Apply Pod Security Standards at the Cluster Level

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.

If you are running a version of Kubernetes other than v1.25, check the documentation for that version.

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:

1. Create a cluster with no Pod Security Standards applied:

   kind create cluster --name psa-wo-cluster-pss --image kindest/node:v1.24.0
   

   The output is similar to this:

   Creating cluster "psa-wo-cluster-pss" ...
   ✓ Ensuring node image (kindest/node:v1.24.0) 🖼
   ✓ 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! 😊
   

2. 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'.
   

3. 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
   

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

   1. 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
   

   2. 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
   

   3. 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:

1. Based on the risk posture applied to a cluster, a stricter Pod Security Standard like restricted might be a better choice.

2. 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:

3. 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/v1
       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
   

   Note: pod-security.admission.config.k8s.io/v1 configuration requires v1.25+. For v1.23 and v1.24, use v1beta1. For v1.22, use v1alpha1.

4. 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.

5. 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:v1.24.0 --config /tmp/pss/cluster-config.yaml
   

   The output is similar to this:

    Creating cluster "psa-with-cluster-pss" ...
     ✓ Ensuring node image (kindest/node:v1.24.0) 🖼 
     ✓ 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 🙂
   

6. 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'.
   

7. 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
   

8. 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 consume 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