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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 v1.24, check the documentation for that version.

Before you begin

Install the following on your workstation:

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
    
    1. 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
    
    1. 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/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
    
  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
    
  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

Last modified June 18, 2022 at 5:15 PM PST: Batch fix links (4) (2f4a067fea)