Mutating policies

The structure os mutating policies is the same as validating ones:

• They have to register validate and validate_settings waPC functions.
• The communication API used between the host and the policy is the same as that used by validating policies.

Mutating policies accept a request and can propose a mutation of the incoming object by returning a ValidationResponse object that looks like this:

{
  "accepted": true,
  "mutated_object": <object to be created>
}

The mutated_object field contains the object the policy wants creating in the Kubernetes cluster, serialized to JSON.

A concrete example

Let's assume the policy received this ValidationRequest:

{
  "settings": {},
  "request": {
    "operation": "CREATE",
    "object": {
      "apiVersion": "v1",
      "kind": "Pod",
      "metadata": {
        "name": "security-context-demo-4"
      },
      "spec": {
        "containers": [
        {
          "name": "sec-ctx-4",
          "image": "gcr.io/google-samples/node-hello:1.0",
          "securityContext": {
            "capabilities": {
              "add": ["NET_ADMIN", "SYS_TIME"]
            }
          }
        }
        ]
      }
    }
  }
}

note

Only important field are in the request object for this example.

This request generation happens because someone tried to create a Pod that would look like this:

apiVersion: v1
kind: Pod
metadata:
  name: security-context-demo-4
spec:
  containers:
  - name: sec-ctx-4
    image: gcr.io/google-samples/node-hello:1.0
    securityContext:
      capabilities:
        add:
        - NET_ADMIN
        - SYS_TIME

Assume the policy replies with the following ValidationResponse:

{
  "accepted": true,
  "mutated_object": {
    "apiVersion": "v1",
    "kind": "Pod",
    "metadata": {
      "name": "security-context-demo-4"
    },
    "spec": {
      "containers": [
        {
          "name": "sec-ctx-4",
          "image": "gcr.io/google-samples/node-hello:1.0",
          "securityContext": {
            "capabilities": {
              "add": [
                "NET_ADMIN",
                "SYS_TIME"
              ],
              "drop": [
                "BPF"
              ]
            }
          }
        }
      ]
    }
  }
}

That would lead to request acceptance, but the final Pod would look like this:

apiVersion: v1
kind: Pod
metadata:
  name: security-context-demo-4
spec:
  containers:
  - name: sec-ctx-4
    image: gcr.io/google-samples/node-hello:1.0
    securityContext:
      capabilities:
        add:
        - NET_ADMIN
        - SYS_TIME
        drop:
        - BPF

As you can see, the policy altered the securityContext.capabilities.drop section of the only container declared in the Pod.

The container is now dropping the BPF capability due to the policy.

Recap

These are the functions a mutating policy must implement:

waPC function name	Input payload	Output payload
validate	{ "request": { // AdmissionReview.request data }, "settings": { // your policy configuration } }	{ // mandatory "accepted": boolean, // optional, ignored if accepted // recommended for rejections "message": string, // optional, ignored if accepted "code": integer, // JSON Object to be created // Can be used only when the // request is accepted "mutated_object": object }
validate_settings	{ // your policy configuration }	{ // mandatory "validate": boolean, // optional, ignored if accepted // recommended for rejections "message": string, }