From Fragile to Resilient: ValidatingAdmissionPolicies Strengthen Kubernetes Kubernetes Community Days Denmark November 19th 2024

Hi 👋, I’m Marcus Noble! I’m a platform engineer at working on release engineering, CI/CD infrastructure and provider-independent Kubernetes development. 6+ years experience running Kubernetes in production environments. Fediverse at: @Marcus@k8s.social Everywhere else: MarcusNoble.com

Dynamic Admission Control ValidatingAdmission MutatingAdmission Webhook Webhook

Purpose / Use Cases Defaulting Policy Enforcement ● Injecting imagePullSecrets dynamically when pods are created ● Injecting sidecars into pods ● Injecting proxy environment variables into pods ● Prevent using latest image tag ● Require all pods to have resource limits set ● Block the use of deprecated Kubernetes APIs (e.g. batch/v1beta1) ● Require a PodDisruptionBudget ● Enforce a standard set of labels / annotations on all resources ● Replace all image registries with an in-house container image proxy / cache ● Block nodes joining the cluster with known CVEs based on the kernel version (e.g. CVE-2022-0185) ● Inject Log4Shell mitigation env var into all pods (CVE-2021-44228) ● Block binding to the cluster-admin role Best Practices Problem Mitigation

Webhooks in Kubernetes are ✨ POWERFUL ✨ But with that power comes Taken from my talk about this at K UK last year 😱 RISK 😱

Wouldn’t it be great if we had a safer alternative? Yes! Yes, it would! 6

Introducing ValidatingAdmissionPolicies Status: Alpha in v1.26, Beta in v1.28, GA in v1.30 Introduced in: KEP-3488 A declarative, in-process alternative to validating admission webhooks.

Introducing ValidatingAdmissionPolicies Status: Alpha in v1.26, Beta in v1.28, GA in v1.30 Introduced in: KEP-3488 A declarative, in-process alternative to validating admission webhooks. Kubernetes manifests

Introducing ValidatingAdmissionPolicies Status: Alpha in v1.26, Beta in v1.28, GA in v1.30 Introduced in: KEP-3488 api-server A declarative, in-process alternative to validating admission webhooks. Kubernetes manifests

Introducing ValidatingAdmissionPolicies Status: Alpha in v1.26, Beta in v1.28, GA in v1.30 Introduced in: KEP-3488 api-server A declarative, in-process alternative to validating admission webhooks. Kubernetes manifests More on this shortly Uses the Common Expression Language (CEL) for the policy language.

Introducing ValidatingAdmissionPolicies Status: Alpha in v1.26, Beta in v1.28, GA in v1.30 Introduced in: KEP-3488 api-server A declarative, in-process alternative to validating admission webhooks. Kubernetes manifests More on this shortly Uses the Common Expression Language (CEL) for the policy language. Consists of two main resources: ● ValidatingAdmissionPolicy describes the policy logic ● ValidatingAdmissionPolicyBinding links the above policy to the resources it applies to

Brief introduction to CEL ● Uses a similar syntax to the expressions in C-based languages, e.g. self.minReplicas <= self.replicas && self.replicas <= self.maxReplicas ● Designed to be embedded into other applications with a focus on “one-liners” of code. ● Small number of built in functions (e.g. split, has) ● Functions expanded through custom libraries (Kubernetes includes several of these) References: ● ● Already used by Kyverno, Tekton, etc. ● Has a concept of “cost” per operation. https://kubernetes.io/docs/reference/usingapi/cel/ ● https://github.com/google/cel-go ● https://playcel.undistro.io/ (CEL playground) This can be calculated ahead of running the expression and if needed, block its execution if too expensive.

