W3cubDocs

Apply Pod Security Standards at the Cluster Level

Note

This tutorial applies only for new clusters.

Pod Security admission (PSA) is enabled by default in v1.23 and later, as it has graduated to beta. Pod Security is an admission controller that carries out checks against the Kubernetes Pod Security Standards when new pods are created. This tutorial shows you how to enforce the baseline Pod Security Standard at the cluster level which applies a standard configuration to all namespaces in a cluster.

To apply Pod Security Standards to specific namespaces, refer to Apply Pod Security Standards at the namespace level.

Before you begin

Install the following on your workstation:

KinD
kubectl

Choose the right Pod Security Standard to apply

Pod Security Admission lets you apply built-in Pod Security Standards with the following modes: enforce, audit, and warn.

To gather information that helps you to choose the Pod Security Standards that are most appropriate for your configuration, do the following:

Create a cluster with no Pod Security Standards applied:

kind create cluster --name psa-wo-cluster-pss --image kindest/node:v1.23.0

The output is similar to this:

Creating cluster "psa-wo-cluster-pss" ...
✓ Ensuring node image (kindest/node:v1.23.0) 🖼
✓ Preparing nodes 📦  
✓ Writing configuration 📜
✓ Starting control-plane 🕹️
✓ Installing CNI 🔌
✓ Installing StorageClass 💾
Set kubectl context to "kind-psa-wo-cluster-pss"
You can now use your cluster with:

kubectl cluster-info --context kind-psa-wo-cluster-pss

Thanks for using kind! 😊

Set the kubectl context to the new cluster:

kubectl cluster-info --context kind-psa-wo-cluster-pss

The output is similar to this:

 Kubernetes control plane is running at https://127.0.0.1:61350

CoreDNS is running at https://127.0.0.1:61350/api/v1/namespaces/kube-system/services/kube-dns:dns/proxy

To further debug and diagnose cluster problems, use 'kubectl cluster-info dump'.

Get a list of namespaces in the cluster:

kubectl get ns

The output is similar to this:

NAME                 STATUS   AGE
default              Active   9m30s
kube-node-lease      Active   9m32s
kube-public          Active   9m32s
kube-system          Active   9m32s
local-path-storage   Active   9m26s

Use --dry-run=server to understand what happens when different Pod Security Standards are applied:

Privileged

kubectl label --dry-run=server --overwrite ns --all \
pod-security.kubernetes.io/enforce=privileged

The output is similar to this:

namespace/default labeled
namespace/kube-node-lease labeled
namespace/kube-public labeled
namespace/kube-system labeled
namespace/local-path-storage labeled

Baseline

kubectl label --dry-run=server --overwrite ns --all \
pod-security.kubernetes.io/enforce=baseline

The output is similar to this:

namespace/default labeled
namespace/kube-node-lease labeled
namespace/kube-public labeled
Warning: existing pods in namespace "kube-system" violate the new PodSecurity enforce level "baseline:latest"
Warning: etcd-psa-wo-cluster-pss-control-plane (and 3 other pods): host namespaces, hostPath volumes
Warning: kindnet-vzj42: non-default capabilities, host namespaces, hostPath volumes
Warning: kube-proxy-m6hwf: host namespaces, hostPath volumes, privileged
namespace/kube-system labeled
namespace/local-path-storage labeled

Restricted

 kubectl label --dry-run=server --overwrite ns --all \
 pod-security.kubernetes.io/enforce=restricted

The output is similar to this:

namespace/default labeled
namespace/kube-node-lease labeled
namespace/kube-public labeled
Warning: existing pods in namespace "kube-system" violate the new PodSecurity enforce level "restricted:latest"
Warning: coredns-7bb9c7b568-hsptc (and 1 other pod): unrestricted capabilities, runAsNonRoot != true, seccompProfile
Warning: etcd-psa-wo-cluster-pss-control-plane (and 3 other pods): host namespaces, hostPath volumes, allowPrivilegeEscalation != false, unrestricted capabilities, restricted volume types, runAsNonRoot != true
Warning: kindnet-vzj42: non-default capabilities, host namespaces, hostPath volumes, allowPrivilegeEscalation != false, unrestricted capabilities, restricted volume types, runAsNonRoot != true, seccompProfile
Warning: kube-proxy-m6hwf: host namespaces, hostPath volumes, privileged, allowPrivilegeEscalation != false, unrestricted capabilities, restricted volume types, runAsNonRoot != true, seccompProfile
namespace/kube-system labeled
Warning: existing pods in namespace "local-path-storage" violate the new PodSecurity enforce level "restricted:latest"
Warning: local-path-provisioner-d6d9f7ffc-lw9lh: allowPrivilegeEscalation != false, unrestricted capabilities, runAsNonRoot != true, seccompProfile
namespace/local-path-storage labeled

From the previous output, you'll notice that applying the privileged Pod Security Standard shows no warnings for any namespaces. However, baseline and restricted standards both have warnings, specifically in the kube-system namespace.

Set modes, versions and standards

In this section, you apply the following Pod Security Standards to the latest version:

baseline standard in enforce mode.
restricted standard in warn and audit mode.

The baseline Pod Security Standard provides a convenient middle ground that allows keeping the exemption list short and prevents known privilege escalations.

