Kyverno Policy As Code Using CDK8S

Abstract

Kyverno Kyverno is a policy engine designed for Kubernetes, Kyverno policies can validate, mutate, and generate Kubernetes resources plus ensure OCI image supply chain security.
This blog, it provides the way to create Kyverno policy as code using CDK8S typescript.
By importing Kyverno CRDs and using CDK8S you can create Kyverno policy manifest using your familiar programming languages such as typescript as scale.

Pre-requisite
Overview of Kyverno
Import Kyverno CRDs
Write code
Build Kyverno policy from code
Apply and test
Test Restart Deployment On Configmap Change
Conclusion

🚀 Pre-requisite

Install typescript, node, and cdk8s as well as projen (optional) which is a tool for managing project configuration as code.
Getting started with cdk8s
EKS/kubernetes cluster to test

🚀 Overview of Kyverno

The features are
- Policies as Kubernetes resources in YAML
- Validate, mutate, or generate any resource using Kustomize overlays
- Match resources using label selectors and wildcards
- Block non-conformant resources using admission controls, or report policy violations
- Test policies and validate resources using the Kyverno CLI, in your CI/CD pipeline, before applying them to your cluster
How does it work?

🚀 Import Kyverno CRDs

Import kyverno CRDs as cdk8s lib

⚡ $ cdk8s import https://raw.githubusercontent.com/kyverno/kyverno/main/config/crds/kyverno.io_clusterpolicies.yaml --output src/imports/
Importing resources, this may take a few moments...
kyverno.io
  kyverno.io/clusterpolicy

Output of importing

    ⚡ $ tree src/imports/
    src/imports/
    └── kyverno.io.ts

    0 directories, 1 file

🚀 Write code

It's much more convenient to use visual code writing Kyverno policies in typescript language. We can read the document and find all references of construct, objects and properties of Kyverno policies through code descriptions.

On top of all policies, there's a simple construct (feel free to implement more the construct) so that each policy just needs to input name, pattern, etc.

The interface of kyverno properties

  export interface KyvernoProps {
    name: string;
    message: string;
    namespace?: string;
    action?: ClusterPolicySpecValidationFailureAction;
    kinds?: Array<string>;
    resources?: {};
    exclude?: ClusterPolicySpecRulesExclude;
    deny?: ClusterPolicySpecRulesValidateDeny;
    pattern?: {};
    anyPatterns?: {};
  };

The constructed class

  export class KyvernoClusterPolicy extends Chart {
    constructor(scope: Construct, name: string, kyvernoProps: KyvernoProps) {
      super(scope, name);

      new ClusterPolicy(this, `${kyvernoProps.name}`, {
        metadata: {
          name: kyvernoProps.name,
          namespace: kyvernoProps.namespace || undefined,
          annotations: {
            'policies.kyverno.io/category': 'Pod Security Standards',
          },
        },
        spec: {
          validationFailureAction: kyvernoProps.action || ClusterPolicySpecValidationFailureAction.ENFORCE,
          rules: [{
            name: kyvernoProps.name,
            match: {
              any: [{
                resources: kyvernoProps.resources || { kinds: ['Pod'] },
              }],
            },
            validate: {
              deny: kyvernoProps.deny || undefined,
              message: kyvernoProps.message,
              pattern: kyvernoProps.pattern || undefined,
              anyPattern: kyvernoProps.anyPatterns || undefined,
            },
            exclude: kyvernoProps.exclude || undefined,
          }],
        },
      });
    }
  }

This blog provides examples of 5 use cases
1. Deny delete objects which have label protected: 'true'
2. require-app-label
3. require-request-limit
4. Require run-as-non-root
5. [Restart Deployment On Configmap Change]

🚀 Build Kyverno policy from code

Source code:

  ⚡ $ tree src/
  src/
  ├── imports
  │   └── kyverno.io.ts
  ├── kyverno-policies
  │   ├── deny-delete-resources.ts
  │   ├── kverno-list.ts
  │   ├── kyvernoProps.ts
  │   ├── require-app-labels.ts
  │   ├── require-requests-limits.ts
  │   └── require-runasnonroot.ts
  ├── main.ts
  └── test-yaml
      ├── inflate-negative-test-deployment.yaml
      └── inflate-positive-test-deployment.yaml

  3 directories, 10 files

Build

  ⚡ $ npx projen build
  👾 build » default | ts-node --project tsconfig.dev.json .projenrc.ts
  👾 build » compile | tsc --build
  👾 build » post-compile » synth | cdk8s synth
  No manifests synthesized
  👾 build » test | jest --passWithNoTests --all --updateSnapshot
  No tests found, exiting with code 0
  ----------|---------|----------|---------|---------|-------------------
  File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
  ----------|---------|----------|---------|---------|-------------------
  All files |       0 |        0 |       0 |       0 |
  ----------|---------|----------|---------|---------|-------------------
  👾 build » test » eslint | eslint --ext .ts,.tsx --fix --no-error-on-unmatched-pattern src test build-tools projenrc .projenrc.ts

Output yaml files

  ⚡ $ tree dist/
  dist/
  └── kyverno
      ├── require-app-label-kyverno-policy.yaml
      ├── require-request-limit-kyverno-policy.yaml
      └── run-as-non-root-kyverno-policy.yaml

