Allow hostNetwork pods to use user namespaces

Author: HirazawaUi

keps/sig-node/5607-hostnetwork-userns/README.md

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+# KEP-5607: Allow HostNetwork Pods to Use User Namespaces
+
+<!-- toc -->
+- [Release Signoff Checklist](#release-signoff-checklist)
+- [Summary](#summary)
+- [Motivation](#motivation)
+  - [Goals](#goals)
+  - [Non-Goals](#non-goals)
+- [Proposal](#proposal)
+  - [User Stories (Optional)](#user-stories-optional)
+    - [Story 1](#story-1)
+  - [Notes/Constraints/Caveats (Optional)](#notesconstraintscaveats-optional)
+  - [Risks and Mitigations](#risks-and-mitigations)
+- [Design Details](#design-details)
+  - [Test Plan](#test-plan)
+      - [Prerequisite testing updates](#prerequisite-testing-updates)
+      - [Unit tests](#unit-tests)
+      - [Integration tests](#integration-tests)
+      - [e2e tests](#e2e-tests)
+  - [Graduation Criteria](#graduation-criteria)
+    - [Alpha](#alpha)
+    - [Beta](#beta)
+    - [GA](#ga)
+  - [Upgrade / Downgrade Strategy](#upgrade--downgrade-strategy)
+  - [Version Skew Strategy](#version-skew-strategy)
+- [Production Readiness Review Questionnaire](#production-readiness-review-questionnaire)
+  - [Feature Enablement and Rollback](#feature-enablement-and-rollback)
+  - [Rollout, Upgrade and Rollback Planning](#rollout-upgrade-and-rollback-planning)
+  - [Monitoring Requirements](#monitoring-requirements)
+  - [Dependencies](#dependencies)
+  - [Scalability](#scalability)
+  - [Troubleshooting](#troubleshooting)
+- [Implementation History](#implementation-history)
+- [Drawbacks](#drawbacks)
+- [Alternatives](#alternatives)
+- [Infrastructure Needed (Optional)](#infrastructure-needed-optional)
+<!-- /toc -->
+
+## Release Signoff Checklist
+
+<!--
+**ACTION REQUIRED:** In order to merge code into a release, there must be an
+issue in [kubernetes/enhancements] referencing this KEP and targeting a release
+milestone **before the [Enhancement Freeze](https://git.k8s.io/sig-release/releases)
+of the targeted release**.
+
+For enhancements that make changes to code or processes/procedures in core
+Kubernetes—i.e., [kubernetes/kubernetes], we require the following Release
+Signoff checklist to be completed.
+
+Check these off as they are completed for the Release Team to track. These
+checklist items _must_ be updated for the enhancement to be released.
+-->
+
+Items marked with (R) are required *prior to targeting to a milestone / release*.
+
+- [ ] (R) Enhancement issue in release milestone, which links to KEP dir in [kubernetes/enhancements] (not the initial KEP PR)
+- [ ] (R) KEP approvers have approved the KEP status as `implementable`
+- [ ] (R) Design details are appropriately documented
+- [ ] (R) Test plan is in place, giving consideration to SIG Architecture and SIG Testing input (including test refactors)
+  - [ ] e2e Tests for all Beta API Operations (endpoints)
+  - [ ] (R) Ensure GA e2e tests meet requirements for [Conformance Tests](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-architecture/conformance-tests.md)
+  - [ ] (R) Minimum Two Week Window for GA e2e tests to prove flake free
+- [ ] (R) Graduation criteria is in place
+  - [ ] (R) [all GA Endpoints](https://github.com/kubernetes/community/pull/1806) must be hit by [Conformance Tests](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-architecture/conformance-tests.md) within one minor version of promotion to GA
+- [ ] (R) Production readiness review completed
+- [ ] (R) Production readiness review approved
+- [ ] "Implementation History" section is up-to-date for milestone
+- [ ] User-facing documentation has been created in [kubernetes/website], for publication to [kubernetes.io]
+- [ ] Supporting documentation—e.g., additional design documents, links to mailing list discussions/SIG meetings, relevant PRs/issues, release notes
+
+<!--
+**Note:** This checklist is iterative and should be reviewed and updated every time this enhancement is being considered for a milestone.
+-->
+
+[kubernetes.io]: https://kubernetes.io/
+[kubernetes/enhancements]: https://git.k8s.io/enhancements
+[kubernetes/kubernetes]: https://git.k8s.io/kubernetes
+[kubernetes/website]: https://git.k8s.io/website
+
+## Summary
+
+This KEP proposes introducing a new feature gate to allow Pods to have both `hostNetwork` enabled and user namespaces enabled (by setting `hostUsers: false`).
+
+## Motivation
+
+The primary motivation is to enhance the security of Kubernetes control plane components. Many control plane components, such as the `kube-apiserver` and `kube-controller-manager` often run as static Pods and are configured with `hostNetwork: true` to bind to node ports or interact directly with the host's network stack.
+
+Currently, a validation rule in the kube-apiserver prevents the combination of `hostNetwork: true` and `hostUsers: false`. This KEP aims to remove that barrier.
