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Chapter 11. Image Policy

11.1. Overview

You can control which images are allowed to run on your cluster using the ImagePolicy admission plug-in (currently considered beta). It allows you to control:

  • The source of images: which registries can be used to pull images
  • Image resolution: force pods to run with immutable digests to ensure the image does not change due to a re-tag
  • Container image label restrictions: force an image to have or not have particular labels
  • Image annotation restrictions: force an image in the integrated container registry to have or not have particular annotations

11.2. Configuring the ImagePolicy Admission Plug-in

To configure which images can run on your cluster, configure the ImagePolicy Admission plug-in in the master-config.yaml file. You can set one or more rules as required.

  • Reject images with a particular annotation:

    Use this rule to reject all images that have a specific annotation set on them. The following rejects all images using the images.openshift.io/deny-execution annotation: 

    - name: execution-denied
  onResources:
  - resource: pods
  - resource: builds
  reject: true
  matchImageAnnotations:
  - key: images.openshift.io/deny-execution 1
    value: "true"
  skipOnResolutionFailure: true
    1
    If a particular image has been deemed harmful, administrators can set this annotation to flag those images.

  • Enable user to run images from Docker Hub:

    Use this rule to allow users to use images from Docker Hub: 

    - name: allow-images-from-dockerhub
  onResources:
    - resource: pods
    - resource: builds
    matchRegistries:
    - docker.io

Following is an example configuration for setting multiple ImagePolicy addmission plugin rules in the master-config.yaml file:

Annotated Example File

admissionConfig:
  pluginConfig:
    openshift.io/ImagePolicy:
      configuration:
        kind: ImagePolicyConfig
        apiVersion: v1
        resolveImages: AttemptRewrite 1
        executionRules: 2
        - name: execution-denied
          # Reject all images that have the annotation images.openshift.io/deny-execution set to true.
          # This annotation may be set by infrastructure that wishes to flag particular images as dangerous
          onResources: 3
          - resource: pods
          - resource: builds
          reject: true 4
          matchImageAnnotations: 5
          - key: images.openshift.io/deny-execution
            value: "true"
          skipOnResolutionFailure: true 6
        - name: allow-images-from-internal-registry
          # allows images from the internal registry and tries to resolve them
          onResources:
          - resource: pods
          - resource: builds
          matchIntegratedRegistry: true
        - name: allow-images-from-dockerhub
          onResources:
          - resource: pods
          - resource: builds
          matchRegistries:
          - docker.io
        resolutionRules: 7
        - targetResource:
            resource: pods
          localNames: true
          policy: AttemptRewrite
        - targetResource: 8
            group: batch
            resource: jobs
          localNames: true 9
          policy: AttemptRewrite

1
Try to resolve images to an immutable image digest and update the image pull specification in the pod.
2
Array of rules to evaluate against incoming resources. If you only have reject: true rules, the default is allow all. If you have any accept rule, that is reject: false in any of the rules, the default behaviour of the ImagePolicy switches to deny-all.
3
Indicates which resources to enforce rules upon. If nothing is specified, the default is pods.
4
Indicates that if this rule matches, the pod should be rejected.
5
List of annotations to match on the image object’s metadata.
6
If you are not able to resolve the image, do not fail the pod.
7
Array of rules allowing use of image streams in Kubernetes resources. The default configuration allows pods, replicationcontrollers, replicasets, statefulsets, daemonsets, deployments, and jobs to use same-project image stream tag references in their image fields.
8
Identifies the group and resource to which this rule applies. If resource is *, this rule will apply to all resources in that group.
9
LocalNames will allow single segment names (for example, ruby:2.4) to be interpreted as namespace-local image stream tags, but only if the resource or target image stream has local name resolution enabled.
Note

If you normally rely on infrastructure images being pulled using a default registry prefix (such as docker.io or registry.access.redhat.com), those images will not match to any matchRegistries value since they will have no registry prefix. To ensure infrastructure images have a registry prefix that can match your image policy, set the imageConfig.format value in your master-config.yaml file.

11.3. Testing the ImagePolicy Admission Plug-in

  1. Use the openshift/image-policy-check to test your configuration.

    For example, use the information above, then test like this: 

    oc import-image openshift/image-policy-check:latest --confirm

  2. Create a pod using this YAML. The pod should be created. 

    apiVersion: v1
kind: Pod
metadata:
  generateName: test-pod
spec:
  containers:
  - image: docker.io/openshift/image-policy-check:latest
    name: first

  3. Create another pod pointing to a different registry. The pod should be rejected. 

    apiVersion: v1
kind: Pod
metadata:
  generateName: test-pod
spec:
  containers:
  - image: different-registry/openshift/image-policy-check:latest
    name: first

  4. Create a pod pointing to the internal registry using the imported image. The pod should be created and if you look at the image specification, you should see a digest in place of the tag. 

    apiVersion: v1
kind: Pod
metadata:
  generateName: test-pod
spec:
  containers:
  - image: <internal registry IP>:5000/<namespace>/image-policy-check:latest
    name: first

  5. Create a pod pointing to the internal registry using the imported image. The pod should be created and if you look at the image specification, you should see the tag unmodified. 

    apiVersion: v1
kind: Pod
metadata:
  generateName: test-pod
spec:
  containers:
  - image: <internal registry IP>:5000/<namespace>/image-policy-check:v1
    name: first

  6. Get the digest from oc get istag/image-policy-check:latest and use it for oc annotate images/<digest> images.openshift.io/deny-execution=true. For example: 

    $ oc annotate images/sha256:09ce3d8b5b63595ffca6636c7daefb1a615a7c0e3f8ea68e5db044a9340d6ba8 images.openshift.io/deny-execution=true

  7. Create this pod again, and you should see the pod rejected: 

    apiVersion: v1
kind: Pod
metadata:
  generateName: test-pod
spec:
  containers:
  - image: <internal registry IP>:5000/<namespace>/image-policy-check:latest
    name: first