Validating Admission Policy

This page provides an overview of Validating Admission Policy.

What is Validating Admission Policy?

Validating admission policies offer a declarative, in-process alternative to validating admission webhooks.

Validating admission policies use the Common Expression Language (CEL) to declare the validation rules of a policy. Validation admission policies are highly configurable, enabling policy authors to define policies that can be parameterized and scoped to resources as needed by cluster administrators.

What Resources Make a Policy

A policy is generally made up of three resources:

• The ValidatingAdmissionPolicy describes the abstract logic of a policy (think: "this policy makes sure a particular label is set to a particular value").

• A ValidatingAdmissionPolicyBinding links the above resources together and provides scoping. If you only want to require an owner label to be set for Pods, the binding is where you would specify this restriction.

• A parameter resource provides information to a ValidatingAdmissionPolicy to make it a concrete statement (think "the owner label must be set to something that ends in .company.com"). A native type such as ConfigMap or a CRD defines the schema of a parameter resource. ValidatingAdmissionPolicy objects specify what Kind they are expecting for their parameter resource.

At least a ValidatingAdmissionPolicy and a corresponding ValidatingAdmissionPolicyBinding must be defined for a policy to have an effect.

If a ValidatingAdmissionPolicy does not need to be configured via parameters, simply leave spec.paramKind in ValidatingAdmissionPolicy not specified.

Getting Started with Validating Admission Policy

Validating Admission Policy is part of the cluster control-plane. You should write and deploy them with great caution. The following describes how to quickly experiment with Validating Admission Policy.

Creating a ValidatingAdmissionPolicy

The following is an example of a ValidatingAdmissionPolicy.

validatingadmissionpolicy/basic-example-policy.yaml

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicy
metadata:
  name: "demo-policy.example.com"
spec:
  failurePolicy: Fail
  matchConstraints:
    resourceRules:
    - apiGroups:   ["apps"]
      apiVersions: ["v1"]
      operations:  ["CREATE", "UPDATE"]
      resources:   ["deployments"]
  validations:
    - expression: "object.spec.replicas <= 5"

spec.validations contains CEL expressions which use the Common Expression Language (CEL) to validate the request. If an expression evaluates to false, the validation check is enforced according to the spec.failurePolicy field.

To configure a validating admission policy for use in a cluster, a binding is required. The following is an example of a ValidatingAdmissionPolicyBinding.:

validatingadmissionpolicy/basic-example-binding.yaml

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicyBinding
metadata:
  name: "demo-binding-test.example.com"
spec:
  policyName: "demo-policy.example.com"
  validationActions: [Deny]
  matchResources:
    namespaceSelector:
      matchLabels:
        environment: test

When trying to create a deployment with replicas set not satisfying the validation expression, an error will return containing message:

ValidatingAdmissionPolicy 'demo-policy.example.com' with binding 'demo-binding-test.example.com' denied request: failed expression: object.spec.replicas <= 5

The above provides a simple example of using ValidatingAdmissionPolicy without a parameter configured.

Validation actions

Each ValidatingAdmissionPolicyBinding must specify one or more validationActions to declare how validations of a policy are enforced.

The supported validationActions are:

• Deny: Validation failure results in a denied request.
• Warn: Validation failure is reported to the request client as a warning.
• Audit: Validation failure is included in the audit event for the API request.

For example, to both warn clients about a validation failure and to audit the validation failures, use:

validationActions: [Warn, Audit]

Deny and Warn may not be used together since this combination needlessly duplicates the validation failure both in the API response body and the HTTP warning headers.

A validation that evaluates to false is always enforced according to these actions. Failures defined by the failurePolicy are enforced according to these actions only if the failurePolicy is set to Fail (or not specified), otherwise the failures are ignored.

See Audit Annotations: validation failures for more details about the validation failure audit annotation.

Parameter resources

Parameter resources allow a policy configuration to be separate from its definition. A policy can define paramKind, which outlines GVK of the parameter resource, and then a policy binding ties a policy by name (via policyName) to a particular parameter resource via paramRef.

