Pod Security Admission

An overview of the Pod Security Admission Controller, which can enforce the Pod Security Standards.
FEATURE STATE: Kubernetes v1.22 [alpha]

The Kubernetes Pod Security Standards define different isolation levels for Pods. These standards let you define how you want to restrict the behavior of pods in a clear, consistent fashion.

As an Alpha feature, Kubernetes offers a built-in Pod Security admission controller, the successor to PodSecurityPolicies. Pod security restrictions are applied at the namespace level when pods are created.

Note: The PodSecurityPolicy API is deprecated and will be removed from Kubernetes in v1.25.

Enabling the Alpha feature

Setting pod security controls by namespace is an alpha feature. You must enable the PodSecurity feature gate in order to use it.

--feature-gates="...,PodSecurity=true"

Pod Security levels

Pod Security admission places requirements on a Pod's Security Context and other related fields according to the three levels defined by the Pod Security Standards: privileged, baseline, and restricted. Refer to the Pod Security Standards page for an in-depth look at those requirements.

Pod Security Admission labels for namespaces

Provided that you have enabled this feature, you can configure namespaces to define the admission control mode you want to use for pod security in each namespace. Kubernetes defines a set of labels that you can set to define which of the predefined Pod Security Standard levels you want to use for a namespace. The label you select defines what action the control plane takes if a potential violation is detected:

Pod Security Admission modes
Mode Description
enforce Policy violations will cause the pod to be rejected.
audit Policy violations will trigger the addition of an audit annotation to the event recorded in the audit log, but are otherwise allowed.
warn Policy violations will trigger a user-facing warning, but are otherwise allowed.

A namespace can configure any or all modes, or even set a different level for different modes.

For each mode, there are two labels that determine the policy used:

# The per-mode level label indicates which policy level to apply for the mode.
#
# MODE must be one of `enforce`, `audit`, or `warn`.
# LEVEL must be one of `privileged`, `baseline`, or `restricted`.
pod-security.kubernetes.io/<MODE>: <LEVEL>

# Optional: per-mode version label that can be used to pin the policy to the
# version that shipped with a given Kubernetes minor version (for example v1.23).
#
# MODE must be one of `enforce`, `audit`, or `warn`.
# VERSION must be a valid Kubernetes minor version, or `latest`.
pod-security.kubernetes.io/<MODE>-version: <VERSION>

Check out Enforce Pod Security Standards with Namespace Labels to see example usage.

Workload resources and Pod templates

Pods are often created indirectly, by creating a workload object such as a Deployment or Job. The workload object defines a Pod template and a controller for the workload resource creates Pods based on that template. To help catch violations early, both the audit and warning modes are applied to the workload resources. However, enforce mode is not applied to workload resources, only to the resulting pod objects.

Exemptions

You can define exemptions from pod security enforcement in order allow the creation of pods that would have otherwise been prohibited due to the policy associated with a given namespace. Exemptions can be statically configured in the Admission Controller configuration.

Exemptions must be explicitly enumerated. Requests meeting exemption criteria are ignored by the Admission Controller (all enforce, audit and warn behaviors are skipped). Exemption dimensions include:

  • Usernames: requests from users with an exempt authenticated (or impersonated) username are ignored.
  • RuntimeClassNames: pods and workload resources specifying an exempt runtime class name are ignored.
  • Namespaces: pods and workload resources in an exempt namespace are ignored.
Caution: Most pods are created by a controller in response to a workload resource, meaning that exempting an end user will only exempt them from enforcement when creating pods directly, but not when creating a workload resource. Controller service accounts (such as system:serviceaccount:kube-system:replicaset-controller) should generally not be exempted, as doing so would implicitly exempt any user that can create the corresponding workload resource.

Updates to the following pod fields are exempt from policy checks, meaning that if a pod update request only changes these fields, it will not be denied even if the pod is in violation of the current policy level:

  • Any metadata updates except changes to the seccomp or AppArmor annotations:
    • seccomp.security.alpha.kubernetes.io/pod (deprecated)
    • container.seccomp.security.alpha.kubernetes.io/* (deprecated)
    • container.apparmor.security.beta.kubernetes.io/*
  • Valid updates to .spec.activeDeadlineSeconds
  • Valid updates to .spec.tolerations

What's next

Last modified August 25, 2021 at 12:57 AM PST : Fix link in pod-security-admission (3dc86945e)