Add Documentation for Kubelet CSR Approver

Co-Authored-By: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

docs/hardening.md

@@ -117,7 +117,8 @@ Let's take a deep look to the resultant **kubernetes** configuration:
 * The `anonymous-auth` (on `kube-apiserver`) is set to `true` by default. This is fine, because it is considered safe if you enable `RBAC` for the `authorization-mode`.
 * The `enable-admission-plugins` has not the `PodSecurityPolicy` admission plugin. This because it is going to be definitely removed from **kubernetes** `v1.25`. For this reason we decided to set the newest `PodSecurity` (for more details, please take a look here: <https://kubernetes.io/docs/concepts/security/pod-security-admission/>). Then, we set the `EventRateLimit` plugin, providing additional configuration files (that are automatically created under the hood and mounted inside the `kube-apiserver` container) to make it work.
 * The `encryption-provider-config` provide encryption at rest. This means that the `kube-apiserver` encrypt data that is going to be stored before they reach `etcd`. So the data is completely unreadable from `etcd` (in case an attacker is able to exploit this).
-* The `rotateCertificates` in `KubeletConfiguration` is set to `true` along with `serverTLSBootstrap`. This could be used in alternative to `tlsCertFile` and `tlsPrivateKeyFile` parameters. Additionally it automatically generates certificates by itself, but you need to manually approve them or at least using an operator to do this (for more details, please take a look here: <https://kubernetes.io/docs/reference/command-line-tools-reference/kubelet-tls-bootstrapping/>).
+* The `rotateCertificates` in `KubeletConfiguration` is set to `true` along with `serverTLSBootstrap`. This could be used in alternative to `tlsCertFile` and `tlsPrivateKeyFile` parameters. Additionally it automatically generates certificates by itself. By default the CSRs are approved automatically via [kubelet-csr-approver](https://github.com/postfinance/kubelet-csr-approver). You can customize approval configuration by modifying Helm values via `kubelet_csr_approver_values`.
+  See <https://kubernetes.io/docs/reference/command-line-tools-reference/kubelet-tls-bootstrapping/> for more information on the subject.
 * If you are installing **kubernetes** in an AppArmor-based OS (eg. Debian/Ubuntu) you can enable the `AppArmor` feature gate uncommenting the lines with the comment `# AppArmor-based OS` on top.
 * The `kubelet_systemd_hardening`, both with `kubelet_secure_addresses` setup a minimal firewall on the system. To better understand how these variables work, here's an explanatory image:
   ![kubelet hardening](img/kubelet-hardening.png)
@@ -134,5 +135,3 @@ ansible-playbook -v cluster.yml \
 ```
 
 **N.B.** The `vars.yaml` contains our general cluster information (SANs, load balancer, dns, etc..) and `hardening.yaml` is the file described above.
-
-Once completed the cluster deployment, don't forget to approve the generated certificates (check them with `kubectl get csr`, approve with `kubectl certificate approve <csr_name>`). This action is necessary because the `secureTLSBootstrap` option and `RotateKubeletServerCertificate` feature gate for `kubelet` are enabled (CIS [4.2.11](https://www.tenable.com/audits/items/CIS_Kubernetes_v1.20_v1.0.0_Level_1_Worker.audit:05af3dfbca8e0c3fb3559c6c7de29191), [4.2.12](https://www.tenable.com/audits/items/CIS_Kubernetes_v1.20_v1.0.0_Level_1_Worker.audit:5351c76f8c5bff8f98c29a5200a35435)).

docs/vars.md

@@ -199,9 +199,9 @@ Stack](https://github.com/kubernetes-sigs/kubespray/blob/master/docs/dns-stack.m
 
 * *kubelet_rotate_server_certificates* - Auto rotate the kubelet server certificates by requesting new certificates
   from the kube-apiserver when the certificate expiration approaches.
-  **Note** that server certificates are **not** approved automatically. Approve them manually
-  (`kubectl get csr`, `kubectl certificate approve`) or implement custom approving controller like
-  [kubelet-rubber-stamp](https://github.com/kontena/kubelet-rubber-stamp).
+  Note that enabling this also activates *kubelet_csr_approver* which approves automatically the CSRs.
+  To customize its behavior, you can override the Helm values via *kubelet_csr_approver_values*.
+  See [kubelet-csr-approver](https://github.com/postfinance/kubelet-csr-approver) for more information.
 
 * *kubelet_streaming_connection_idle_timeout* - Set the maximum time a streaming connection can be idle before the connection is automatically closed.