  1 directory, 3 files

🚀 Apply and test

Apply policies and check the result

  ⚡ $ kubectl apply -f dist/kyverno/
  clusterpolicy.kyverno.io/require-app-label configured
  clusterpolicy.kyverno.io/require-request-limit configured
  clusterpolicy.kyverno.io/run-as-non-root configured

Test negative, the deployment inflate-negative-test-deployment.yaml does not have resource limit and request and enable runAsNonRoot

  ⚡ $ kubectl apply -f src/test-yaml/inflate-negative-test-deployment.yaml
  Error from server: error when creating "src/test-yaml/inflate-negative-test-deployment.yaml": admission webhook "validate.kyverno.svc-fail" denied the request:

  policy Deployment/default/inflate-negative-test for resource violations:

  require-app-label: {}
  require-request-limit:
    autogen-require-request-limit: 'validation error: All containers must have CPU and
      memory resource requests and limits defined. rule autogen-require-request-limit
      failed at path /spec/template/spec/containers/0/resources/limits/'

Test positive

  kubectl apply -f src/test-yaml/inflate-positive-test-deployment.yaml
  deployment.apps/inflate-positive-test created

Test without non-root user enabled because the validation failure action is AUDIT so the deployment is applied successfully

  ⚡ $ kubectl apply -f src/test-yaml/inflate-without-nonroot-test-deployment.yaml
  deployment.apps/inflate-without-nonroot-test created

But let's view the policy violations

  ⚡ $ kubectl describe polr polr-ns-default | grep inflate -A15 -B10| grep "Result: \+fail" -B10
      Seconds:  1661326749
    Category:   Pod Security Standards
    Message:    validation error: Containers must be required to run as non-root users. This policy ensures runAsNonRoot is set to true. rule autogen-run-as-non-root[0] failed at path /spec/template/spec/securityContext/runAsNonRoot/ rule autogen-run-as-non-root[1] failed at path /spec/template/spec/containers/0/securityContext/
    Policy:     run-as-non-root
    Resources:
      API Version:  apps/v1
      Kind:         Deployment
      Name:         inflate-without-nonroot-test
      Namespace:    default
      UID:          b05068c1-425c-41f4-ae0f-c913100a1c9c
    Result:         fail

🚀 Test Restart Deployment On Configmap Change

Changing configmap requires rollout restart of deployments which reference to that configmap. We can use kyverno to automate this for us.
Create kyverno policy to watch a Configmap and if it changes will write an annotation to one or more target Deployments thus triggering a new rollout and thereby refreshing the referred Configmap
First we need to grant additional privileges to the Kyverno ServiceAccount for updating apps.deployments resources through Aggregated ClusterRoles
- Kyverno has clusterrole with aggregationRule which will combine all clusterrole with label app: kyverno into one in aggregation
```
  aggregationRule:
    clusterRoleSelectors:
    - matchLabels:
        app: kyverno
```
- Create a new kyverno cluster role to inject into the main one kyverno-clusterrole.ts.
Kyverno policy to Restart Deployment On Configmap Change: restart-on-configmap-changes.ts

Rebuild the project to generate manifest yaml files. npx projen build

  ⚡ $ tree dist/
  dist/
  ├── kyverno
  │   ├── require-app-label-kyverno-policy.yaml
  │   ├── require-request-limit-kyverno-policy.yaml
  │   ├── restart-on-configmap-change-policy.yaml
  │   └── run-as-non-root-kyverno-policy.yaml
  └── role
      └── kyverno-create-deployments-clusterrole.yaml

  2 directories, 5 files

Apply clusterrole and policy then test using inflate-positive-test-deployment.yaml and inflate-test-configmap.yaml

  ⚡ $ kv7 get cpol restart-on-configmap-change
  NAME                          BACKGROUND   ACTION    READY
  restart-on-configmap-change   true         audit     true

  ⚡ $ kv7 get deploy -l app=inflate-positive-test
  NAME                    READY   UP-TO-DATE   AVAILABLE   AGE
  inflate-positive-test   1/1     1            1           62m

  ⚡ $ kv7 get cm -l app=inflate-test-configmap
  NAME                     DATA   AGE
  inflate-test-configmap   2      64m

We now update the configmap to see kyverno rollout restart the deployment

  ⚡ $ kv7 apply -f inflate-test-configmap.yaml
  configmap/inflate-test-configmap configured

  ~ $ kv7 get pod -l app=inflate-positive-test --watch
  NAME                                     READY   STATUS    RESTARTS   AGE
  inflate-positive-test-668477b686-cdggl   1/1     Running   0          3m3s
  inflate-positive-test-59bb77549c-lxcjx   0/1     Pending   0          0s
  inflate-positive-test-668477b686-cdggl   1/1     Terminating   0          3m9s
  inflate-positive-test-59bb77549c-lxcjx   0/1     Pending       0          0s
  inflate-positive-test-59bb77549c-lxcjx   0/1     ContainerCreating   0          0s
  inflate-positive-test-59bb77549c-lxcjx   1/1     Running             0          1s
  inflate-positive-test-668477b686-cdggl   1/1     Terminating         0          3m11s
  inflate-positive-test-668477b686-cdggl   1/1     Terminating         0          3m11s

Reference: https://kyverno.io/policies/other/restart_deployment_on_secret_change/restart_deployment_on_secret_change/

🚀 Conclusion

Someone said Kyverno policy as code but the code is in yaml language, it's not an actual programming language.
Using CDK8S to generate Kyverno policy help to leverage the strong programming skill of the developer and structure project more efficiently.

Vu Dao