If parameter configuration is needed, the following is an example of a ValidatingAdmissionPolicy with parameter configuration.

validatingadmissionpolicy/policy-with-param.yaml

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicy
metadata:
  name: "replicalimit-policy.example.com"
spec:
  failurePolicy: Fail
  paramKind:
    apiVersion: rules.example.com/v1
    kind: ReplicaLimit
  matchConstraints:
    resourceRules:
    - apiGroups:   ["apps"]
      apiVersions: ["v1"]
      operations:  ["CREATE", "UPDATE"]
      resources:   ["deployments"]
  validations:
    - expression: "object.spec.replicas <= params.maxReplicas"
      reason: Invalid

The spec.paramKind field of the ValidatingAdmissionPolicy specifies the kind of resources used to parameterize this policy. For this example, it is configured by ReplicaLimit custom resources. Note in this example how the CEL expression references the parameters via the CEL params variable, e.g. params.maxReplicas. spec.matchConstraints specifies what resources this policy is designed to validate. Note that the native types such like ConfigMap could also be used as parameter reference.

The spec.validations fields contain CEL expressions. If an expression evaluates to false, the validation check is enforced according to the spec.failurePolicy field.

The validating admission policy author is responsible for providing the ReplicaLimit parameter CRD.

To configure an validating admission policy for use in a cluster, a binding and parameter resource are created. The following is an example of a ValidatingAdmissionPolicyBinding that uses a cluster-wide param - the same param will be used to validate every resource request that matches the binding:

validatingadmissionpolicy/binding-with-param.yaml

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicyBinding
metadata:
  name: "replicalimit-binding-test.example.com"
spec:
  policyName: "replicalimit-policy.example.com"
  validationActions: [Deny]
  paramRef:
    name: "replica-limit-test.example.com"
    namespace: "default"
  matchResources:
    namespaceSelector:
      matchLabels:
        environment: test

Notice this binding applies a parameter to the policy for all resources which are in the test environment.

The parameter resource could be as following:

validatingadmissionpolicy/replicalimit-param.yaml

apiVersion: rules.example.com/v1
kind: ReplicaLimit
metadata:
  name: "replica-limit-test.example.com"
  namespace: "default"
maxReplicas: 3

This policy parameter resource limits deployments to a max of 3 replicas.

An admission policy may have multiple bindings. To bind all other environments to have a maxReplicas limit of 100, create another ValidatingAdmissionPolicyBinding:

validatingadmissionpolicy/binding-with-param-prod.yaml

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicyBinding
metadata:
  name: "replicalimit-binding-nontest"
spec:
  policyName: "replicalimit-policy.example.com"
  validationActions: [Deny]
  paramRef:
    name: "replica-limit-prod.example.com"
    namespace: "default"
  matchResources:
    namespaceSelector:
      matchExpressions:
      - key: environment
        operator: NotIn
        values:
        - test

Notice this binding applies a different parameter to resources which are not in the test environment.

And have a parameter resource:

validatingadmissionpolicy/replicalimit-param-prod.yaml

apiVersion: rules.example.com/v1
kind: ReplicaLimit
metadata:
  name: "replica-limit-prod.example.com"
maxReplicas: 100

For each admission request, the API server evaluates CEL expressions of each (policy, binding, param) combination that match the request. For a request to be admitted it must pass all evaluations.

If multiple bindings match the request, the policy will be evaluated for each, and they must all pass evaluation for the policy to be considered passed.

If multiple parameters match a single binding, the policy rules will be evaluated for each param, and they too must all pass for the binding to be considered passed. Bindings can have overlapping match criteria. The policy is evaluated for each matching binding-parameter combination. A policy may even be evaluated multiple times if multiple bindings match it, or a single binding that matches multiple parameters.

The params object representing a parameter resource will not be set if a parameter resource has not been bound, so for policies requiring a parameter resource, it can be useful to add a check to ensure one has been bound. A parameter resource will not be bound and params will be null if paramKind of the policy, or paramRef of the binding are not specified.

