Configuring Supply Chain Security Tools - Policy

This topic describes how you can configure Supply Chain Security Tools - Policy. SCST - Policy requires extra configuration steps to verify your container images.

Admission of Images

An image is admitted after it is validated against all policies with matching image patterns, and where at least one valid signature is obtained from the authorities provided in the matched ClusterImagePolicy later in the topic. Within a single policy, every signature must be valid. When more than one policy has a matching image pattern, the image must match at least one signature from each ClusterImagePolicy.

Including Namespaces

The Policy Controller only validates resources in namespaces that have chosen to opt-in. This is done by adding the label policy.sigstore.dev/include: "true" to the namespace resource.

kubectl label namespace my-secure-namespace policy.sigstore.dev/include=true

Caution
Without a Policy Controller ClusterImagePolicy applied, there are fallback behaviors where images are validated against the public Sigstore Rekor and Fulcio servers by using a keyless authority flow. Therefore, if the deploying image is signed publicly by a third-party using the keyless authority flow, the image is admitted as it can validate against the public Rekor and Fulcio. To avoid this behavior, develop, and apply a ClusterImagePolicy that applies to the images being deployed in the namespace.

Create a `ClusterImagePolicy` resource

The cluster image policy is a custom resource containing the following properties:

images: The images block defines the patterns of images that must be subject to the ClusterImagePolicy. If multiple policies match a particular image, ALL of those policies must be satisfied for the image to be admitted.

Policy Controller by default defines if the following globs are specified:
- If * is specified, the glob matching behavior is index.docker.io/library/*.
- If */* is specified, the glob matching behavior is index.docker.io/*/*. With these defaults, you require the glob pattern ** to match against all images. If your image is hosted on Docker Hub, include index.docker.io as the host for the glob.
authorities: The authorities block defines the rules for discovering and validating signatures. Discovery is done by using the sources text box, and is specified on any entry. Signatures are cryptographically verified using one of the key or keyless text boxes.

When a policy is selected to be evaluated against the matched image, the authorities are used to validate signatures. If at least one authority is satisfied and a signature is validated, the policy is validated.

`mode`

In a ClusterImagePolicy, spec.mode specifies the action of a policy:

enforce: The default behavior. If the policy fails to validate the image, the policy fails.
warn: If the policy fails to validate the image, validation error messages are converted to Warnings and the policy passes.

A sample of a ClusterImagePolicy which has warn mode configured.

---
apiVersion: policy.sigstore.dev/v1beta1
kind: ClusterImagePolicy
metadata:
  name: POLICY-NAME
spec:
  mode: warn

When enforce mode rejects an image, the image is not admitted.

Sample output message:

error: failed to patch: admission webhook "policy.sigstore.dev" denied the request: validation failed: failed policy: POLICY-NAME: spec.template.spec.containers[0].image
IMAGE-REFERENCE signature key validation failed for authority authority-0 for IMAGE-REFERENCE: GET IMAGE-SIGNATURE-REFERENCE: DENIED: denied; denied
failed policy: <POLICY_NAME>: spec.template.spec.containers[1].image
IMAGE-REFERENCE signature key validation failed for authority authority-0 for IMAGE-REFERENCE: GET IMAGE-SIGNATURE-REFERENCE: DENIED: denied; denied

When warn mode rejects an image, the image is admitted.

Sample output message:

Warning: failed policy: POLICY-NAME: spec.template.spec.containers[0].image
Warning: IMAGE-REFERENCE signature key validation failed for authority authority-0 for IMAGE-REFERENCE: GET IMAGE-SIGNATURE-REFERENCE: DENIED: denied; denied
Warning: failed policy: POLICY-NAME: spec.template.spec.containers[1].image
Warning: IMAGE-REFERENCE signature key validation failed for authority authority-0 for IMAGE-REFERENCE: GET IMAGE-SIGNATURE-REFERENCE: DENIED: denied; denied

If a namespace contains both signed and unsigned images, utilizing two ClusterImagePolicies can address this. One policy can be configured with enforce for images that are signed and the other policy can be configured with warn to allow expected unsigned images.

For example, allowing unsigned tap-packages images required for the platform through a warn policy. However, the signed images produced from Tanzu Build Service are verified with an enforce policy.

