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Version: 1.14

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 fieldcel-policy field
1failurePolicyfailurePolicyBoth inform on behaviour when the policy server errors. Not to confuse with (5).
2matchConstraintsrulesBoth accept the same RuleWithOperations that informs on what kind of Resource the policy applies to.
3variablessettings.variablesIn Kubewarden's cel-policy, expressions that define variables are separated into settings.variables. Apart from that, they are equivalent.
4validationssettings.validationsIn Kubewarden's cel-policy, expressions that define validations are separated into settings.validations. Apart from that, they are equivalent.
5validationActionsmodemode has as options protect and monitor. Auditing is more full featured in Kubewarden, see (7).
6matchResourcesnamespaceSelector, objectSelectorDefine ways to constraint using Selectors. Kubewarden's policies have them as namespaceSelector and objectSelector.
7auditAnnotationsbackgroundAudit, annotationsThese Kubewarden fields set the policy usage in Audit Scanner, and its category and severity for PolicyReports.
---Kubewarden-only featuresFor other features, see the rest of tutorial CEL examples.
tip

The kwctl tool can be used to migrate a VAP policy to Kubewarden.

This is described inside of this howto.

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:

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