For the use cases require parameter configuration, we recommend to add a param check in spec.validations[0].expression:

- expression: "params != null"
  message: "params missing but required to bind to this policy"

Optional parameters

It can be convenient to be able to have optional parameters as part of a parameter resource, and only validate them if present. CEL provides has(), which checks if the key passed to it exists. CEL also implements Boolean short-circuiting. If the first half of a logical OR evaluates to true, it won’t evaluate the other half (since the result of the entire OR will be true regardless).

Combining the two, we can provide a way to validate optional parameters:

!has(params.optionalNumber) || (params.optionalNumber >= 5 && params.optionalNumber <= 10)

Here, we first check that the optional parameter is present with !has(params.optionalNumber).

• If optionalNumber hasn’t been defined, then the expression short-circuits since !has(params.optionalNumber) will evaluate to true.
• If optionalNumber has been defined, then the latter half of the CEL expression will be evaluated, and optionalNumber will be checked to ensure that it contains a value between 5 and 10 inclusive.

Per-namespace Parameters

As the author of a ValidatingAdmissionPolicy and its ValidatingAdmissionPolicyBinding, you can choose to specify cluster-wide, or per-namespace parameters. If you specify a namespace for the binding's paramRef, the control plane only searches for parameters in that namespace.

However, if namespace is not specified in the ValidatingAdmissionPolicyBinding, the API server can search for relevant parameters in the namespace that a request is against. For example, if you make a request to modify a ConfigMap in the default namespace and there is a relevant ValidatingAdmissionPolicyBinding with no namespace set, then the API server looks for a parameter object in default. This design enables policy configuration that depends on the namespace of the resource being manipulated, for more fine-tuned control.

Parameter selector

In addition to specify a parameter in a binding by name, you may choose instead to specify label selector, such that all resources of the policy's paramKind, and the param's namespace (if applicable) that match the label selector are selected for evaluation. See selector for more information on how label selectors match resources.

If multiple parameters are found to meet the condition, the policy's rules are evaluated for each parameter found and the results will be ANDed together.

If namespace is provided, only objects of the paramKind in the provided namespace are eligible for selection. Otherwise, when namespace is empty and paramKind is namespace-scoped, the namespace used in the request being admitted will be used.

Authorization checks

We introduced the authorization check for parameter resources. User is expected to have read access to the resources referenced by paramKind in ValidatingAdmissionPolicy and paramRef in ValidatingAdmissionPolicyBinding.

Note that if a resource in paramKind fails resolving via the restmapper, read access to all resources of groups is required.

Failure Policy

failurePolicy defines how mis-configurations and CEL expressions evaluating to error from the admission policy are handled. Allowed values are Ignore or Fail.

• Ignore means that an error calling the ValidatingAdmissionPolicy is ignored and the API request is allowed to continue.
• Fail means that an error calling the ValidatingAdmissionPolicy causes the admission to fail and the API request to be rejected.

Note that the failurePolicy is defined inside ValidatingAdmissionPolicy:

validatingadmissionpolicy/failure-policy-ignore.yaml

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicy
spec:
...
failurePolicy: Ignore # The default is "Fail"
validations:
- expression: "object.spec.xyz == params.x"  

Validation Expression

spec.validations[i].expression represents the expression which will be evaluated by CEL. To learn more, see the CEL language specification CEL expressions have access to the contents of the Admission request/response, organized into CEL variables as well as some other useful variables:

• 'object' - The object from the incoming request. The value is null for DELETE requests.
• 'oldObject' - The existing object. The value is null for CREATE requests.
• 'request' - Attributes of the admission request.
• 'params' - Parameter resource referred to by the policy binding being evaluated. The value is null if ParamKind is not specified.
• namespaceObject - The namespace, as a Kubernetes resource, that the incoming object belongs to. The value is null if the incoming object is cluster-scoped.
• authorizer - A CEL Authorizer. May be used to perform authorization checks for the principal (authenticated user) of the request. See AuthzSelectors and Authz in the Kubernetes CEL library documentation for more details.
• authorizer.requestResource - A shortcut for an authorization check configured with the request resource (group, resource, (subresource), namespace, name).

