Kubernetes 1.25: alpha support for running Pods with user namespaces

Authors: Rodrigo Campos (Microsoft), Giuseppe Scrivano (Red Hat)

Kubernetes v1.25 introduces the support for user namespaces.

This is a major improvement for running secure workloads in Kubernetes. Each pod will have access only to a limited subset of the available UIDs and GIDs on the system, thus adding a new security layer to protect from other pods running on the same system.

How does it work?

A process running on Linux can use up to 4294967296 different UIDs and GIDs.

User namespaces is a Linux feature that allows mapping a set of users in the container to different users in the host, thus restricting what IDs a process can effectively use. Furthermore, the capabilities granted in a new user namespace do not apply in the host initial namespaces.

Why is it important?

There are mainly two reasons why user namespaces are important:

• improve security since they restrict the IDs a pod can use, so each pod can run in its own separate environment with unique IDs.

• enable running workloads as root in a safer manner.

In a user namespace we can map the root user inside the pod to a non-zero ID outside the container, containers believe in running as root while they are a regular unprivileged ID from the host point of view.

The process can keep capabilities that are usually restricted to privileged pods and do it in a safe way since the capabilities granted in a new user namespace do not apply in the host initial namespaces.

How do I enable user namespaces?

At the moment, user namespaces support is opt-in, so you must enable it for a pod setting hostUsers to false under the pod spec stanza:

apiVersion: v1
kind: Pod
spec:
  hostUsers: false
  containers:
  - name: nginx
    image: docker.io/nginx

The feature is behind a feature gate, so make sure to enable the UserNamespacesStatelessPodsSupport gate before you can use the new feature.

The runtime must also support user namespaces:

• containerd: support is planned for the 1.7 release. See containerd issue #7063 for more details.

• CRI-O: v1.25 has support for user namespaces.

Support for this in cri-dockerd is not planned yet.

How do I get involved?

You can reach SIG Node by several means:

You can also contact us directly:

• GitHub / Slack: @rata @giuseppe