Policy As Code

Policy as Code

Automated policy enforcement for infrastructure, Kubernetes, and CI/CD pipelines.

When to Use

• Enforcing security policies in Kubernetes
• Validating Terraform configurations
• Implementing compliance requirements
• Pre-commit infrastructure validation
• Admission control in clusters

Tools Overview

OPA Gatekeeper (Kubernetes)

Installation

kubectl apply -f https://raw.githubusercontent.com/open-policy-agent/gatekeeper/master/deploy/gatekeeper.yaml

Constraint Template

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])
        }

Constraint

apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sRequiredLabels
metadata:
  name: require-team-label
spec:
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Namespace"]
      - apiGroups: ["apps"]
        kinds: ["Deployment"]
  parameters:
    labels:
      - "team"
      - "environment"

Common Policies

# Require resource limits
apiVersion: templates.gatekeeper.sh/v1
kind: ConstraintTemplate
metadata:
  name: k8scontainerlimits
spec:
  crd:
    spec:
      names:
        kind: K8sContainerLimits
  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])
        }
---
# Block privileged containers
apiVersion: templates.gatekeeper.sh/v1
kind: ConstraintTemplate
metadata:
  name: k8spsprivilegedcontainer
spec:
  crd:
    spec:
      names:
        kind: K8sPSPPrivilegedContainer
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8spsprivilegedcontainer

        violation[{"msg": msg}] {
          container := input.review.object.spec.containers[_]
          container.securityContext.privileged
          msg := sprintf("Privileged container not allowed: %v", [container.name])
        }

Kyverno (Kubernetes)

Installation

kubectl create -f https://github.com/kyverno/kyverno/releases/latest/download/install.yaml

Policies

# Require labels
apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: require-labels
spec:
  validationFailureAction: Enforce
  rules:
    - name: check-team-label
      match:
        any:
          - resources:
              kinds:
                - Deployment
                - StatefulSet
      validate:
        message: "Label 'team' is required"
        pattern:
          metadata:
            labels:
              team: "?*"
---
# Add default labels (mutating)
apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: add-default-labels
spec:
  rules:
    - name: add-managed-by
      match:
        any:
          - resources:
              kinds:
                - Deployment
      mutate:
        patchStrategicMerge:
          metadata:
            labels:
              managed-by: kyverno
---
# Block latest tag
apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: disallow-latest-tag
spec:
  validationFailureAction: Enforce
  rules:
    - name: validate-image-tag
      match:
        any:
          - resources:
              kinds:
                - Pod
      validate:
        message: "Using 'latest' tag is not allowed"
        pattern:
          spec:
            containers:
              - image: "!*:latest"
---
# Require resource limits
apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: require-resources
spec:
  validationFailureAction: Enforce
  rules:
    - name: validate-resources
      match:
        any:
          - resources:
              kinds:
                - Pod
      validate:
        message: "CPU and memory limits are required"
        pattern:
          spec:
            containers:
              - resources:
                  limits:
                    memory: "?*"
                    cpu: "?*"
                  requests:
                    memory: "?*"
                    cpu: "?*"

Checkov (Terraform/IaC)

Installation

pip install checkov
# or
brew install checkov

Usage

# Scan Terraform directory
checkov -d ./terraform

# Scan specific file
checkov -f main.tf

# Output formats
checkov -d . -o json
checkov -d . -o sarif

# Skip checks
checkov -d . --skip-check CKV_AWS_1,CKV_AWS_2

# Custom policies
checkov -d . --external-checks-dir ./custom-policies

Custom Policy

# custom-policies/check_s3_versioning.py
from checkov.terraform.checks.resource.base_resource_check import BaseResourceCheck
from checkov.common.models.enums import CheckResult, CheckCategories

class S3Versioning(BaseResourceCheck):
    def __init__(self):
        name = "Ensure S3 bucket has versioning enabled"
        id = "CKV_CUSTOM_1"
        supported_resources = ["aws_s3_bucket"]
        categories = [CheckCategories.BACKUP_AND_RECOVERY]
        super().__init__(name=name, id=id, categories=categories, supported_resources=supported_resources)

    def scan_resource_conf(self, conf):
        versioning = conf.get("versioning", [{}])
        if versioning and versioning[0].get("enabled", [False])[0]:
            return CheckResult.PASSED
        return CheckResult.FAILED

check = S3Versioning()

.checkov.yaml

# .checkov.yaml
soft-fail: false
compact: true
framework:
  - terraform
  - kubernetes
skip-check:
  - CKV_AWS_1  # S3 access logging
  - CKV_K8S_1  # Pod security
check:
  - CKV_AWS_*
  - CKV_K8S_*

Conftest (CI/CD)

Installation

brew install conftest
# or
go install github.com/open-policy-agent/conftest@latest

Terraform Policy

# policy/terraform.rego
package main

deny[msg] {
  resource := input.resource_changes[_]
  resource.type == "aws_s3_bucket"
  resource.change.after.versioning[0].enabled != true
  msg := sprintf("S3 bucket %v must have versioning enabled", [resource.address])
}

deny[msg] {
  resource := input.resource_changes[_]
  resource.type == "aws_security_group_rule"
  resource.change.after.cidr_blocks[_] == "0.0.0.0/0"
  resource.change.after.from_port == 22
  msg := sprintf("Security group rule %v allows SSH from 0.0.0.0/0", [resource.address])
}

CI/CD Integration

# .github/workflows/policy.yml
name: Policy Check
on: [pull_request]

jobs:
  terraform-policy:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Setup Terraform
        uses: hashicorp/setup-terraform@v3

      - name: Terraform Init
        run: terraform init

      - name: Terraform Plan
        run: terraform plan -out=tfplan

      - name: Convert to JSON
        run: terraform show -json tfplan > tfplan.json

      - name: Run Conftest
        uses: instrumenta/conftest-action@master
        with:
          files: tfplan.json
          policy: policy/

  kubernetes-policy:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Run Checkov
        uses: bridgecrewio/checkov-action@master
        with:
          directory: ./kubernetes
          framework: kubernetes

Pre-commit Hooks

# .pre-commit-config.yaml
repos:
  - repo: https://github.com/antonbabenko/pre-commit-terraform
    rev: v1.86.0
    hooks:
      - id: terraform_fmt
      - id: terraform_validate
      - id: terraform_tflint
      - id: terraform_checkov
        args:
          - --args=--skip-check CKV_AWS_1

  - repo: https://github.com/open-policy-agent/conftest
    rev: v0.46.0
    hooks:
      - id: conftest
        args: [--policy, policy/]

Best Practices

1. Start with audit mode before enforcing
2. Document exceptions with rationale
3. Version policies in Git
4. Test policies before deployment
5. Monitor policy violations for trends
6. Regular policy reviews for relevance

Integration

Works with:

• /k8s - Kubernetes policies
• /terraform - IaC validation
• /security - Security policies
• /devops - CI/CD integration