If Warning is undesirable, you might configure a static.action pass authority to allow expected unsigned images. For information about static action authorities, see the Static Action documentation.

`images`

In a ClusterImagePolicy, spec.images specifies a list of glob matching patterns. These patterns are matched against the image digest in PodSpec for resources attempting deployment.

Policy Controller defines the following globs by default: - If * is specified, the glob matching behavior is index.docker.io/library/*. - If */* is specified, the glob matching behavior is index.docker.io/*/*.

With these defaults, you require the glob pattern ** to match against all images. If your image is hosted on Docker Hub, include index.docker.io as the host for the glob.

A sample of a ClusterImagePolicy which matches against all images using glob:

apiVersion: policy.sigstore.dev/v1beta1
kind: ClusterImagePolicy
metadata:
  name: image-policy
spec:
  images:
  - glob: "**"

`authorities`

Authorities listed in the authorities block of the ClusterImagePolicy are key or keyless specifications.

Each key authority can contain a PEM-encoded ECDSA public key, a secretRef, or a kms path.

Important
Only ECDSA public keys are supported.

spec:
  authorities:
    - key:
        data: |
          -----BEGIN PUBLIC KEY-----
          ...
          -----END PUBLIC KEY-----
    - key:
        secretRef:
          name: secretName
    - key:
        kms: KMSPATH

Note
The secret referenced in key.secretRef.name must be created in the cosign-system namespace or the namespace where the Policy Controller is installed. Such secret must only contain one data entry with the public key.

Each keyless authority can contain a Fulcio URL, a Rekor URL, a certificate, or an array of identities.

spec:
  authorities:
    - keyless:
        url: https://fulcio.example.com
        ca-cert:
          data: Certificate Data
      ctlog:
        url: https://rekor.example.com
    - keyless:
        url: https://fulcio.example.com
        ca-cert:
          secretRef:
            name: secretName
    - keyless:
        identities:
          - issuer: https://accounts.google.com
            subject: .*@example.com
          - issuer: https://token.actions.githubusercontent.com
            subject: https://github.com/mycompany/*/.github/workflows/*@*

The authorities are evaluated using the “any of” operator to admit container images. For each pod, the Policy Controller iterates over the list of containers and init containers. For every policy that matches against the images, they must each have at least one valid signature obtained using the authorities specified. If an image does not match any policy, the Policy Controller does not admit the image.

`static.action`

ClusterImagePolicy authorities are configured to always pass or fail with static.action.

Sample ClusterImagePolicy with static action fail.

apiVersion: policy.sigstore.dev/v1beta1
kind: ClusterImagePolicy
metadata:
  name: POLICY-NAME
spec:
  authorities:
  - static:
      action: fail

A sample output of static action fail:

error: failed to patch: admission webhook "policy.sigstore.dev" denied the request: validation failed: failed policy: POLICY-NAME: spec.template.spec.containers[0].image
IMAGE-REFERENCE disallowed by static policy
failed policy: POLICY-NAME: spec.template.spec.containers[1].image
IMAGE-REFERENCE disallowed by static policy

Images that are unsigned in a namespace with validation enabled are admitted with an authority with static action pass.

A scenario where this applies is configuring a policy with static.action pass for tap-packages images. Another policy is then configured to validate signed images produced by Tanzu Build Service. This allows images from tap-packages, which are unsigned and required by the platform, to be admitted while still validating signed built images from Tanzu Build Service. See Configure your supply chain to sign and verify your image builds for an example.

If Warning messages are desirable for admitted images where validation failed, you can configure a policy with warn mode and valid authorities. For information about ClusterImagePolicy modes, see the Mode documentation.

Provide credentials for the package

There are three ways the package reads credentials to authenticate to registries protected by authentication:

Reading imagePullSecrets directly from the resource being admitted. See Container image pull secrets in the Kubernetes documentation.
Reading imagePullSecrets from the service account the resource is running as. See Arranging for imagePullSecrets to be automatically attached in the Kubernetes documentation.
Reading a secretRef from the ClusterImagePolicy resource’s signaturePullSecrets when specifying the cosign signature source.