Additionally, to prevent pods from failing in kube-system, you'll exempt the namespace from having Pod Security Standards applied.

When you implement Pod Security Admission in your own environment, consider the following:

Based on the risk posture applied to a cluster, a stricter Pod Security Standard like restricted might be a better choice.
Exempting the kube-system namespace allows pods to run as privileged in this namespace. For real world use, the Kubernetes project strongly recommends that you apply strict RBAC policies that limit access to kube-system, following the principle of least privilege. To implement the preceding standards, do the following:

Create a configuration file that can be consumed by the Pod Security Admission Controller to implement these Pod Security Standards:

mkdir -p /tmp/pss
cat <<EOF > /tmp/pss/cluster-level-pss.yaml 
apiVersion: apiserver.config.k8s.io/v1
kind: AdmissionConfiguration
plugins:
- name: PodSecurity
  configuration:
    apiVersion: pod-security.admission.config.k8s.io/v1beta1
    kind: PodSecurityConfiguration
    defaults:
      enforce: "baseline"
      enforce-version: "latest"
      audit: "restricted"
      audit-version: "latest"
      warn: "restricted"
      warn-version: "latest"
    exemptions:
      usernames: []
      runtimeClasses: []
      namespaces: [kube-system]
EOF

Configure the API server to consume this file during cluster creation:

cat <<EOF > /tmp/pss/cluster-config.yaml 
kind: Cluster
apiVersion: kind.x-k8s.io/v1alpha4
nodes:
- role: control-plane
  kubeadmConfigPatches:
  - |
    kind: ClusterConfiguration
    apiServer:
        extraArgs:
          admission-control-config-file: /etc/config/cluster-level-pss.yaml
        extraVolumes:
          - name: accf
            hostPath: /etc/config
            mountPath: /etc/config
            readOnly: false
            pathType: "DirectoryOrCreate"
  extraMounts:
  - hostPath: /tmp/pss
    containerPath: /etc/config
    # optional: if set, the mount is read-only.
    # default false
    readOnly: false
    # optional: if set, the mount needs SELinux relabeling.
    # default false
    selinuxRelabel: false
    # optional: set propagation mode (None, HostToContainer or Bidirectional)
    # see https://kubernetes.io/docs/concepts/storage/volumes/#mount-propagation
    # default None
    propagation: None
EOF

Note: If you use Docker Desktop with KinD on macOS, you can add /tmp as a Shared Directory under the menu item Preferences > Resources > File Sharing.

Create a cluster that uses Pod Security Admission to apply these Pod Security Standards:

 kind create cluster --name psa-with-cluster-pss --image kindest/node:v1.23.0 --config /tmp/pss/cluster-config.yaml

The output is similar to this:

 Creating cluster "psa-with-cluster-pss" ...
  ✓ Ensuring node image (kindest/node:v1.23.0) 🖼 
  ✓ Preparing nodes 📦  
  ✓ Writing configuration 📜 
  ✓ Starting control-plane 🕹️ 
  ✓ Installing CNI 🔌 
  ✓ Installing StorageClass 💾 
 Set kubectl context to "kind-psa-with-cluster-pss"
 You can now use your cluster with:

 kubectl cluster-info --context kind-psa-with-cluster-pss

 Have a question, bug, or feature request? Let us know! https://kind.sigs.k8s.io/#community 🙂

Point kubectl to the cluster

 kubectl cluster-info --context kind-psa-with-cluster-pss

The output is similar to this:

 Kubernetes control plane is running at https://127.0.0.1:63855
 CoreDNS is running at https://127.0.0.1:63855/api/v1/namespaces/kube-system/services/kube-dns:dns/proxy

 To further debug and diagnose cluster problems, use 'kubectl cluster-info dump'.

Create the following Pod specification for a minimal configuration in the default namespace:

cat <<EOF > /tmp/pss/nginx-pod.yaml
apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  containers:
    - image: nginx
      name: nginx
      ports:
        - containerPort: 80
EOF

Create the Pod in the cluster:

 kubectl apply -f /tmp/pss/nginx-pod.yaml

The output is similar to this:

 Warning: would violate PodSecurity "restricted:latest": allowPrivilegeEscalation != false (container "nginx" must set securityContext.allowPrivilegeEscalation=false), unrestricted capabilities (container "nginx" must set securityContext.capabilities.drop=["ALL"]), runAsNonRoot != true (pod or container "nginx" must set securityContext.runAsNonRoot=true), seccompProfile (pod or container "nginx" must set securityContext.seccompProfile.type to "RuntimeDefault" or "Localhost")
 pod/nginx created

Clean up

Run kind delete cluster --name psa-with-cluster-pss and kind delete cluster --name psa-wo-cluster-pss to delete the clusters you created.

What's next

Run a shell script to perform all the preceding steps at once:
1. Create a Pod Security Standards based cluster level Configuration
2. Create a file to let API server consumes this configuration
3. Create a cluster that creates an API server with this configuration
4. Set kubectl context to this new cluster
5. Create a minimal pod yaml file
6. Apply this file to create a Pod in the new cluster
Pod Security Admission
Pod Security Standards
Apply Pod Security Standards at the namespace level

© 2022 The Kubernetes Authors
Documentation Distributed under CC BY 4.0.
https://kubernetes.io/docs/tutorials/security/cluster-level-pss/