attest-provider/validation/attest-constraint-template.yaml

apiVersion: templates.gatekeeper.sh/v1
kind: ConstraintTemplate
metadata:
  name: k8sattestexternaldata
spec:
  crd:
    spec:
      names:
        kind: K8sAttestExternalData
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8sexternaldata

        external_data_response := response {
          # build a list of keys containing images
          images := [img | img = input.review.object.spec.containers[_].image]

          # send external data request
          response := external_data({"provider": "attest-provider-validate", "keys": images})
        }

        violation[{"msg": msg}] {
          errs := response_with_error(external_data_response)

          msg := sprintf("invalid response: %v", [errs])
        }

        violation[{"msg": msg}] {
          response := external_data_response.responses[_]
          # a response is a list: [key, result, error]
          image := response[0]
          result := response[1]

          # here we could also check for no_policy if we wanted to fail closed if there is no policy for this image
          result.outcome == "failure"

          msg := sprintf("policy failure for image %v:\n%v", [image, yaml.marshal({"violations": result.violations})])
        }

        response_with_error(response) := errs {
          count(response.errors) > 0
          errs := response.errors
        }

        response_with_error(response) := errs {
          count(response.system_error) > 0
          errs := response.system_error
        }