Reusing ValidatingAdmissionPolicies

Kubernetes vanilla Validating policies consist of the following resources:

• ValidatingAdmissionPolicy: describes the logic in CEL. It optionally accepts also parameters in spec.paramKind.
• ValidatingAdmissionPolicyBinding: scopes the policy.

Let's see a concrete example. These and others can be reused with Kubewarden's cel-policy with little effort.

The following ValidatingAdmissionPolicy comes from the Kubernetes docs. This policy checks that the number of Replicas in Deployments is less or equal to 5. It is bound with a ValidatingAdmissionPolicyBinding so it only affects Namespaces that havel a label environment set to test.

ValidatingAdmissionPolicy

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicy
metadata:
  name: "replicalimit-policy.example.com"
spec:
  failurePolicy: Fail # (1)
  matchConstraints: # (2)
    resourceRules:
      - apiGroups: ["apps"]
        apiVersions: ["v1"]
        operations: ["CREATE", "UPDATE"]
        resources: ["deployments"]
  variables: # (3)
    - name: maxreplicas
      expression: int(5)
  validations: # (4)
    - expression: "object.spec.replicas <= variables.maxreplicas"
      messageExpression: "'the number of replicast must be less than or equal to ' + string(variables.maxreplicas)"
---
apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicyBinding
metadata:
  name: "replicalimit-binding-test.example.com"
spec:
  policyName: "replicalimit-policy.example.com"
  validationActions: [Deny] # (5)
  matchResources: # (6)
    namespaceSelector:
      matchLabels:
        environment: test

Here we have an equivalent Kubewarden policy:

Kubewarden's cel-policy

./cel-policy-example.yaml

apiVersion: policies.kubewarden.io/v1
kind: ClusterAdmissionPolicy
metadata:
  annotations:
    io.kubewarden.policy.category: Resource validation # (7)
    io.kubewarden.policy.severity: low # (7)
  name: "cel-policy-replica-example"
spec:
  module: registry://ghcr.io/kubewarden/policies/cel-policy:v1.0.0
  failurePolicy: Fail # (1). Defaults to "Fail"
  mode: protect # (5). Defaults to "protect"
  rules: # (2)
    - apiGroups: ["apps"]
      apiVersions: ["v1"]
      operations: ["CREATE", "UPDATE"]
      resources: ["deployments"]
  settings:
    variables: # (3)
      - name: "replicas"
        expression: "object.spec.replicas"
      - name: maxreplicas
        expression: int(5)
    validations: # (4)
      - expression: "variables.replicas <= variables.maxreplicas"
        messageExpression: "'the number of replicast must be less than or equal to ' + string(variables.maxreplicas)"
  backgroundAudit: true # (7). Defaults to "true"
  namespaceSelector: # (6)
    matchLabels:
      environment: test

Notice the commented numbers on both the YAML manifests. Let's expand on them:

#	VAP field	cel-policy field
1	failurePolicy	failurePolicy	Both inform on behaviour when the policy server errors. Not to confuse with (5).
2	matchConstraints	rules	Both accept the same RuleWithOperations that informs on what kind of Resource the policy applies to.
3	variables	settings.variables	In Kubewarden's cel-policy, expressions that define variables are separated into settings.variables. Apart from that, they are equivalent.
4	validations	settings.validations	In Kubewarden's cel-policy, expressions that define validations are separated into settings.validations. Apart from that, they are equivalent.
5	validationActions	mode	mode has as options protect and monitor. Auditing is more full featured in Kubewarden, see (7).
6	matchResources	namespaceSelector, objectSelector	Define ways to constraint using Selectors. Kubewarden's policies have them as namespaceSelector and objectSelector.
7	auditAnnotations	backgroundAudit, annotations	These Kubewarden fields set the policy usage in Audit Scanner, and its category and severity for PolicyReports.
	---	Kubewarden-only features	For other features, see the rest of tutorial CEL examples.

Yet to be implemented equivalences

There are some VAP features that aren't yet implemented. If look forward to them, please get in contact with us. These are:

• VAP authorizer library.
• VAP matchConditions (spec.matchConditions).
• VAP Parameters (ValidatingAdmissionPolicy spec.paramKind & ValidatingAdmissionPolicyBinding spec.paramRef).
• VAP Audit Annotations (ValidatingAdmissionPolicy spec.auditAnnotations when ValidatingAdmissionPolicyBinding spec.validationActions is set to "Audit"). This is covered by Kubewarden's Audit Scanner and PolicyReports, which allows to audit resources already in the cluster.
• CEL resource constraints and estimated cost limit. This is partially covered by Kubewarden's general policy timeout protection.

Applying the policy

As normal, we can deploy our policy by instantiating its manifest:

$ kubectl apply -f ./cel-policy-example.yaml

And then test it by instantiating a deployment:

$ kubectl apply -f - <<EOF
apiVersion: v1
kind: Namespace
metadata:
  name: test
  labels:
    environment: test
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  namespace: test
spec:
  replicas: 6
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
EOF

namespace/test created
Error from server: error when creating "STDIN":
  admission webhook "clusterwide-cel-policy-replica-example.kubewarden.admission" denied the request:
  The number of replicas must be less than or equal to 5