The apiVersion, kind, metadata.name and metadata.generateName are always accessible from the root of the object. No other metadata properties are accessible.

Equality on arrays with list type of 'set' or 'map' ignores element order, i.e. [1, 2] == [2, 1]. Concatenation on arrays with x-kubernetes-list-type use the semantics of the list type:

• 'set': X + Y performs a union where the array positions of all elements in X are preserved and non-intersecting elements in Y are appended, retaining their partial order.
• 'map': X + Y performs a merge where the array positions of all keys in X are preserved but the values are overwritten by values in Y when the key sets of X and Y intersect. Elements in Y with non-intersecting keys are appended, retaining their partial order.

Validation expression examples

Read Supported evaluation on CEL for more information about CEL rules.

spec.validation[i].reason represents a machine-readable description of why this validation failed. If this is the first validation in the list to fail, this reason, as well as the corresponding HTTP response code, are used in the HTTP response to the client. The currently supported reasons are: Unauthorized, Forbidden, Invalid, RequestEntityTooLarge. If not set, StatusReasonInvalid is used in the response to the client.

Matching requests: matchConditions

You can define match conditions for a ValidatingAdmissionPolicy if you need fine-grained request filtering. These conditions are useful if you find that match rules, objectSelectors and namespaceSelectors still doesn't provide the filtering you want. Match conditions are CEL expressions. All match conditions must evaluate to true for the resource to be evaluated.

Here is an example illustrating a few different uses for match conditions:

access/validating-admission-policy-match-conditions.yaml

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicy
metadata:
  name: "demo-policy.example.com"
spec:
  failurePolicy: Fail
  matchConstraints:
    resourceRules:
      - apiGroups:   ["*"]
        apiVersions: ["*"]
        operations:  ["CREATE", "UPDATE"]
        resources:   ["*"]
  matchConditions:
    - name: 'exclude-leases' # Each match condition must have a unique name
      expression: '!(request.resource.group == "coordination.k8s.io" && request.resource.resource == "leases")' # Match non-lease resources.
    - name: 'exclude-kubelet-requests'
      expression: '!("system:nodes" in request.userInfo.groups)' # Match requests made by non-node users.
    - name: 'rbac' # Skip RBAC requests.
      expression: 'request.resource.group != "rbac.authorization.k8s.io"'
  validations:
    - expression: "!object.metadata.name.contains('demo') || object.metadata.namespace == 'demo'"

Match conditions have access to the same CEL variables as validation expressions.

In the event of an error evaluating a match condition the policy is not evaluated. Whether to reject the request is determined as follows:

1. If any match condition evaluated to false (regardless of other errors), the API server skips the policy.
2. Otherwise:
   • for failurePolicy: Fail, reject the request (without evaluating the policy).
   • for failurePolicy: Ignore, proceed with the request but skip the policy.

Audit annotations

auditAnnotations may be used to include audit annotations in the audit event of the API request.

For example, here is an admission policy with an audit annotation:

access/validating-admission-policy-audit-annotation.yaml

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicy
metadata:
  name: "demo-policy.example.com"
spec:
  failurePolicy: Fail
  matchConstraints:
    resourceRules:
    - apiGroups:   ["apps"]
      apiVersions: ["v1"]
      operations:  ["CREATE", "UPDATE"]
      resources:   ["deployments"]
  validations:
    - expression: "object.spec.replicas > 50"
      messageExpression: "'Deployment spec.replicas set to ' + string(object.spec.replicas)"
  auditAnnotations:
    - key: "high-replica-count"
      valueExpression: "'Deployment spec.replicas set to ' + string(object.spec.replicas)"

When an API request is validated with this admission policy, the resulting audit event will look like:

# the audit event recorded
{
    "kind": "Event",
    "apiVersion": "audit.k8s.io/v1",
    "annotations": {
        "demo-policy.example.com/high-replica-count": "Deployment spec.replicas set to 128"
        # other annotations
        ...
    }
    # other fields
    ...
}

In this example the annotation will only be included if the spec.replicas of the Deployment is more than 50, otherwise the CEL expression evaluates to null and the annotation will not be included.

