Implementing Policy As Code With Open Policy Agent

# Install Gatekeeper via Helm
helm repo add gatekeeper https://open-policy-agent.github.io/gatekeeper/charts
helm install gatekeeper gatekeeper/gatekeeper \
  --namespace gatekeeper-system --create-namespace \
  --set replicas=3 \
  --set audit.replicas=1 \
  --set audit.writeToRAMDisk=true

# templates/k8s-required-labels.yaml
apiVersion: templates.gatekeeper.sh/v1
kind: ConstraintTemplate
metadata:
  name: k8srequiredlabels
spec:
  crd:
    spec:
      names:
        kind: K8sRequiredLabels
      validation:
        openAPIV3Schema:
          type: object
          properties:
            labels:
              type: array
              items:
                type: string
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8srequiredlabels
        violation[{"msg": msg}] {
          provided := {label | input.review.object.metadata.labels[label]}
          required := {label | label := input.parameters.labels[_]}
          missing := required - provided
          count(missing) > 0
          msg := sprintf("Missing required labels: %v", [missing])
        }

---
# templates/k8s-container-limits.yaml
apiVersion: templates.gatekeeper.sh/v1
kind: ConstraintTemplate
metadata:
  name: k8scontainerlimits
spec:
  crd:
    spec:
      names:
        kind: K8sContainerLimits
      validation:
        openAPIV3Schema:
          type: object
          properties:
            cpu:
              type: string
            memory:
              type: string
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8scontainerlimits
        violation[{"msg": msg}] {
          container := input.review.object.spec.containers[_]
          not container.resources.limits.cpu
          msg := sprintf("Container %v has no CPU limit", [container.name])
        }
        violation[{"msg": msg}] {
          container := input.review.object.spec.containers[_]
          not container.resources.limits.memory
          msg := sprintf("Container %v has no memory limit", [container.name])
        }

---
# templates/k8s-block-privileged.yaml
apiVersion: templates.gatekeeper.sh/v1
kind: ConstraintTemplate
metadata:
  name: k8sblockprivileged
spec:
  crd:
    spec:
      names:
        kind: K8sBlockPrivileged
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8sblockprivileged
        violation[{"msg": msg}] {
          container := input.review.object.spec.containers[_]
          container.securityContext.privileged == true
          msg := sprintf("Privileged container not allowed: %v", [container.name])
        }
        violation[{"msg": msg}] {
          container := input.review.object.spec.initContainers[_]
          container.securityContext.privileged == true
          msg := sprintf("Privileged init container not allowed: %v", [container.name])
        }

# constraints/require-labels.yaml
apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sRequiredLabels
metadata:
  name: require-team-labels
spec:
  enforcementAction: deny
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Namespace"]
      - apiGroups: ["apps"]
        kinds: ["Deployment", "StatefulSet"]
    excludedNamespaces:
      - kube-system
      - gatekeeper-system
  parameters:
    labels:
      - "team"
      - "environment"
      - "cost-center"

---
# constraints/block-privileged.yaml
apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sBlockPrivileged
metadata:
  name: block-privileged-containers
spec:
  enforcementAction: deny
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Pod"]
      - apiGroups: ["apps"]
        kinds: ["Deployment", "DaemonSet", "StatefulSet"]
    excludedNamespaces:
      - kube-system

# Install conftest
brew install conftest

# Test Kubernetes manifests against OPA policies locally
conftest test deployment.yaml --policy policies/ --output json

# Test Terraform against OPA policies
conftest test terraform/main.tf --policy policies/terraform/ --parser hcl2

# Test Dockerfiles
conftest test Dockerfile --policy policies/docker/

# policies/kubernetes/deny_latest_tag.rego
package kubernetes

deny[msg] {
  input.kind == "Deployment"
  container := input.spec.template.spec.containers[_]
  endswith(container.image, ":latest")
  msg := sprintf("Container %v uses :latest tag. Pin to specific version.", [container.name])
}

deny[msg] {
  input.kind == "Deployment"
  container := input.spec.template.spec.containers[_]
  not contains(container.image, ":")
  msg := sprintf("Container %v has no tag. Pin to specific version.", [container.name])
}

# .github/workflows/policy-test.yml
name: Policy Validation

on:
  pull_request:
    paths: ['k8s/**', 'terraform/**', 'policies/**']

jobs:
  conftest:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Install conftest
        run: |
          wget -q https://github.com/open-policy-agent/conftest/releases/download/v0.50.0/conftest_0.50.0_Linux_x86_64.tar.gz
          tar xzf conftest_0.50.0_Linux_x86_64.tar.gz
          sudo mv conftest /usr/local/bin/
      - name: Test K8s manifests
        run: conftest test k8s/**/*.yaml --policy policies/kubernetes/ --output json
      - name: Test Terraform
        run: conftest test terraform/*.tf --policy policies/terraform/ --parser hcl2