Kyverno And Cosign

Kyverno And Cosign

Anurag Kumar's photo
Anurag Kumar
·May 5, 2022·

4 min read

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Table of contents

  • Signing the image
  • Creating a Pod with the signed Image

In my previous article, I have written about kyverno. Continuing that, In this short article, I’m going to explore Kyverno and Cosign together. Kyverno is a Kubernetes native policy engine, and cosign is a tool used for signing container images. This article assumes that you know the basics of Kyverno and Cosign.

This article is more hands on oriented, so I will encourage you to try this hands on.

  • What we are going to do in this blog

    1. Creating a container image
    2. Pushing the image to container registry
    3. Signing the Image with Cosign
    4. Creating a ClusterPolicy that will enforce only signed images to be used while creating k8s resources.
    5. Creating a pod with the same image that we had made above.
  • For the example purpose, I will be creating a very simple container image based on nginx:alpine image.

  • Create a new directory and name it anything you want.
  • First, let's create an index.html page that we will be using in the image.
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Cosign and Kyverno</title>
</head>
<body>
    <h1> Having fun with Cosign and Kyverno </h1>
</body>
</html>
  • Now let's create a Dockerfile in the same directory.
    FROM nginx:alpine
    COPY index.html /usr/share/nginx/html
    
  • Now we need to build the image. To build the image, execute the command
    podman build -t kranurag7/kyverno-cosign .
    
  • I'm using Podman here. If you use any other tool like docker, then replace podman with docker.

Signing the image

  • Now comes the part of signing the image.
  • To sign the image, we will use cosign.
  • Execute the command cosign sign --key cosign.key kranurag7/kyverno-cosign
  • If everything is correct, you should see this message.

Screenshot from 2022-04-26 14-48-20.png

  • After signing when you go to docker hub or your respective container registry then you will notice a tag pushed to the repo as soon as you sign the image.

kyverno-cosign.png

  • I'm not going into the details of cosign in this article. I will write another one for that.
  • If verifies that your image is signed now.
  • You can also verify your image from the terminal.
    cosign verify --key cosign.pub kranurag7/kyverno-cosign | jq
    
  • You should see something like this.

Screenshot from 2022-04-26 14-55-14.png

  • If you don't have jq installed then skip jq

Creating a Pod with the signed Image

  • Now that we have our image ready, let's create a ClusterPolicy that will only allow the pods whose container images are signed with cosign.
  • To do that, we will go to kyverno.io/policies and copy-paste one policy named verify_image. You can find it here
apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: verify-image
  annotations:
    policies.kyverno.io/title: Verify Image
    policies.kyverno.io/category: Sample
    policies.kyverno.io/severity: medium
    policies.kyverno.io/subject: Pod
    policies.kyverno.io/minversion: 1.4.2
    policies.kyverno.io/description: >-
      Using the Cosign project, OCI images may be signed to ensure supply chain
      security is maintained. Those signatures can be verified before pulling into
      a cluster. This policy checks the signature of an image repo called
      ghcr.io/kyverno/test-verify-image to ensure it has been signed by verifying
      its signature against the provided public key. This policy serves as an illustration for
      how to configure a similar rule and will require replacing with your image(s) and keys.      
spec:
  validationFailureAction: enforce
  background: false
  rules:
    - name: verify-image
      match:
        any:
        - resources:
            kinds:
              - Pod
      verifyImages:
        - image: "*"
          key: |-
            -----BEGIN PUBLIC KEY-----
            MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEBgkz0hpHwOmEXaRaPPFMqhczFGRw
            wcQnj0jnRdsB0U6npFtHCJLIzALIYag9iHEz6RGArOvLa0eFLOkQKRvpg==
            -----END PUBLIC KEY-----
  • Now we will execute the command kubectl apply -f verify_image.yaml
  • After this, we will try running our signed image and one unsigned image as well.
  • Let's first start with an image which is not signed by cosign.
  • For illustration purpose, we will run the nginx image
    kubectl run my-pod --image=nginx
    
  • You will notice that the object is not getting created, and it's throwing an error.
  • The error will look something like this.

Screenshot from 2022-04-26 14-27-04.png

  • You can also check this in the policy reports.
    kubectl get polr
    

Screenshot from 2022-04-26 14-29-56.png

  • It's showing that one policy is failing.
  • For looking at policies, I generally use policy-reporter. In the policy-reporter Dashboard, you will notice that one failing policy is there. For me, this is much simpler to visualize what's happening in the cluster.

Screenshot from 2022-04-26 14-32-04.png

  • You can see in the image above that one policy is failing and something is wrong.
  • Now let's deploy a pod with the image we have signed a few minutes back.
  • kubectl run my-pod --image=kranurag7/kyverno-cosign

kyverno-cosign.gif

  • Now let's try executing the command for the simple web page that we had created.
    kubectl exec -it my-pod -- curl localhost:80
    
  • You should see some output like this

Screenshot from 2022-04-26 15-21-24.png

  • Now let's see this in browser
    kubectl port-forward my-pod 8081:80
    
  • Go to your browser and type localhost:8081. You should see this in your browser. Screenshot from 2022-04-26 15-25-12.png

  • That's all I have for this article. I hope you enjoyed reading and practicing it. Stay tuned for more.

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