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In this article, we explore various Kubernetes security context scenarios and demonstrate how to update pod configurations to meet specific security requirements. You’ll learn how to check the running user inside pods, update configurations to run processes as non-root or root with additional capabilities, and understand container-level versus pod-level security context inheritance.

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Checking the User of a Running Pod

To determine which user is executing the sleep process inside an Ubuntu sleeper pod, follow these steps:
  1. List the Pods
    Identify the Ubuntu sleeper pod by listing all pods:     
    kubectl get pods
  2. Access the Pod
    Use kubectl exec to access the pod’s shell:     
    kubectl exec -it ubuntu-sleeper -- bash
  3. Verify the User Inside the Container
    Once inside, run the command:     
    whoami
    The expected output should be: 
    root
Below is a sample console output demonstrating these steps: 
kubectl get pods
NAME             READY   STATUS    RESTARTS   AGE
ubuntu-sleeper   1/1     Running   0          7m58s
And, inside the container: 
whoami
root
This confirms that the sleep process is running as the root user.

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Updating the Pod to Run as a Specific Non-Root User (UID 1010)

To update the Ubuntu sleeper pod so that the process runs with the user ID 1010, follow these steps:
  1. Export the Current Pod Configuration
    Retrieve the current configuration and save it to a file:     
    kubectl get pod ubuntu-sleeper -o yaml > ubuntu-sleeper.yaml
  2. Edit the Configuration File
    Open ubuntu-sleeper.yaml in your favorite text editor. Locate the pod’s securityContext, which should currently look like:     
    securityContext: {}
  3. Modify the Security Context
    Update the file by adding the runAsUser property with UID 1010:     
    securityContext:
  runAsUser: 1010
  4. Apply the Updated Configuration
    Save the changes, then delete the existing pod (forcing deletion if necessary) and reapply the configuration:     
    kubectl delete pod ubuntu-sleeper --force
kubectl apply -f ubuntu-sleeper.yaml
An excerpt of the modified configuration is as follows: 
apiVersion: v1
kind: Pod
metadata:
  name: ubuntu-sleeper
  namespace: default
spec:
  containers:
  - command:
    - sleep
    - "4800"
    image: ubuntu
    imagePullPolicy: Always
    name: ubuntu
    resources: {}
    terminationMessagePath: /dev/termination-log
    terminationMessagePolicy: File
    volumeMounts:
    - mountPath: /var/run/secrets/kubernetes.io/serviceaccount
      name: kube-api-access-vwn5z
      readOnly: true
  dnsPolicy: ClusterFirst
  enableServiceLinks: true
  nodeName: controlplane
  preemptionPolicy: PreemptLowerPriority
  priority: 0
  restartPolicy: Always
  schedulerName: default-scheduler
  securityContext:
    runAsUser: 1010
  serviceAccount: default
  serviceAccountName: default
  terminationGracePeriodSeconds: 30
  tolerations:
  - effect: NoExecute
    key: node.kubernetes.io/not-ready
    operator: Exists
    tolerationSeconds: 300
  - effect: NoExecute
    key: node.kubernetes.io/unreachable
    operator: Exists
After applying these changes, you can verify that the pod is running: 
kubectl get pods
At the container level, the securityContext settings override those defined at the pod level.

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Determining the User for Multiple Containers in a Pod

When dealing with pods that contain multiple containers, it’s important to understand how user contexts are inherited. Consider the following pod definition from the multi-pod.yaml file: 
apiVersion: v1
kind: Pod
metadata:
  name: multi-pod
spec:
  securityContext:
    runAsUser: 1001
  containers:
    - image: ubuntu
      name: web
      command: ["sleep", "5000"]
      securityContext:
        runAsUser: 1002
    - image: ubuntu
      name: sidecar
      command: ["sleep", "5000"]
Key observations:
  • The pod-level security context sets runAsUser to 1001.
  • The web container defines its own securityContext with runAsUser: 1002.
Since the container-level security context takes precedence, the processes in the web container run as user 1002. The sidecar container, which lacks a container-specific security context, inherits the pod-level setting, so its processes run as user 1001. The final configuration is provided below for reference: 
apiVersion: v1
kind: Pod
metadata:
  name: multi-pod
spec:
  securityContext:
    runAsUser: 1001
  containers:
  - image: ubuntu
    name: web
    command: ["sleep", "5000"]
    securityContext:
      runAsUser: 1002
  - image: ubuntu
    name: sidecar
    command: ["sleep", "5000"]