Example Blocking the use of the ‘latest’ image tag prevent-latest-vap.yaml apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicy metadata: name: “prevent-latest-image-tag” spec: failurePolicy: Fail matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”, “UPDATE”] resources: [“deployments”, “daemonsets”, “statefulsets”] validations: - message: “The use of the ‘latest’ tag is not allowed” expression: | object.spec.template.spec.containers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) && ( !has(object.spec.template.spec.initContainers) || object.spec.template.spec.initContainers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) )

Example Blocking the use of the ‘latest’ image tag prevent-latest-vap.yaml apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicy metadata: name: “prevent-latest-image-tag” spec: failurePolicy: Fail matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”, “UPDATE”] resources: [“deployments”, “daemonsets”, “statefulsets”] validations: - message: “The use of the ‘latest’ tag is not allowed” expression: | object.spec.template.spec.containers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) && ( !has(object.spec.template.spec.initContainers) || object.spec.template.spec.initContainers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) ) The name of our policy. We’ll reference this in our ValidatingAdmission PolicyBinding

Example Blocking the use of the ‘latest’ image tag prevent-latest-vap.yaml apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicy metadata: name: “prevent-latest-image-tag” spec: failurePolicy: Fail matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”, “UPDATE”] resources: [“deployments”, “daemonsets”, “statefulsets”] validations: - message: “The use of the ‘latest’ tag is not allowed” expression: | object.spec.template.spec.containers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) && ( !has(object.spec.template.spec.initContainers) || object.spec.template.spec.initContainers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) ) We want our policy to block any incoming requests that don’t meet this policies requirements (default). The alternative is Ignore if you want to disable enforcement of this policy.

Example Blocking the use of the ‘latest’ image tag prevent-latest-vap.yaml apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicy metadata: name: “prevent-latest-image-tag” spec: failurePolicy: Fail matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”, “UPDATE”] resources: [“deployments”, “daemonsets”, “statefulsets”] validations: - message: “The use of the ‘latest’ tag is not allowed” expression: | object.spec.template.spec.containers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) && ( !has(object.spec.template.spec.initContainers) || object.spec.template.spec.initContainers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) ) We define what resources this policy applies to. Multiple resource types can be defined here but if they don’t have the same general API the CEL expression will quickly become very complex.

Example Blocking the use of the ‘latest’ image tag prevent-latest-vap.yaml apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicy metadata: name: “prevent-latest-image-tag” spec: failurePolicy: Fail matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”, “UPDATE”] resources: [“deployments”, “daemonsets”, “statefulsets”] validations: - message: “The use of the ‘latest’ tag is not allowed” expression: | object.spec.template.spec.containers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) && ( !has(object.spec.template.spec.initContainers) || object.spec.template.spec.initContainers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) ) We’ll add a human-friendly message to be shown when the API request has been blocked by this policy. If we don’t include this, a generic message that include the whole expression is shown instead.

Example Blocking the use of the ‘latest’ image tag prevent-latest-vap.yaml apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicy metadata: name: “prevent-latest-image-tag” spec: failurePolicy: Fail matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”, “UPDATE”] resources: [“deployments”, “daemonsets”, “statefulsets”] validations: - message: “The use of the ‘latest’ tag is not allowed” expression: | object.spec.template.spec.containers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) && ( !has(object.spec.template.spec.initContainers) || object.spec.template.spec.initContainers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) ) Finally we have our expression. This is the expression of allowed resources, not those to block. (I keep getting caught out by this 😅)

Example Blocking the use of the ‘latest’ image tag prevent-latest-vap.yaml apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicy metadata: name: “prevent-latest-image-tag” spec: failurePolicy: Fail matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”, “UPDATE”] resources: [“deployments”, “daemonsets”, “statefulsets”] validations: - message: “The use of the ‘latest’ tag is not allowed” expression: | object.spec.template.spec.containers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) && ( !has(object.spec.template.spec.initContainers) || object.spec.template.spec.initContainers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) ) object is the incoming resource from the API call.

