaml. Alternatively the OPA process can itself run in the container in which case the configuration file should be mounted as a volume and referred from the entrypoint.

• WASM. Policies can be built into a WASM binary module. This method does not support feeding additional data and dynamic policy reload as of now. Unfortunately there is also a memory access violation error in the wasmtime crate happening for the big policy files. The investigation is in progress, so it is preferred not to rely on this method anyway. While running OPA as a WASM eliminates any networking communication, it heavily reduces feature set. In particular hot policy reload, decision logging, external calls done by the policies themselves are not possible by design. Using this way of policy enforcement requires wasm feature enabled.

All the policies currently are using the same policy names and definitions as the original Keystone to keep the deviation as less as possible. For the newly added APIs this is not anymore the case.

With the Open Policy Agent it is not only possible to define a decision (allowed or forbidden), but also to produce additional information describing i.e. reason of the request refusal. This is currently being used by the policies by defining an array of "violation" objects explaining missing permissions.

Sample policy for updating the federated IDP mapping:

package identity.mapping_update

# update mapping.

default allow := false

allow if {
	"admin" in input.credentials.roles
}

allow if {
	own_mapping
	"manager" in input.credentials.roles
}

own_mapping if {
	input.target.domain_id != null
	input.target.domain_id == input.credentials.domain_id
}

violation contains {"field": "domain_id", "msg": "updating mapping for other domain requires `admin` role."} if {
	identity.foreign_mapping
	not "admin" in input.credentials.roles
}

violation contains {"field": "role", "msg": "updating global mapping requires `admin` role."} if {
	identity.global_mapping
	not "admin" in input.credentials.roles
}

violation contains {"field": "role", "msg": "updating mapping requires `manager` role."} if {
	identity.own_mapping
	not "member" in input.credentials.roles
}

As can be guessed such policy would permit the API request when admin role is present in the current credentials roles or the mapping in scope is owned by the domain the user is currently scoped to with the manager role.`

Additional improvement from the legacy Keystone is the time and data when the policies are evaluated. For list operation policy input is populated with the credentials and all query parameters. For show operation the input additionally contain the target object previously fetched so that the policy can additionally consider current resource attributes. create operation also gets the complete input. update operation first fetch the target resource and pass it as the target, while the updated properties are passed as the "update" object into the policy. The delete operation also fetches the to be deleted object passing it into the policy. This approach allow advanced cases where operations may need to be prohibited by certain resource attributes.