Enforce SBOM attestation with Policy Controller

This guide demonstrates how to use the Sigstore Policy Controller to verify image attestations before admitting an image into a Kubernetes cluster. In this guide, you will create a ClusterImagePolicy that checks the existence of a SBOM attestation attached to a container image, and then test the admission controller by running a registry.enforce.dev/chainguard/node image with SBOM attestations.

Prerequisites

To follow along with this guide, you will need the following:

• A Kubernetes cluster with administrative access. You can set up a local cluster using kind or use an existing cluster.
• kubectl — to work with your cluster. Install kubectl for your operating system by following the official Kubernetes kubectl documentation.
• Sigstore Policy Controller installed in your cluster. Follow our How To Install Sigstore Policy Controller guide if you do not have it installed, and be sure to label any namespace that you intend to use with the policy.sigstore.dev/include=true label.

Once you have everything in place you can continue to the first step and confirm that the Policy Controller is working as expected.

Step 1 - Checking the Policy Controller is Denying Admission

Before creating a ClusterImagePolicy, check that the Policy Controller is deployed and that your default namespace is labeled correctly. Run the following to check that the deployment is complete:

kubectl -n cosign-system wait --for=condition=Available deployment/policy-controller-webhook && \
kubectl -n cosign-system wait --for=condition=Available deployment/policy-controller-policy-webhook

When both deployments are finished, verify the default namespace is using the Policy Controller:

kubectl get ns -l policy.sigstore.dev/include=true

You should receive output like the following:

NAME      STATUS   AGE
default   Active   24s

Once you are sure that the Policy Controller is deployed and your default namespace is configured to use it, run a pod to make sure admission requests are handled and denied by default:

kubectl run --image k8s.gcr.io/pause:3.9 test

Since there is no ClusterImagePolicy defined yet, the Policy Controller will deny the admission request with a message like the following:

Error from server (BadRequest): admission webhook "policy.sigstore.dev" denied the request: validation failed: no matching policies: spec.containers[0].image
k8s.gcr.io/pause@sha256:7031c1b283388d2c2e09b57badb803c05ebed362dc88d84b480cc47f72a21097

In the next step, you will define a policy that verifies Chainguard Images have a SBOM attestation and apply it to your cluster.

Step 2 — Creating a ClusterImagePolicy

Now that you have the Policy Controller running in your cluster, and have the default namespace configured to use it, you can now define a ClusterImagePolicy to admit images.

Open a new file with nano or your preferred editor:

nano /tmp/cip.yaml

Copy the following policy to the /tmp/cip.yaml file:

# Copyright 2022 Chainguard, Inc.
# SPDX-License-Identifier: Apache-2.0
apiVersion: policy.sigstore.dev/v1beta1
kind: ClusterImagePolicy
metadata:
  name: must-have-spdx-cue
  annotations:
    catalog.chainguard.dev/title: Enforce SBOM attestation
    catalog.chainguard.dev/description: Enforce a signed SPDX SBOM attestation from a custom key
    catalog.chainguard.dev/labels: attestation,cue
spec:
  images:
    - glob: "**"
  authorities:
    - name: my-authority
      keyless:
        identities:
          - issuer: "https://token.actions.githubusercontent.com"
            subject: "https://github.com/chainguard-images/images/.github/workflows/release.yaml@refs/heads/main"
      attestations:
        - name: must-have-spdx-attestation
          predicateType: https://spdx.dev/Document
          policy:
            type: cue
            data: |
                            predicateType: "https://spdx.dev/Document"

The glob: ** line, working in combination with the authorities and policy sections, will allow any image that has at least a SBOM attestation with predicate type https://spdx.dev/Document to be admitted into your cluster.

Save the file and then apply the policy:

kubectl apply -f /tmp/cip.yaml

You will receive output showing the policy is created:

clusterimagepolicy.policy.sigstore.dev/must-have-spdx-cue created

Now run the k8s.gcr.io/pause:3.9 image which does not have a SBOM attestation:

kubectl run --image k8s.gcr.io/pause:3.9 noattestedimage

Since the image does not contain any attached SBOM, you will receive a message that the pod was rejected:

Error from server (BadRequest): admission webhook "policy.sigstore.dev" denied the request: validation failed: failed policy: demo: spec.containers[0].image
k8s.gcr.io/pause:3.9 no matching attestations with type https://spdx.dev/Document

Finally, we run registry.enforce.dev/chainguard/node image which contains a SBOM attestation of type https://spdx.dev/Document:

kubectl run --image registry.enforce.dev/chainguard/node mysbomattestedimage

Since the image has now a SBOM attestation, you will receive a message that the pod was created successfully:

pod/mysbomattestedimage created

Delete the pod once you’re done experimenting with it:

kubectl delete pod mysbomattestedimage

To learn more about how the Policy Controller uses Cosign to verify and admit images, review the Cosign Sigstore documentation.