Example Blocking the use of the ‘latest’ image tag prevent-latest-vap.yaml apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicy metadata: name: “prevent-latest-image-tag” spec: failurePolicy: Fail matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”, “UPDATE”] resources: [“deployments”, “daemonsets”, “statefulsets”] validations: - message: “The use of the ‘latest’ tag is not allowed” expression: | object.spec.template.spec.containers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) && ( !has(object.spec.template.spec.initContainers) || object.spec.template.spec.initContainers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) ) First we check all containers don’t have the latest tag (or no tag specified).

Example Blocking the use of the ‘latest’ image tag prevent-latest-vap.yaml apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicy metadata: name: “prevent-latest-image-tag” spec: failurePolicy: Fail matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”, “UPDATE”] resources: [“deployments”, “daemonsets”, “statefulsets”] validations: - message: “The use of the ‘latest’ tag is not allowed” expression: | object.spec.template.spec.containers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) && ( !has(object.spec.template.spec.initContainers) || object.spec.template.spec.initContainers.all(container, !container.image.endsWith(“:latest”) && container.image.contains(“:”) ) ) Then we check if this resource defines initContainers and if so we check they also don’t use latest.

Example Blocking the use of the ‘latest’ image tag prevent-latest-binding.yaml Our policy does nothing until we bind it to some conditions. apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicyBinding metadata: name: “prevent-latest-image-tag” spec: policyName: “prevent-latest-image-tag” validationActions: [Deny] matchResources: {}

Example Blocking the use of the ‘latest’ image tag prevent-latest-binding.yaml The name of the policy we have just created. apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicyBinding metadata: name: “prevent-latest-image-tag” spec: policyName: “prevent-latest-image-tag” validationActions: [Deny] matchResources: {}

Example Blocking the use of the ‘latest’ image tag prevent-latest-binding.yaml The action to take when a policy isn’t met. Available options are: Deny, Warn & Audit apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicyBinding metadata: name: “prevent-latest-image-tag” spec: policyName: “prevent-latest-image-tag” validationActions: [Deny] Note: this is an array as you matchResources: {} can specify both warn and audit together

Example Blocking the use of the ‘latest’ image tag prevent-latest-binding.yaml We’re not defining any filtering so this policy applies cluster-wide. We could limit our policy to specific namespaces or labels, for example. apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicyBinding metadata: name: “prevent-latest-image-tag” spec: policyName: “prevent-latest-image-tag” validationActions: [Deny] matchResources: {}

Example Blocking the use of the ‘latest’ image tag prevent-latest-binding.yaml We’re not defining any filtering so this policy applies cluster-wide. We could limit our policy to specific namespaces or labels, for example. apiVersion: admissionregistration.k8s.io/v1 kind: ValidatingAdmissionPolicyBinding metadata: name: “prevent-latest-image-tag” spec: policyName: “prevent-latest-image-tag” validationActions: [Deny] matchResources: namespaceSelector: matchLabels: environment: prod

Example Blocking the use of the ‘latest’ image tag deployment.yaml apiVersion: apps/v1 kind: Deployment metadata: name: nginx-blocked labels: app: nginx spec: replicas: 1 selector: matchLabels: app: nginx template: metadata: labels: app: nginx spec: containers: - name: nginx image: nginx:latest

kubectl apply -f deployment.yaml The deployments “nginx-blocked” is invalid: : ValidatingAdmissionPolicy ‘prevent-latest-image-tag’ with binding ‘prevent-latest-image-tag’ denied request: The use of the ‘latest’ tag is not allowed

Our friendly message ⛔ Blocked! ⛔

More advanced features

More advanced features ● More context values - Along with object you also have oldObject & request you can use in your expressions matchConditions: - name: “exclude-kubelet-requests” expression: “!(“system:nodes” in request.userInfo.groups)”

