policyDefinitions/Kubernetes/ensure-read-only-access-to-root-filesystem-in-a-kubernetes-cluster/template.yaml

apiVersion: templates.gatekeeper.sh/v1beta1
kind: ConstraintTemplate
metadata:
  name: k8sazurereadonlyrootfilesystem
spec:
  crd:
    spec:
      names:
        kind: K8sAzureReadOnlyRootFilesystem
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8sazurereadonlyrootfilesystem

        violation[{"msg": msg, "details": {}}] {
            c := input_containers[_]
            input_read_only_root_fs(c)
            msg := sprintf("only read-only root filesystem container is allowed: %v", [c.name])
        }

        input_read_only_root_fs(c) {
            not has_field(c, "securityContext")
        }
        input_read_only_root_fs(c) {
            not c.securityContext.readOnlyRootFilesystem == true
        }

        input_containers[c] {
            c := input.review.object.spec.containers[_]
        }
        input_containers[c] {
            c := input.review.object.spec.initContainers[_]
        }

        # has_field returns whether an object has a field
        has_field(object, field) = true {
            object[field]
        }