+
+### Goals
+
+* Introduce a new, separate alpha feature gate: `UserNamespacesHostNetworkSupport`.
+
+* When this feature gate is enabled, modify the Pod validation logic to allow Pod specs where `spec.hostNetwork` is true and `spec.hostUsers` is false.
+
+### Non-Goals
+
+Including this functionality as part of the `UserNamespacesSupport` feature gate. As `UserNamespacesSupport` is nearing GA, it would be unwise to add a new, unstable feature with external dependencies.
+
+## Proposal
+
+We propose the introduction of a new feature gate named `UserNamespacesHostNetworkSupport`.
+
+When this feature gate is disabled (the default state), the kube-apiserver will maintain the current validation behavior, rejecting any Pod spec that includes both `spec.hostNetwork: true` and `spec.hostUsers: false`.
+
+When the `UserNamespacesHostNetworkSupport` feature gate is enabled, we will relax this validation check. 
+The kube-apiserver will accept such a Pod spec and pass it on to the kubelet. 
+At this point, the responsibility for successfully creating and running the Pod shifts to the container runtime. 
+If the low-level container runtime (e.g., containerd/runc) does not support this combination, the pod will remain stuck in the `ContainerCreating` state and report an exception event, which is the expected behavior.
+
+This change will primarily involve modifying the Pod validation function in pkg/apis/core/validation/validation.go to account for the state of the new feature gate.
+
+### User Stories (Optional)
+
+#### Story 1
+As a cluster administrator, I want to enable user namespaces for my control plane static Pods (e.g., kube-apiserver, kube-controller-manager) to follow the principle of least privilege and reduce the attack surface. These Pods need to use hostNetwork to interact correctly with the cluster network. By enabling the new feature gate, I can add a critical layer of security isolation to these vital components without changing their networking model.
+
+
+### Notes/Constraints/Caveats (Optional)
+
+### Risks and Mitigations
+
+
+## Design Details
+
+The core design change is very simple: in the apiserver's Pod validation logic, locate the code block that prevents the `hostNetwork: true` and `hostUsers: false` combination, and wrap it in a conditional that only executes the validation if the `UserNamespacesHostNetworkSupport` feature gate is disabled.
+```
+func validateHostUsers(spec *core.PodSpec, fldPath *field.Path, opts PodValidationOptions) field.ErrorList {
+	allErrs := field.ErrorList{}
+
+	// ... existing validations ...
+
+	// Note we already validated above spec.SecurityContext is not nil.
+	if !utilfeature.DefaultFeatureGate.Enabled(features.UserNamespacesHostNetworkSupport) && spec.SecurityContext.HostNetwork {
+		allErrs = append(allErrs, field.Forbidden(fldPath.Child("hostNetwork"), "when `hostUsers` is false"))
+	}
+
+  // ... existing validations ...
+
+	return allErrs
+}
+
+```
+
+### Test Plan
+
+[ ] I/we understand the owners of the involved components may require updates to
+existing tests to make this code solid enough prior to committing the changes necessary
+to implement this enhancement.
+
+##### Prerequisite testing updates
+
+##### Unit tests
+
+- `pkg/apis/core/validation`: `2025-10-03` - `85.1%`
+
+##### Integration tests
+
+##### e2e tests
+
+- Add e2e tests to ensure that pods with the combination of `hostNetwork: true` and `hostUsers: false` can run properly.
+
+### Graduation Criteria
+
+#### Alpha
+
+- The `UserNamespacesHostNetworkSupport` feature gate is implemented and disabled by default.
+
+#### Beta
+
+- At least one mainstream container runtime and one low-level container runtime (e.g., containerd/runc) have released official versions supporting the simultaneous enabling of hostNetwork and user namespaces.
+- Add e2e tests to ensure feature availability.
+
+#### GA
+
+- The feature has been stable in Beta for at least 2 Kubernetes releases.
+- Multiple major container runtimes support the feature.
+
+
+### Upgrade / Downgrade Strategy
+
+Upgrade: After upgrading to a version that supports this KEP, the `UserNamespacesHostNetworkSupport` feature gate can be enabled at any time.
+
+Downgrade: If downgrading to a version that does not support this KEP, the kube-apiserver will revert to strict validation. Pods already running with this combination will be unaffected, but new or updated Pod requests attempting to use this combination will be rejected.
+
+### Version Skew Strategy
+
+A newer kube-apiserver with this feature enabled will accept such a Pod.
+
+An older kubelet will still get the Pod definition from the kube-apiserver. 
+It will attempt to create the Pod, and the success or failure will depend on the version of the container runtime it is using.
+
+## Production Readiness Review Questionnaire
+
+### Feature Enablement and Rollback
+
+###### How can this feature be enabled / disabled in a live cluster?
+
+- [ ] Feature gate (also fill in values in `kep.yaml`)
+  - Feature gate name: `UserNamespacesHostNetworkSupport`
+  - Components depending on the feature gate: `kube-apiserver`
+- [ ] Other
+  - Describe the mechanism:
+  - Will enabling / disabling the feature require downtime of the control
+    plane?