More advanced features ● More context values - Along with object you also have oldObject & request you can use in your expressions ● Parameters - make your policies configurable by allowing parameter resources to be applied by the binding policy.yaml apiVersion: admissionregistration.k8s… kind: ValidatingAdmissionPolicy metadata: name: “param-example” spec: paramKind: apiVersion: rules.example.com/v1 kind: ReplicaLimit validations: - expression: object.spec.replicas <= params.maxReplicas policy-binding.yaml apiVersion: admissionregistration.k8s… kind: ValidatingAdmissionPolicyBinding metadata: name: “param-example” spec: policyName: “param-example” paramRef: name: “my-parameters” namespace: “default” params.yaml apiVersion: rules.example.com/v1 kind: ReplicaLimit metadata: name: “my-parameters” namespace: “default” maxReplicas: 3

More advanced features ● More context values - Along with object you also have oldObject & request you can use in your expressions ● Parameters - make your policies configurable by allowing parameter resources to be applied by the binding ● Variables - reusable CEL expressions to simplify your validation expressions. spec: Must be a valid CEL identifier (e.g. no -) variables: - name: teamLabel expression: “‘team’ in object.spec.metadata.labels ? object.spec.metadata.labels[‘team’] : ‘no-team’” validations: - expression: variables.teamLabel == ‘rel-eng’

More advanced features ● More context values - Along with object you also have oldObject & request you can use in your expressions ● Parameters - make your policies configurable by allowing parameter resources to be applied by the binding ● Variables - reusable CEL expressions to simplify your validation expressions. ● Audit annotations - Add extra metadata to the audit logs, dynamic values using CEL auditAnnotations: - key: “replica-count” valueExpression: | “‘Deployment spec.replicas set to ’ + string(object.spec.replicas)” { “kind”: “Event”, “apiVersion”: “audit.k8s.io/v1”, “annotations”: { “demo-policy.example.com/replica-count”: “Deployment spec.replicas set to 128” … } }

More advanced features ● More context values - Along with object you also have oldObject & request you can use in your expressions ● Parameters - make your policies configurable by allowing parameter resources to be applied by the binding ● Variables - reusable CEL expressions to simplify your validation expressions. ● Audit annotations - Add extra metadata to the audit logs, dynamic values using CEL ● Message expressions - Leverage some CEL in your message to have dynamic validation messages messageExpression: “‘object.spec.replicas must be no greater than ’ + string(params.maxReplicas)”

So that’s validation covered, what about mutating? Well… 34

MutatingAdmissionPolicies Status: Alpha in v1.32 Introduced in: KEP-3962 ● Follows the same idea as ValidatingAdmissionPolicies. ● Introduces MutatingAdmissionPolicy and MutatingAdmissionPolicyBinding ● Supports two patch strategies: ApplyConfiguration and JSONPatch ● Phase 1 will only support adding and updating of values (unsetting values planned for phase 2 but will need to get creative as CEL doesn’t natively support such an operation) ● Not yet finalized or available for testing (unless you want to build from source)

Example Injecting proxy values as env vars proxy-vars-map.yaml apiVersion: admissionregistration.k8s.io/v1alpha1 kind: MutatingAdmissionPolicy metadata: name: “proxy-values” spec: matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”] resources: [“pods”] mutations: - patchType: “ApplyConfiguration” mutation: > Object{ spec: Object.spec{ containers: object.spec.containers.map(c, Object.spec.containers.item{ name: c.name, env: [ Object.spec.containers.env{ name: “HTTP_PROXY”, value: “http://proxy.proxy.svc:3128” } + c.env ] } ) } }

Example Injecting proxy values as env vars Same as ValidatingAdmissionPolicies proxy-vars-map.yaml apiVersion: admissionregistration.k8s.io/v1alpha1 kind: MutatingAdmissionPolicy metadata: name: “proxy-values” spec: matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”] resources: [“pods”] mutations: - patchType: “ApplyConfiguration” mutation: > Object{ spec: Object.spec{ containers: object.spec.containers.map(c, Object.spec.containers.item{ name: c.name, env: [ Object.spec.containers.env{ name: “HTTP_PROXY”, value: “http://proxy.proxy.svc:3128” } + c.env ] } ) } }