Note that audit annotation keys are prefixed by the name of the ValidatingAdmissionWebhook and a /. If another admission controller, such as an admission webhook, uses the exact same audit annotation key, the value of the first admission controller to include the audit annotation will be included in the audit event and all other values will be ignored.

Message expression

To return a more friendly message when the policy rejects a request, we can use a CEL expression to composite a message with spec.validations[i].messageExpression. Similar to the validation expression, a message expression has access to object, oldObject, request, params, and namespaceObject. Unlike validations, message expression must evaluate to a string.

For example, to better inform the user of the reason of denial when the policy refers to a parameter, we can have the following validation:

access/deployment-replicas-policy.yaml

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicy
metadata:
  name: "deploy-replica-policy.example.com"
spec:
  paramKind:
    apiVersion: rules.example.com/v1
    kind: ReplicaLimit
  matchConstraints:
    resourceRules:
    - apiGroups:   ["apps"]
      apiVersions: ["v1"]
      operations:  ["CREATE", "UPDATE"]
      resources:   ["deployments"]
  validations:
  - expression: "object.spec.replicas <= params.maxReplicas"
    messageExpression: "'object.spec.replicas must be no greater than ' + string(params.maxReplicas)"
    reason: Invalid

After creating a params object that limits the replicas to 3 and setting up the binding, when we try to create a deployment with 5 replicas, we will receive the following message.

$ kubectl create deploy --image=nginx nginx --replicas=5
error: failed to create deployment: deployments.apps "nginx" is forbidden: ValidatingAdmissionPolicy 'deploy-replica-policy.example.com' with binding 'demo-binding-test.example.com' denied request: object.spec.replicas must be no greater than 3

This is more informative than a static message of "too many replicas".

The message expression takes precedence over the static message defined in spec.validations[i].message if both are defined. However, if the message expression fails to evaluate, the static message will be used instead. Additionally, if the message expression evaluates to a multi-line string, the evaluation result will be discarded and the static message will be used if present. Note that static message is validated against multi-line strings.

Type checking

When a policy definition is created or updated, the validation process parses the expressions it contains and reports any syntax errors, rejecting the definition if any errors are found. Afterward, the referred variables are checked for type errors, including missing fields and type confusion, against the matched types of spec.matchConstraints. The result of type checking can be retrieved from status.typeChecking. The presence of status.typeChecking indicates the completion of type checking, and an empty status.typeChecking means that no errors were detected.

For example, given the following policy definition:

validatingadmissionpolicy/typechecking.yaml

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicy
metadata:
  name: "deploy-replica-policy.example.com"
spec:
  matchConstraints:
    resourceRules:
    - apiGroups:   ["apps"]
      apiVersions: ["v1"]
      operations:  ["CREATE", "UPDATE"]
      resources:   ["deployments"]
  validations:
  - expression: "object.replicas > 1" # should be "object.spec.replicas > 1"
    message: "must be replicated"
    reason: Invalid

The status will yield the following information:

status:
  typeChecking:
    expressionWarnings:
    - fieldRef: spec.validations[0].expression
      warning: |-
        apps/v1, Kind=Deployment: ERROR: <input>:1:7: undefined field 'replicas'
         | object.replicas > 1
         | ......^        

If multiple resources are matched in spec.matchConstraints, all of matched resources will be checked against. For example, the following policy definition

validatingadmissionpolicy/typechecking-multiple-match.yaml

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicy
metadata:
  name: "replica-policy.example.com"
spec:
  matchConstraints:
    resourceRules:
    - apiGroups:   ["apps"]
      apiVersions: ["v1"]
      operations:  ["CREATE", "UPDATE"]
      resources:   ["deployments","replicasets"]
  validations:
  - expression: "object.replicas > 1" # should be "object.spec.replicas > 1"
    message: "must be replicated"
    reason: Invalid

will have multiple types and type checking result of each type in the warning message.

