sagikazarmark / kube-secrets-init

Kubernetes mutating webhook for `secrets-init` injection

Blog Post

Kubernetes and Secrets Management In The Cloud

kube-secrets-init

The kube-secrets-init is a Kubernetes mutating admission webhook, that mutates any K8s Pod that is using specially prefixed environment variables, directly or from Kubernetes as Secret or ConfigMap.

kube-secrets-init mutation

The kube-secrets-init injects a copy-secrets-init initContainer into a target Pod, mounts /helper/bin (default; can be changed with the volume-path flag) and copies the secrets-init tool into the mounted volume. It also modifies Pod entrypoint to secrets-init init system, following original command and arguments, extracted either from Pod specification or from Docker image.

What secrets-init does

secrets-init runs as PID 1, acting like a simple init system. It launches a single process and then proxies all received signals to a session rooted at that child process.

secrets-init also passes almost all environment variables without modification, replacing secret variables with values from secret management services.

Integration with AWS Secrets Manager

User can put AWS secret ARN as environment variable value. The secrets-init will resolve any environment value, using specified ARN, to referenced secret value.

# environment variable passed to `secrets-init`
MY_DB_PASSWORD=arn:aws:secretsmanager:$AWS_REGION:$AWS_ACCOUNT_ID:secret:mydbpassword-cdma3

# environment variable passed to child process, resolved by `secrets-init`
MY_DB_PASSWORD=very-secret-password

Integration with AWS Systems Manager Parameter Store

It is possible to use AWS Systems Manager Parameter Store to store application parameters and secrets.

User can put AWS Parameter Store ARN as environment variable value. The secrets-init will resolve any environment value, using specified ARN, to referenced parameter value.

# environment variable passed to `secrets-init`
MY_API_KEY=arn:aws:ssm:$AWS_REGION:$AWS_ACCOUNT_ID:parameter/api/key

# environment variable passed to child process, resolved by `secrets-init`
MY_API_KEY=key-123456789

Integration with Google Secret Manager

User can put Google secret name (prefixed with gcp:secretmanager:) as environment variable value. The secrets-init will resolve any environment value, using specified name, to referenced secret value.

# environment variable passed to `secrets-init`
MY_DB_PASSWORD=gcp:secretmanager:projects/$PROJECT_ID/secrets/mydbpassword
# OR versioned secret (with version or 'latest')
MY_DB_PASSWORD=gcp:secretmanager:projects/$PROJECT_ID/secrets/mydbpassword/versions/2

# environment variable passed to child process, resolved by `secrets-init`
MY_DB_PASSWORD=very-secret-password

Requirement

AWS

In order to resolve AWS secrets from AWS Secrets Manager and Parameter Store, secrets-init should run under IAM role that has permission to access desired secrets.

This can be achieved by assigning IAM Role to Kubernetes Pod. It's possible to assign IAM Role to EC2 instance, where container is running, but this option is less secure.

Google Cloud

In order to resolve Google secrets from Google Secret Manager, secrets-init should run under IAM role that has permission to access desired secrets. For example, you can assign the following 2 predefined Google IAM roles to a Google Service Account: Secret Manager Viewer and Secret Manager Secret Accessor role.

This can be achieved by assigning IAM Role to Kubernetes Pod with Workload Identity. It's possible to assign IAM Role to GCE instance, where container is running, but this option is less secure.

Uncomment --provider=google flag in the deployment.yaml file.

The kube-secrets-init deployment

Deploy with Helm Chart

Consider using the kube-secrets-init Helm Chart, authored and managed by Márk Sági-Kazár.

helm repo add skm https://charts.sagikazarmark.dev
helm install --generate-name --wait skm/kube-secrets-init

Check chart GitHub repository

Manual Deployment

1. To deploy the kube-secrets-init server, we need to create a webhook service and a deployment in our Kubernetes cluster. It’s pretty straightforward, except one thing, which is the server’s TLS configuration. If you’d care to examine the deployment.yaml file, you’ll find that the certificate and corresponding private key files are read from command line arguments, and that the path to these files comes from a volume mount that points to a Kubernetes secret:

[...]
      args:
      [...]
      - --tls-cert-file=/etc/webhook/certs/cert.pem
      - --tls-private-key-file=/etc/webhook/certs/key.pem
      volumeMounts:
      - name: webhook-certs
        mountPath: /etc/webhook/certs
        readOnly: true
[...]
   volumes:
   - name: webhook-certs
     secret:
       secretName: secrets-init-webhook-certs

The most important thing to remember is to set the corresponding CA certificate later in the webhook configuration, so the apiserver will know that it should be accepted. For now, we’ll reuse the script originally written by the Istio team to generate a certificate signing request. Then we’ll send the request to the Kubernetes API, fetch the certificate, and create the required secret from the result.

First, run webhook-create-signed-cert.sh script and check if the secret holding the certificate and key has been created:

./deployment/webhook-create-signed-cert.sh

creating certs in tmpdir /var/folders/vl/gxsw2kf13jsf7s8xrqzcybb00000gp/T/tmp.xsatrckI71
Generating RSA private key, 2048 bit long modulus
.........................+++
....................+++
e is 65537 (0x10001)
certificatesigningrequest.certificates.k8s.io/secrets-init-webhook-svc.default created
NAME                         AGE   REQUESTOR              CONDITION
secrets-init-webhook-svc.default   1s    [e-mail]   Pending
certificatesigningrequest.certificates.k8s.io/secrets-init-webhook-svc.default approved
secret/secrets-init-webhook-certs configured

Once the secret is created, we can create deployment and service. These are standard Kubernetes deployment and service resources. Up until this point we’ve produced nothing but an HTTP server that’s accepting requests through a service on port 443:

kubectl create -f deployment/deployment.yaml

kubectl create -f deployment/service.yaml

configure mutating admission webhook

Now that our webhook server is running, it can accept requests from the apiserver. However, we should create some configuration resources in Kubernetes first. Let’s start with our validating webhook, then we’ll configure the mutating webhook later. If you take a look at the webhook configuration, you’ll notice that it contains a placeholder for CA_BUNDLE:

[...]
      service:
        name: secrets-init-webhook-svc
        namespace: default
        path: "/pods"
      caBundle: ${CA_BUNDLE}
[...]

There is a small script that substitutes the CA_BUNDLE placeholder in the configuration with this CA. Run this command before creating the validating webhook configuration:

cat ./deployment/mutatingwebhook.yaml | ./deployment/webhook-patch-ca-bundle.sh > ./deployment/mutatingwebhook-bundle.yaml

Create mutating webhook configuration:

kubectl create -f deployment/mutatingwebhook-bundle.yaml

configure RBAC for secrets-init-webhook

Create Kubernetes Service Account to be used with secrets-init-webhook:

kubectl create -f deployment/service-account.yaml

Define RBAC permission for webhook service account:

# create a cluster role
kubectl create -f deployment/clusterrole.yaml
# define a cluster role binding
kubectl create -f deployment/clusterrolebinding.yaml