admission-controller

Admission Controller

The controller applies sensible defaults for pod's securityContext runAsUser value. It validates that security context runAsNonRoot is not defined when runAsUser is set to 0, as you cannot run as nonRoot and run as user 0. If no runAsUser value is defined, it will assign a default value of 2000.

• Prepare Images
• Deploy Admission Controller
• Verify Admission Controller

Prepare Images

The project can be built by running make. The images are pushed by running make push-image.

Deploy Admission Controller

To deploy, run the prepare.sh in your terminal to create a CA, certificate, and private key for the controller and generate the secret, and the deployment manifests.

kubectl apply -f k8s/ns.yaml

kubectl apply -f k8s/secret-webhook-server-tls.yaml

kubectl apply -f k8s/admission-controller.yaml

Verify Admission Controller

First, apply a pod with a conflicting securityContext. We will define runAsUser equal to 0, and runAsNonRoot.

kubectl apply -f -<<EOF
apiVersion: v1
kind: Pod
metadata:
  name: conflict
  labels:
    app: conflict
spec:
  restartPolicy: Never
  securityContext:
    runAsNonRoot: true
    runAsUser: 0
  containers:
    - name: busybox
      image: busybox
      command: ["sh", "-c", "echo Running as user $(id -u)"]
EOF

output

Error from server: error when creating "STDIN": admission webhook "admission-controller.admission.svc" denied the request: runAsNonRoot specified, but runAsUser set to 0 (the root user, contradictory)

The admission controller can be ignore with an annotation of admission-ignore, test this

Apply a pod with a conflicting securityContext. We will define runAsUser equal to 0, and runAsNonRoot, and add an annotation of admission-ignore.

kubectl apply -f -<<EOF
apiVersion: v1
kind: Pod
metadata:
  annotations:
    admission-ignore: ""
  name: conflict
  labels:
    app: conflict
spec:
  restartPolicy: Never
  securityContext:
    runAsNonRoot: true
    runAsUser: 0
  containers:
    - name: busybox
      image: busybox
      command: ["sh", "-c", "echo Running as user $(id -u)"]
EOF

output

pod/conflict created

Apply a pod without runAsUser defined

kubectl apply -f -<<EOF
apiVersion: v1
kind: Pod
metadata:
  name: defaults
  labels:
    app: defaults
spec:
  restartPolicy: OnFailure
  containers:
    - name: busybox
      image: nginx
      command: ["sh","-c","sleep 9999"]
EOF

kubectl wait --for=condition=ready pod/defaults

kubectl get po defaults -ojsonpath='{ .spec.securityContext }' | jq

output

{
  "runAsNonRoot": true,
  "runAsUser": 20000
}

Define runAsUser and override admission controller

kubectl apply -f -<<EOF
apiVersion: v1
kind: Pod
metadata:
  name: override
  labels:
    app: override
spec:
  restartPolicy: OnFailure
  securityContext:
    runAsNonRoot: false
  containers:
    - name: root
      securityContext:
        runAsUser: 0
      image: nginx
      command: ["sh","-c","sleep 9999"]
EOF

kubectl wait --for=condition=ready pod/override

kubectl get po override -ojsonpath='{ .spec.containers[*].securityContext }' | jq

kubectl get po override -ojsonpath='{ .spec.securityContext }' | jq

output

{
  "runAsUser": 0
}

output

{
  "runAsNoneRoot": false
}