Kubernetes 1.27: Efficient SELinux volume relabeling (Beta)

The problem

On Linux with Security-Enhanced Linux (SELinux) enabled, it's traditionally the container runtime that applies SELinux labels to a Pod and all its volumes. Kubernetes only passes the SELinux label from a Pod's securityContext fields to the container runtime.

The container runtime then recursively changes SELinux label on all files that are visible to the Pod's containers. This can be time-consuming if there are many files on the volume, especially when the volume is on a remote filesystem.

If a Pod does not have any SELinux label assigned in Kubernetes API, the container runtime assigns a unique random one, so a process that potentially escapes the container boundary cannot access data of any other container on the host. The container runtime still recursively relabels all pod volumes with this random SELinux label.

Improvement using mount options

If a Pod and its volume meet all of the following conditions, Kubernetes will mount the volume directly with the right SELinux label. Such mount will happen in a constant time and the container runtime will not need to recursively relabel any files on it.

1. The operating system must support SELinux.

   Without SELinux support detected, kubelet and the container runtime do not do anything with regard to SELinux.

2. The feature gates ReadWriteOncePod and SELinuxMountReadWriteOncePod must be enabled. These feature gates are Beta in Kubernetes 1.27 and Alpha in 1.25.

   With any of these feature gates disabled, SELinux labels will be always applied by the container runtime by a recursive walk through the volume (or its subPaths).

3. The Pod must have at least seLinuxOptions.level assigned in its Pod Security Context or all Pod containers must have it set in their Security Contexts. Kubernetes will read the default user, role and type from the operating system defaults (typically system_u, system_r and container_t).

   Without Kubernetes knowing at least the SELinux level, the container runtime will assign a random one after the volumes are mounted. The container runtime will still relabel the volumes recursively in that case.

4. The volume must be a Persistent Volume with Access Mode ReadWriteOncePod.

   This is a limitation of the initial implementation. As described above, two Pods can have a different SELinux label and still use the same volume, as long as they use a different subPath of it. This use case is not possible when the volumes are mounted with the SELinux label, because the whole volume is mounted and most filesystems don't support mounting a single volume multiple times with multiple SELinux labels.

   If running two Pods with two different SELinux contexts and using different subPaths of the same volume is necessary in your deployments, please comment in the KEP issue (or upvote any existing comment - it's best not to duplicate). Such pods may not run when the feature is extended to cover all volume access modes.

5. The volume plugin or the CSI driver responsible for the volume supports mounting with SELinux mount options.

   These in-tree volume plugins support mounting with SELinux mount options: fc, iscsi, and rbd.

   CSI drivers that support mounting with SELinux mount options must announce that in their CSIDriver instance by setting seLinuxMount field.

   Volumes managed by other volume plugins or CSI drivers that don't set seLinuxMount: true will be recursively relabelled by the container runtime.

Mounting with SELinux context

When all aforementioned conditions are met, kubelet will pass -o context=<SELinux label> mount option to the volume plugin or CSI driver. CSI driver vendors must ensure that this mount option is supported by their CSI driver and, if necessary, the CSI driver appends other mount options that are needed for -o context to work.

For example, NFS may need -o context=<SELinux label>,nosharecache, so each volume mounted from the same NFS server can have a different SELinux label value. Similarly, CIFS may need -o context=<SELinux label>,nosharesock.

It's up to the CSI driver vendor to test their CSI driver in a SELinux enabled environment before setting seLinuxMount: true in the CSIDriver instance.

How can I learn more?

SELinux in containers: see excellent visual SELinux guide by Daniel J Walsh. Note that the guide is older than Kubernetes, it describes Multi-Category Security (MCS) mode using virtual machines as an example, however, a similar concept is used for containers.

See a series of blog posts for details how exactly SELinux is applied to containers by container runtimes:

• How SELinux separates containers using Multi-Level Security
• Why you should be using Multi-Category Security for your Linux containers

Read the KEP: Speed up SELinux volume relabeling using mounts

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