SPIFFE CSI Driver

A Container Storage Interface driver for Kubernetes that facilitates injection of the SPIFFE Workload API.

The SPIFFE Workload API is nominally served over a Unix domain socket. Some SPIFFE implementations (e.g. SPIRE) rely on DaemonSets to run one Workload API server instance per host. In these cases, it is necessary to inject the Workload API socket into each pod. The primary motivation for using a CSI driver for this purpose is to avoid the use of hostPath volumes in workload containers, which is commonly disallowed or limited by policy due to inherent security concerns. Note that hostPath volumes are still required for the CSI driver to interact with the Kubelet (see Limitations).

This driver mounts a directory containing a SPIFFE Workload API socket provided by a SPIFFE implementation (e.g. SPIRE) as an ephemeral inline volume into workload pods.

How it Works

This component is generally deployed as a container in the DaemonSet that provides the Workload API implementation (e.g SPIRE agent) and registered with the kubelet using the official CSI Node Driver Registrar image. The SPIFFE CSI Driver and the Workload API implementation share the directory hosting the Workload API Unix Domain Socket using an hostPath volume. An emptyDir volume cannot be used since the backing directory would be removed if the SPIFFE CSI Driver pod is restarted, invalidating the mount into workload containers.

When pods declare an ephemeral inline mount using this driver, the driver is invoked to mount the volume. The driver does a read-only bind mount of the directory containing the Workload API Unix Domain Socket into the container at the requested target path.

Similarly, when the pod is destroyed, the driver is invoked and removes the bind mount.

Dependencies

CSI Ephemeral Inline Volumes require at least Kubernetes 1.15 (enabled via the CSIInlineVolume feature gate) or 1.16 (enabled by default).

Limitations

CSI drivers are registered as plugins and otherwise interact with the Kubelet, which requires several hostPath volumes. As such, this driver cannot be used in environments where hostPath volumes are forbidden.

Example

An example deployment can be found here.

Troubleshooting

This component has a fairly simple design and function but some of the following problems may manifest.

Failure to Register with the Kubelet

This problem can be diagnosed by dumping the logs of the kubelet (if possible), the driver registrar container, and the SPIFFE CSI driver container. Likely suspects are a misconfiguratoin of the various volume mounts needed for communication between the register, the SPIFFE CSI driver, and the kubelet.

Failure to Mount the Socket Directory

This problem can be diagnosed by dumping the SPIFFE CSI driver logs.

Failure to Terminate Pods when Driver is Unhealthy Or Removed

If the SPIFFE CSI Driver is removed (or is otherwise unhealthy), any pods that contain a volume mounted by the driver will fail to fully terminate until driver health is restored. The describe command (i.e. kubectl describe) will show the failure to unmount the volume. Kubernetes will continue to retry to unmount the volume via the CSI driver. Once the driver has been restored, the unmounting will eventually succeed and the pod will be fully terminated.

Broken Mount when the CSI Driver Pod is Restarted

Ensure that the Workload API socket directory is shared with the SPIFFE CSI Driver via a hostPath volume. The directory backing emptyDir volumes are tied to the pod instance and invalidated when the pod is restarted.

Reporting a Vulnerability

Vulnerabilities can be reported by sending an email to [email protected]. A confirmation email will be sent to acknowledge the report within 72 hours. A second acknowledgement will be sent within 7 days when the vulnerability has been positively or negatively confirmed.