kata-containers/docs/how-to/how-to-use-the-kata-agent-policy.md

Kata Agent Policy

Agent Policy is a Kata Containers feature that enables the Guest VM to perform additional validation for each ttRPC API request.

The Policy is commonly used for implementing confidential containers, where the Kata Shim and the Kata Agent have different trust properties. However, the Policy can be used for non-confidential containers too - e.g., for a basic defense in depth step of blocking the Host from starting an application on the Guest. However, for non-confidential containers, the Host might be able to modify the Policy and/or replace the Agent and disable its Policy rules, so a Policy is more helpful for confidential containers.

Enabling the Kata Containers Policy code

When compiled with default settings, the Kata Containers code doesn't include the Policy feature. AGENT_POLICY=yes must be specified when building the Guest rootfs (by using rootfs.sh). Specifying that build parameter has the following effects:

1. The Open Policy Agent (OPA) binary gets installed in rootfs.

2. If the AGENT_INIT=yes build parameter was not specified, the kata-opa service gets included in the Guest rootfs. OPA will be started by systemd during the Kata Containers sandbox creation.

3. The Kata Agent gets built using AGENT_POLICY=yes, and therefore includes Policy support. If the AGENT_INIT=yes build parameter was specified in addition to AGENT_POLICY=yes, the Kata Agent will start OPA during the Kata Containers sandbox creation.

Providing the Policy to the Kata Agent

There are two methods for providing the Policy document to the Kata Agent:

Include Policy in the Guest rootfs

When building using AGENT_POLICY=yes, a default Policy file gets automatically included in the Kata Containers Guest rootfs. That default Policy is very permissive - allowing all the ttRPC API requests to be executed. To change the permissions granted by the default Policy, users must add their own Policy file and change the /etc/kata-opa/default-policy.rego symbolic link to point to their custom Policy file, in the Guest image.

Specify Policy as a Kubernetes YAML annotation

Kubernetes users can encode in base64 format their Policy documents, and add the encoded string as an annotation. Example:

Encode a Policy file

For example, the allow-all-except-exec-process.rego sample policy file is different from the default Policy because it rejects any ExecProcess requests. You can encode this policy file:

$ base64 -w 0 allow-all-except-exec-process.rego
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

Attach the Policy to a pod

Add the encoded Policy to your YAML file - e.g., pod1.yaml:

apiVersion: v1
kind: Pod
metadata:
  name: policy-exec-rejected
  annotations:
    io.katacontainers.config.agent.policy: 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
spec:
  runtimeClassName: kata
  containers:
    - name: first-test-container
      image: quay.io/prometheus/busybox:latest
      command:
        - sleep
        - "120"

Create the pod:

$ kubectl apply -f pod1.yaml

While creating the Pod sandbox, the Kata Shim will notice the io.katacontainers.config.agent.policy annotation and will send the Policy document to the Kata Agent - by sending a SetPolicy request. Note that this request will fail if the default Policy, included in the Guest image, doesn't allow this SetPolicy request. If the SetPolicy request is rejected by the Guest, the Kata Shim will fail to start the Pod sandbox.

How is the Policy being enforced?

The Kata Agent is responsible for enforcing the Policy, working together with OPA. The Agent checks the Policy for each ttRPC API request. Before carrying out the actions corresponding to the request, the Agent uses the OPA REST API to check if the Policy allows or blocks the call. The Agent rejects requests that are not allowed by the Policy.