Authentication can fail for the following scenarios:

A not valid credential is specified in the imagePullSecrets of the resource or in the service account the resource runs as.
A not valid credential is specified in the ClusterImagePolicy signaturePullSecrets text box.

Provide secrets for authentication in your policy

You can provide secrets for authentication as part of the policy configuration. The oci location is the image location or a remote location where signatures are configured to be stored during signing. The signaturePullSecrets is available in the cosign-system namespace or the namespace where the Policy Controller is installed.

By default, imagePullSecrets from the resource or service account is used while the default oci location is the image location.

See the following example:

spec:
  authorities:
    - key:
        data: |
          -----BEGIN PUBLIC KEY-----
          ...
          -----END PUBLIC KEY-----
      source:
        - oci: registry.example.com/project/signature-location
          signaturePullSecrets:
            - name: mysecret
    - keyless:
        url: https://fulcio.example.com
      source:
        - oci: registry.example.com/project/signature-location
          signaturePullSecrets:
            - name: mysecret

VMware recommends using a set of credentials with the least amount of privilege that allows reading the signature stored in your registry.

Verify your configuration

A sample policy:

apiVersion: policy.sigstore.dev/v1beta1
kind: ClusterImagePolicy
metadata:
  name: image-policy
spec:
  images:
  - glob: "gcr.io/projectsigstore/cosign*"
  authorities:
  - name: official-cosign-key
    key:
      data: |
        -----BEGIN PUBLIC KEY-----
        MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEhyQCx0E9wQWSFI9ULGwy3BuRklnt
        IqozONbbdbqz11hlRJy9c7SG+hdcFl9jE9uE/dwtuwU2MqU9T/cN0YkWww==
        -----END PUBLIC KEY-----

When using the sample policy, run these commands to verify your configuration:

Verify that the Policy Controller admits the signed image that validates with the configured public key. Run:

kubectl run cosign \
  --image=gcr.io/projectsigstore/cosign:v1.2.1 \
  --dry-run=server

For example:

$ kubectl run cosign \
  --image=gcr.io/projectsigstore/cosign:v1.2.1 \
  --dry-run=server
pod/cosign created (server dry run)

Verify that the Policy Controller rejects the unmatched image. Run:

kubectl run busybox --image=busybox --dry-run=server

For example:

$ kubectl run busybox --image=busybox --dry-run=server
  Error from server (BadRequest): admission webhook "policy.sigstore.dev" denied the request: validation failed: no matching policies: spec.containers[0].image
  index.docker.io/library/busybox@sha256:3614ca5eacf0a3a1bcc361c939202a974b4902b9334ff36eb29ffe9011aaad83

In the output, it did not specify which authorities were used as there was no policy found that matched the image. Therefore, the image fails to validate for a signature and fails to deploy.

Verify that the Policy Controller rejects a matched image signed with a different key than the one configured. Run:

kubectl run cosign-fail \
  --image=gcr.io/projectsigstore/cosign:v0.3.0 \
  --dry-run=server

For example:

$ kubectl run cosign-fail \
    --image=gcr.io/projectsigstore/cosign:v0.3.0 \
    --dry-run=server
  Error from server (BadRequest): admission webhook "policy.sigstore.dev" denied the request: validation failed: failed policy: image-policy: spec.containers[0].image
  gcr.io/projectsigstore/cosign@sha256:135d8c5e27bdc917f04b415fc947d7d5b1137f99bb8fa00bffc3eca1856e9c52 failed to validate public keys with authority official-cosign-key for gcr.io/projectsigstore/cosign@sha256:135d8c5e27bdc917f04b415fc947d7d5b1137f99bb8fa00bffc3eca1856e9c52: no matching signatures:

In the output, it specifies which authorities were used for validation when a policy was found that matched the image. In this case, the authority used was official-cosign-key. If no name is specified, it is defaulted to authority-#.

Configuring Supply Chain Security Tools - Policy

Admission of Images

Including Namespaces

Create a ClusterImagePolicy resource

mode

images

authorities

static.action

Provide credentials for the package

Provide secrets for authentication in your policy

Verify your configuration

Create a `ClusterImagePolicy` resource

`mode`

`images`

`authorities`

`static.action`