Design notes

Kernel Module Management brings a new Module CRD.

Module represents an out-of-tree kernel module that must be inserted into the cluster nodes’ kernel through a ModuleLoader scheduled by a DaemonSet.

KMM optionally builds and runs DriverContainers on the cluster.

It picks the right ModuleLoader image by leveraging kernel mappings in the Module CRD that describe which image should be used for which kernel.

A kernel mapping maps either a literal kernel name or a regex with a kernel’s node.

This allows for more flexibility when targeting a set of kernels (e.g. “for Ubuntu nodes, build from that repository”).

Variables available at build time still reflect the actual kernel version.

Module CRD

apiVersion: kmm.sigs.x-k8s.io/v1beta1
kind: Module
metadata:
  name: module-sample
spec:
  additionalVolumes:
    # All volumes here are available into the DriverContainer / DevicePlugin pod.
    # Users should mount those by adding corresponding volumeMounts under .spec.devicePlugin
    # and .spec.moduleLoader.
    - name: some-volume
      configMap:
        name: some-configmap
  devicePlugin: # is a Container spec
    # This container will be privileged and will mount
    # /var/lib/kubelet/device-plugins automatically.
    name: overwritten-anyway
    image: some-image
  moduleLoader: # is a Container spec
    # This container will not be privileged by default.
    # It will mount /lib/modules and /usr/lib/modules automatically.
    name: overwritten-anyway
    image: overwritten-anyway
    securityContext:
      capabilities:
        add: [SYS_MODULE] # this is enough in most cases
  kernelMappings:
    - literal: 5.16.11-200.fc35.x86_64
      containerImage: quay.io/vendor/module-sample:fedora-5.16.11-200.fc35.x86_64
    - literal: 5.4.0-1054-gke
      containerImage: quay.io/vendor/module-sample:ubuntu-5.4.0-1054-gke
    - regexp: '^.*\-gke$'
      build:
        buildArgs:
          - name: SOME_KEY
            value: SOME_VALUE
        pull:
          insecure: false
        push:
          insecure: false
        dockerfile: |
          FROM some-image
          RUN some-command
      containerImage: quay.io/vendor/module-sample:gke
  selector: # top-level selector
    feature.node.kubernetes.io/cpu-cpuid.VMX: true
status:
  conditions:
    - lastTransitionTime: "2022-03-07T14:01:00Z"
      status: False
      type: Progressing
    - lastTransitionTime: "2022-03-07T14:01:01Z"
      status: True
      type: Ready

In-cluster builds

Virtually any Kubernetes distribution ships with its own kernel. It is thus challenging for a kernel module vendor to make DriverContainer images available for all kernels available in all Kubernetes distributions out there (or even for a subset of them).

KMM supports in-cluster builds of DriverContainer images when those are not made available by the vendor. The in-cluster build system is able to build an image from any Git repository that contains a Dockerfile. It can then push said image to a user-provided repository, optionally using authentication provided in a Kubernetes secret.

Optional: on upstream Kubernetes, we may want to deploy an in-cluster registry to host in-cluster built images.

On OCP, the build mechanism would be BuildConfig (maybe Shipwright in the future) and we can leverage the integrated in-cluster registry.

Security

DriverContainer privileges

By default, the operator would only grant the CAP_SYS_MODULE capability to DriverContainer DaemonSets.

Kubernetes API privileges

The KMM Operator would only be granted a limited set of Kubernetes API privileges:

• Read, modify (for kernel version labeling) and watch Nodes;
• Read and watch Modules, update their status;
• Read, create, modify and watch DaemonSets;
• Read, create, modify and watch Pod objects (used for building and signing);
• Read Secrets for pull and build secrets.
• Read Configmaps for build pods