Add blog for Efficient SELinux volume relabeling (Beta)

content/en/blog/_posts/2023-04-18-efficient-selinux-relabeling-beta.md

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+---
+layout: blog
+title: "Kubernetes 1.27: Efficient SELinux volume relabeling (Beta)"
+date: 2023-04-18T10:00:00-08:00
+slug: kubernetes-1-27-efficient-selinux-relabeling-beta
+---
+
+**Author:** Jan Šafránek (Red Hat)
+
+# 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.
+
+{{% alert title="Note" color="info" %}}
+If a container uses `subPath` of a volume, only that `subPath` of the whole
+volume is relabeled. This allows two pods that have two different SELinux labels
+to use the same volume, as long as they use different subpaths of it.
+{{% /alert %}}
+
+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.
+
+1. The [feature gates](/docs/reference/command-line-tools-reference/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).
+
+1. The Pod must have at least `seLinuxOptions.level` assigned in its [Pod Security Context](/docs/reference/kubernetes-api/workload-resources/pod-v1/#security-context) or all Pod containers must have it set in their [Security Contexts](/docs/reference/kubernetes-api/workload-resources/pod-v1/#security-context-1).
+   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.
+
+1. The volume must be a Persistent Volume with
+   [Access Mode](/docs/concepts/storage/persistent-volumes/#access-modes)
+   `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](https://github.com/kubernetes/enhancements/issues/1710)
+   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.
+
+1. 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](/docs/reference/kubernetes-api/config-and-storage-resources/csi-driver-v1/)
+   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](https://opensource.com/business/13/11/selinux-policy-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](https://www.redhat.com/en/blog/how-selinux-separates-containers-using-multi-level-security)
+* [Why you should be using Multi-Category Security for your Linux containers](https://www.redhat.com/en/blog/why-you-should-be-using-multi-category-security-your-linux-containers)
+
+Read the KEP: [Speed up SELinux volume relabeling using mounts](https://github.com/kubernetes/enhancements/tree/master/keps/sig-storage/1710-selinux-relabeling)