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Updating the Ubuntu Sleeper Pod to Run as Root with SYS_TIME Capability

To modify the Ubuntu sleeper pod so that:
  1. The process runs as the default root user.
  2. The pod is granted the SYS_TIME capability,
follow these steps:
  1. Edit the Pod Configuration
    Open the current configuration file (e.g., ubuntu-sleeper.yaml), and remove any pod-level securityContext that enforces a non-root user.
  2. Add Capabilities at the Container Level
    Under the container specification responsible for running the sleep command, add a securityContext that includes the required capability:     
    securityContext:
  capabilities:
    add: ['SYS_TIME']
A sample updated configuration is shown below: 
apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: "2023-07-26T22:12:07Z"
  name: ubuntu-sleeper
  namespace: default
  resourceVersion: "945"
  uid: 34c800d-d278-498b-b6be-f9e41086be9f
spec:
  containers:
  - command:
    - sleep
    - "4800"
    image: ubuntu
    imagePullPolicy: Always
    name: ubuntu
    securityContext:
      capabilities:
        add: ['SYS_TIME']
    resources: {}
    terminationMessagePath: /dev/termination-log
    terminationMessagePolicy: File
    volumeMounts:
    - mountPath: /var/run/secrets/kubernetes.io/serviceaccount
      name: kube-api-access-km279
      readOnly: true
  dnsPolicy: ClusterFirst
  enableServiceLinks: true
  nodeName: controlplane
  preemptionPolicy: PreemptLowerPriority
  priority: 0
  restartPolicy: Always
  schedulerName: default-scheduler
  serviceAccount: default
  serviceAccountName: default
  terminationGracePeriodSeconds: 30
  tolerations:
  - effect: NoExecute
    operator: Exists
    tolerationSeconds: 300
  - effect: NoExecute
After saving the configuration, delete the existing pod and apply the new configuration: 
kubectl delete pod ubuntu-sleeper --force
kubectl apply -f ubuntu-sleeper.yaml
Verify that the pod is running as expected: 
kubectl get pods

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Adding the NET_ADMIN Capability

To enhance the security context further by adding the NET_ADMIN capability in addition to SYS_TIME, follow these steps:
  1. Modify the Container’s Security Context
    Open the ubuntu-sleeper.yaml file and update the securityContext under the container section to include both capabilities:     
    securityContext:
  capabilities:
    add: ['SYS_TIME', 'NET_ADMIN']
  2. Apply the Changes
    Save the file and reapply the pod configuration by deleting the current pod and applying the updated configuration:     
    kubectl delete pod ubuntu-sleeper --force
kubectl apply -f ubuntu-sleeper.yaml
  3. Verify the Update
    Optionally, check that the pod is running with the updated capabilities:     
    kubectl get pods
A sample console interaction may look like this: 
controlplane ~ ➜ vi ubuntu-sleeper.yaml
controlplane ~ ➜ kubectl delete pod ubuntu-sleeper --force
Warning: Immediate deletion does not wait for confirmation that the running resource has been terminated. The resource may continue to run on the cluster indefinitely.
pod "ubuntu-sleeper" force deleted

controlplane ~ ➜ kubectl apply -f ubuntu-sleeper.yaml
pod/ubuntu-sleeper created

controlplane ~ ➜ kubectl get pods
NAME             READY   STATUS    RESTARTS   AGE
ubuntu-sleeper   1/1     Running   0          10s
This completes the guide on updating Kubernetes pod configurations with various security contexts and capabilities. For more detailed information on Kubernetes security practices, see the Kubernetes Documentation. Happy securing!

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