Read Only Root Filesystem

Description

Requires the use of a read-only root file system by pod containers. Corresponds to the readOnlyRootFilesystem field in a PodSecurityPolicy. For more information, see https://kubernetes.io/docs/concepts/policy/pod-security-policy/#volumes-and-file-systems

Template

apiVersion: templates.gatekeeper.sh/v1
kind: ConstraintTemplate
metadata:
  name: k8spspreadonlyrootfilesystem
  annotations:
    metadata.gatekeeper.sh/title: "Read Only Root Filesystem"
    metadata.gatekeeper.sh/version: 1.0.1
    description: >-
      Requires the use of a read-only root file system by pod containers.
      Corresponds to the `readOnlyRootFilesystem` field in a
      PodSecurityPolicy. For more information, see
      https://kubernetes.io/docs/concepts/policy/pod-security-policy/#volumes-and-file-systems
spec:
  crd:
    spec:
      names:
        kind: K8sPSPReadOnlyRootFilesystem
      validation:
        # Schema for the `parameters` field
        openAPIV3Schema:
          type: object
          description: >-
            Requires the use of a read-only root file system by pod containers.
            Corresponds to the `readOnlyRootFilesystem` field in a
            PodSecurityPolicy. For more information, see
            https://kubernetes.io/docs/concepts/policy/pod-security-policy/#volumes-and-file-systems
          properties:
            exemptImages:
              description: >-
                Any container that uses an image that matches an entry in this list will be excluded
                from enforcement. Prefix-matching can be signified with `*`. For example: `my-image-*`.

                It is recommended that users use the fully-qualified Docker image name (e.g. start with a domain name)
                in order to avoid unexpectedly exempting images from an untrusted repository.
              type: array
              items:
                type: string
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8spspreadonlyrootfilesystem

        import data.lib.exclude_update.is_update
        import data.lib.exempt_container.is_exempt

        violation[{"msg": msg, "details": {}}] {
            # spec.containers.readOnlyRootFilesystem field is immutable.
            not is_update(input.review)

            c := input_containers[_]
            not is_exempt(c)
            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[_]
        }
        input_containers[c] {
            c := input.review.object.spec.ephemeralContainers[_]
        }

        # has_field returns whether an object has a field
        has_field(object, field) = true {
            object[field]
        }
      libs:
        - |
          package lib.exclude_update

          is_update(review) {
              review.operation == "UPDATE"
          }
        - |
          package lib.exempt_container

          is_exempt(container) {
              exempt_images := object.get(object.get(input, "parameters", {}), "exemptImages", [])
              img := container.image
              exemption := exempt_images[_]
              _matches_exemption(img, exemption)
          }

          _matches_exemption(img, exemption) {
              not endswith(exemption, "*")
              exemption == img
          }

          _matches_exemption(img, exemption) {
              endswith(exemption, "*")
              prefix := trim_suffix(exemption, "*")
              startswith(img, prefix)
          }

Usage

kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper-library/master/library/pod-security-policy/read-only-root-filesystem/template.yaml

Examples

require-read-only-root-filesystem

constraint

apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sPSPReadOnlyRootFilesystem
metadata:
  name: psp-readonlyrootfilesystem
spec:
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Pod"]

Usage

kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper-library/master/library/pod-security-policy/read-only-root-filesystem/samples/psp-readonlyrootfilesystem/constraint.yaml

example-disallowed

apiVersion: v1
kind: Pod
metadata:
  name: nginx-readonlyrootfilesystem-disallowed
  labels:
    app: nginx-readonlyrootfilesystem
spec:
  containers:
  - name: nginx
    image: nginx
    securityContext:
      readOnlyRootFilesystem: false

Usage

kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper-library/master/library/pod-security-policy/read-only-root-filesystem/samples/psp-readonlyrootfilesystem/example_disallowed.yaml

example-allowed

apiVersion: v1
kind: Pod
metadata:
  name: nginx-readonlyrootfilesystem-allowed
  labels:
    app: nginx-readonlyrootfilesystem
spec:
  containers:
  - name: nginx
    image: nginx
    securityContext:
      readOnlyRootFilesystem: true

Usage

kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper-library/master/library/pod-security-policy/read-only-root-filesystem/samples/psp-readonlyrootfilesystem/example_allowed.yaml

disallowed-ephemeral

apiVersion: v1
kind: Pod
metadata:
  name: nginx-readonlyrootfilesystem-disallowed
  labels:
    app: nginx-readonlyrootfilesystem
spec:
  ephemeralContainers:
  - name: nginx
    image: nginx
    securityContext:
      readOnlyRootFilesystem: false

Usage

kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper-library/master/library/pod-security-policy/read-only-root-filesystem/samples/psp-readonlyrootfilesystem/disallowed_ephemeral.yaml