Azure Key Vault Provider for Secrets Store CSI Driver

In the previous post, I talked about akv2k8s. akv2k8s is a Kubernetes controller that synchronizes secrets and certificates from Key Vault. Besides synchronizing to a regular secret, it can also inject secrets into pods.

Instead of akv2k8s, you can also use the secrets store CSI driver with the Azure Key Vault provider. As a CSI driver, its main purpose is to mount secrets and certificates as storage volumes. Next to that, it can also create regular Kubernetes secrets that can be used with an ingress controller or mounted as environment variables. That might be required if the application was not designed to read the secret from the file system.

In the previous post, I used akv2k8s to grab a certificate from Key Vault, create a Kubernetes secret and use that secret with nginx ingress controller:

certificate in Key Vault ------akv2aks periodic sync -----> Kubernetes secret ------> nginx ingress controller

Let’s briefly look at how to do this with the secrets store CSI driver.

Installation

Follow the guide to install the Helm chart with Helm v3:

helm repo add csi-secrets-store-provider-azure https://raw.githubusercontent.com/Azure/secrets-store-csi-driver-provider-azure/master/charts
helm install csi-secrets-store-provider-azure/csi-secrets-store-provider-azure --generate-name

This will install the components in the current Kubernetes namespace.

Easy no?

Syncing the certificate

Following the same example as with akv2aks, we need to point at the certificate in Key Vault, set the right permissions, and bring the certificate down to Kubernetes.

You will first need to decide how to access Key Vault. You can use the managed identity of your AKS cluster or be more granular and use pod identity. If you have setup AKS with a managed identity, that is the simplest solution. You just need to grab the clientId of the managed identity like so:

az aks show -g <resource group> -n <aks cluster name> --query identityProfile.kubeletidentity.clientId -o tsv

Next, create a file with the content below and apply it to your cluster in a namespace of your choosing.

apiVersion: secrets-store.csi.x-k8s.io/v1alpha1
kind: SecretProviderClass
metadata:
  name: azure-gebakv
  namespace: YOUR NAMESPACE
spec:
  provider: azure
  secretObjects:
  - secretName: nginx-cert
    type: kubernetes.io/tls
    data:
    - objectName: nginx
      key: tls.key
    - objectName: nginx
      key: tls.crt
  parameters:
    useVMManagedIdentity: "true"
    userAssignedIdentityID: "CLIENTID YOU OBTAINED ABOVE" 
    keyvaultName: "gebakv"         
    objects:  |
      array:
        - |
          objectName: nginx
          objectType: secret        
    tenantId: "ID OF YOUR AZURE AD TENANT"

Compared to the akv2k8s controller, the above configuration is a bit more complex. In the parameters section, in the objects array, you specify the name of the certificate in Key Vault and its object type. Yes, you saw that correctly, the objectType actually has to be secret for this to work.

The other settings are self-explanatory: we use the managed identity, set its clientId and in keyvaultName we set the short name of our Key Vault.

The settings in the parameters section are actually sufficient to mount the secret/certificate in a pod. With the secretObjects section though, we can also ask for the creation of regular Kubernetes secrets. Here, we ask for a secret of type kubernetes.io/tls with name nginx-cert to be created. You need to explicitly set both the tls.key and the tls.crt value and correctly reference the objectName in the array.

The akv2k8s controller is simpler to use as you only need to point it to your certificate in Key Vault (and specify it’s a certificate, not a secret) and set a secret name. There is no need to set the different values in the secret.

Using the secret

The advantage of the secrets store CSI driver is that the secret is only mounted/created when an application requires it. That also means we have to instruct our application to mount the secret explicitly. You do that via a volume as the example below illustrates (part of a deployment):

spec:
      containers:
      - name: realtimeapp
        image: gbaeke/fluxapp:1.0.2
        volumeMounts:
          - mountPath: "/mnt/secrets-store"
            name: secrets-store-inline
            readOnly: true
        env:
        - name: REDISHOST
          value: "redis:6379"
        resources:
          requests:
            cpu: 25m
            memory: 50Mi
          limits:
            cpu: 150m
            memory: 150Mi
        ports:
        - containerPort: 8080
      volumes:
      - name: secrets-store-inline
        csi:
          driver: secrets-store.csi.k8s.io
          readOnly: true
          volumeAttributes:
            secretProviderClass: "azure-gebakv"

In the above YAML, the following happens:

• in volumes: we create a volume called secrets-store-inline and use the csi driver to mount the secrets we specified in the SecretProviderClass we created earlier (azure-gebakv)
• in volumeMounts: we mount the volume on /mnt/secrets-store

Because we used secretObjects in our SecretProviderClass, this mount is accompanied by the creation of a regular Kubernetes secret as well.

When you remove the deployment, the Kubernetes secret will be removed instead of lingering behind for all to see.

Of course, the pods in my deployment do not need the mounted volume. It was not immediately clear to me how to avoid the mount but still create the Kubernetes secret (not exactly the point of a CSI driver 😀). On the other hand, there is a way to have the secret created as part of ingress controller creation. That approach is more useful in this case because we want our ingress controller to use the certificate. More information can be found here. In short, it roughly works as follows:

• instead of creating and mounting a volume in your application pod, a volume should be created and mounted on the ingress controller
• to do so, you modify the deployment of your ingress controller (e.g. ingress-nginx) with extraVolumes: and extraVolumeMounts: sections; depending on the ingress controller you use, other settings might be required

Be aware that you need to enable auto rotation of secrets manually and that it is an alpha feature at this point (December 2020). The akv2k8s controller does that for you out of the box.

Conclusion

Both the akv2k8s controller and the Secrets Store CSI driver (for Azure) can be used to achieve the same objective: syncing secrets, keys and certificates from Key Vault to AKS. In my experience, the akv2k8s controller is easier to use. The big advantage of the Secrets Store CSI driver is that it is a broader solution (not just for AKS) and supports multiple secret stores. Next to Azure Key Vault, it also supports Hashicorp’s Vault for example. My recommendation: for Azure Key Vault and AKS, keep it simple and try akv2k8s first!