status:
  typeChecking:
    expressionWarnings:
    - fieldRef: spec.validations[0].expression
      warning: |-
        apps/v1, Kind=Deployment: ERROR: <input>:1:7: undefined field 'replicas'
         | object.replicas > 1
         | ......^
        apps/v1, Kind=ReplicaSet: ERROR: <input>:1:7: undefined field 'replicas'
         | object.replicas > 1
         | ......^        

Type Checking has the following limitation:

• No wildcard matching. If spec.matchConstraints.resourceRules contains "*" in any of apiGroups, apiVersions or resources, the types that "*" matches will not be checked.
• The number of matched types is limited to 10. This is to prevent a policy that manually specifying too many types. to consume excessive computing resources. In the order of ascending group, version, and then resource, 11th combination and beyond are ignored.
• Type Checking does not affect the policy behavior in any way. Even if the type checking detects errors, the policy will continue to evaluate. If errors do occur during evaluate, the failure policy will decide its outcome.
• Type Checking does not apply to CRDs, including matched CRD types and reference of paramKind. The support for CRDs will come in future release.

Variable composition

If an expression grows too complicated, or part of the expression is reusable and computationally expensive to evaluate, you can extract some part of the expressions into variables. A variable is a named expression that can be referred later in variables in other expressions.

spec:
  variables:
    - name: foo
      expression: "'foo' in object.spec.metadata.labels ? object.spec.metadata.labels['foo'] : 'default'"
  validations:
    - expression: variables.foo == 'bar'

A variable is lazily evaluated when it is first referred. Any error that occurs during the evaluation will be reported during the evaluation of the referring expression. Both the result and potential error are memorized and count only once towards the runtime cost.

The order of variables are important because a variable can refer to other variables that are defined before it. This ordering prevents circular references.

The following is a more complex example of enforcing that image repo names match the environment defined in its namespace.

access/image-matches-namespace-environment.policy.yaml

# This policy enforces that all containers of a deployment has the image repo match the environment label of its namespace.
# Except for "exempt" deployments, or any containers that do not belong to the "example.com" organization (e.g. common sidecars).
# For example, if the namespace has a label of {"environment": "staging"}, all container images must be either staging.example.com/*
# or do not contain "example.com" at all, unless the deployment has {"exempt": "true"} label.
apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingAdmissionPolicy
metadata:
  name: "image-matches-namespace-environment.policy.example.com"
spec:
  failurePolicy: Fail
  matchConstraints:
    resourceRules:
    - apiGroups:   ["apps"]
      apiVersions: ["v1"]
      operations:  ["CREATE", "UPDATE"]
      resources:   ["deployments"]
  variables:
  - name: environment
    expression: "'environment' in namespaceObject.metadata.labels ? namespaceObject.metadata.labels['environment'] : 'prod'"
  - name: exempt
    expression: "'exempt' in object.metadata.labels && object.metadata.labels['exempt'] == 'true'"
  - name: containers
    expression: "object.spec.template.spec.containers"
  - name: containersToCheck
    expression: "variables.containers.filter(c, c.image.contains('example.com/'))"
  validations:
  - expression: "variables.exempt || variables.containersToCheck.all(c, c.image.startsWith(variables.environment + '.'))"
    messageExpression: "'only ' + variables.environment + ' images are allowed in namespace ' + namespaceObject.metadata.name"

With the policy bound to the namespace default, which is labeled environment: prod, the following attempt to create a deployment would be rejected.

kubectl create deploy --image=dev.example.com/nginx invalid

The error message is similar to this.

error: failed to create deployment: deployments.apps "invalid" is forbidden: ValidatingAdmissionPolicy 'image-matches-namespace-environment.policy.example.com' with binding 'demo-binding-test.example.com' denied request: only prod images are allowed in namespace default