Example Injecting proxy values as env vars Replace validations with mutations proxy-vars-map.yaml apiVersion: admissionregistration.k8s.io/v1alpha1 kind: MutatingAdmissionPolicy metadata: name: “proxy-values” spec: matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”] resources: [“pods”] mutations: - patchType: “ApplyConfiguration” mutation: > Object{ spec: Object.spec{ containers: object.spec.containers.map(c, Object.spec.containers.item{ name: c.name, env: [ Object.spec.containers.env{ name: “HTTP_PROXY”, value: “http://proxy.proxy.svc:3128” } + c.env ] } ) } }

Example Injecting proxy values as env vars Indicates the patch strategy Possible values: ● ApplyConfiguration ● JSONPatch proxy-vars-map.yaml apiVersion: admissionregistration.k8s.io/v1alpha1 kind: MutatingAdmissionPolicy metadata: name: “proxy-values” spec: matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”] resources: [“pods”] mutations: - patchType: “ApplyConfiguration” mutation: > Object{ spec: Object.spec{ containers: object.spec.containers.map(c, Object.spec.containers.item{ name: c.name, env: [ Object.spec.containers.env{ name: “HTTP_PROXY”, value: “http://proxy.proxy.svc:3128” } + c.env ] } ) } }

Example Injecting proxy values as env vars Introduction of named types The Object refers to the type of the incoming resource (Pod in this example). This may change in later releases to something like v1.Pod.spec for example. This has memory consumption implications. proxy-vars-map.yaml apiVersion: admissionregistration.k8s.io/v1alpha1 kind: MutatingAdmissionPolicy metadata: name: “proxy-values” spec: matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”] resources: [“pods”] mutations: - patchType: “ApplyConfiguration” mutation: > Object{ spec: Object.spec{ containers: object.spec.containers.map(c, Object.spec.containers.item{ name: c.name, env: [ Object.spec.containers.env{ name: “HTTP_PROXY”, value: “http://proxy.proxy.svc:3128” } + c.env ] } ) } }

Example Injecting proxy values as env vars Merge our proxy vars with any existing vars in all containers. Note: We’re missing initContainers and ephermeralContainers here due to limited space. proxy-vars-map.yaml apiVersion: admissionregistration.k8s.io/v1alpha1 kind: MutatingAdmissionPolicy metadata: name: “proxy-values” spec: matchConstraints: resourceRules: - apiGroups: [“apps”] apiVersions: [“v1”] operations: [“CREATE”] resources: [“pods”] mutations: - patchType: “ApplyConfiguration” mutation: > Object{ spec: Object.spec{ containers: object.spec.containers.map(c, Object.spec.containers.item{ I’m not 100% name: c.name, env: [ sure about Object.spec.containers.env{ this type name: “HTTP_PROXY”, value: “http://proxy.proxy.svc:3128” } + c.env ] } ) } }

Example Injecting proxy values as env vars proxy-vars-binding.yaml apiVersion: admissionregistration.k8s.io/v1beta1 kind: MutatingAdmissionPolicyBinding metadata: name: “proxy-values” spec: policyName: “proxy-values” matchResources: {} Nothing too surprising about our binding resource. All properties are the same as Validating except for the lack of a validationActions property.

What about the future beyond that? 🔮 43

• All these are speculative / wishful thinking right now ● Generative policies ● Resource lookup ● Policy exceptions ● Policy reports ● More abstractions (e.g. Kyverno already working on this with VAP)

Summary ● Time to start replacing those risky webhooks with in-process policies. ● ValidatingAdmissionPolicies generally available for use from v1.30 for safe validation logic. ● v1.32 for alpha release of MutatingAdmissionPolicies. ● More abstractions, generative policies and api lookups hopefully coming in the future.

Wrap-up Slides and resources available at: https://go-get.link/kcddk24 Thoughts, comments and feedback: feedback@marcusnoble.co.uk https://k8s.social/@Marcus Thank you