+  - Will enabling / disabling the feature require downtime or reprovisioning
+    of a node?
+
+###### Does enabling the feature change any default behavior?
+No. The behavior only changes when a user explicitly sets both `hostNetwork: true` and `hostUsers: false` in a Pod spec. 
+The behavior of all existing Pods is unaffected.
+
+###### Can the feature be disabled once it has been enabled (i.e. can we roll back the enablement)?
+
+Yes. It can be disabled by setting the feature gate to false and restarting the kube-apiserver.
+This restores the old validation logic. 
+It will not affect any Pods already running with this combination but will prevent new ones from being created.
+
+###### What happens if we reenable the feature if it was previously rolled back?
+The kube-apiserver will once again begin to accept the combination of `hostNetwork: true` and `hostUsers: false`.
+This is a stateless change, and reenabling is safe.
+
+###### Are there any tests for feature enablement/disablement?
+
+### Rollout, Upgrade and Rollback Planning
+
+###### How can a rollout or rollback fail? Can it impact already running workloads?
+
+The [Version Skew Strategy](#version-skew-strategy) section covers this point.
+
+###### What specific metrics should inform a rollback?
+
+N/A
+
+###### Were upgrade and rollback tested? Was the upgrade->downgrade->upgrade path tested?
+
+This will be validated via manual testing. 
+
+###### Is the rollout accompanied by any deprecations and/or removals of features, APIs, fields of API types, flags, etc.?
+
+No.
+
+### Monitoring Requirements
+
+<!--
+This section must be completed when targeting beta to a release.
+
+For GA, this section is required: approvers should be able to confirm the
+previous answers based on experience in the field.
+-->
+
+###### How can an operator determine if the feature is in use by workloads?
+
+<!--
+Ideally, this should be a metric. Operations against the Kubernetes API (e.g.,
+checking if there are objects with field X set) may be a last resort. Avoid
+logs or events for this purpose.
+-->
+
+###### How can someone using this feature know that it is working for their instance?
+
+<!--
+For instance, if this is a pod-related feature, it should be possible to determine if the feature is functioning properly
+for each individual pod.
+Pick one more of these and delete the rest.
+Please describe all items visible to end users below with sufficient detail so that they can verify correct enablement
+and operation of this feature.
+Recall that end users cannot usually observe component logs or access metrics.
+-->
+
+- [ ] Events
+  - Event Reason: 
+- [ ] API .status
+  - Condition name: 
+  - Other field: 
+- [ ] Other (treat as last resort)
+  - Details:
+
+###### What are the reasonable SLOs (Service Level Objectives) for the enhancement?
+
+<!--
+This is your opportunity to define what "normal" quality of service looks like
+for a feature.
+
+It's impossible to provide comprehensive guidance, but at the very
+high level (needs more precise definitions) those may be things like:
+  - per-day percentage of API calls finishing with 5XX errors <= 1%
+  - 99% percentile over day of absolute value from (job creation time minus expected
+    job creation time) for cron job <= 10%
+  - 99.9% of /health requests per day finish with 200 code
+
+These goals will help you determine what you need to measure (SLIs) in the next
+question.
+-->
+
+###### What are the SLIs (Service Level Indicators) an operator can use to determine the health of the service?
+
+<!--
+Pick one more of these and delete the rest.
+-->
+
+- [ ] Metrics
+  - Metric name:
+  - [Optional] Aggregation method:
+  - Components exposing the metric:
+- [ ] Other (treat as last resort)
+  - Details:
+
+###### Are there any missing metrics that would be useful to have to improve observability of this feature?
+
+<!--
+Describe the metrics themselves and the reasons why they weren't added (e.g., cost,
+implementation difficulties, etc.).
+-->
+
+### Dependencies
+
+###### Does this feature depend on any specific services running in the cluster?
+
+No
+
+### Scalability
+
+###### Will enabling / using this feature result in any new API calls?
+No.
+
+###### Will enabling / using this feature result in introducing new API types?
+No.
+
+###### Will enabling / using this feature result in any new calls to the cloud provider?
+No.
+
+###### Will enabling / using this feature result in increasing size or count of the existing API objects?
+No.
+
+###### Will enabling / using this feature result in increasing time taken by any operations covered by existing SLIs/SLOs?
+No.
+
+###### Will enabling / using this feature result in non-negligible increase of resource usage (CPU, RAM, disk, IO, ...) in any components?
+No.
+
+###### Can enabling / using this feature result in resource exhaustion of some node resources (PIDs, sockets, inodes, etc.)?
+No.
+
+### Troubleshooting
+
+###### How does this feature react if the API server and/or etcd is unavailable?
+No impact to the running workloads
+
+###### What are other known failure modes?
+If the container runtime or low-level runtime (e.g., containerd/runc) does not support the combination of hostNetwork and user namespaces, the pod will remain stuck in the `ContainerCreating` state and fail to be created.
+
+###### What steps should be taken if SLOs are not being met to determine the problem?
+
+N/A
+
+## Implementation History
+
+* 2025-10-03: Initial proposal
+
+## Drawbacks
+
+There are no known drawbacks at this time.
+
+
+## Alternatives
+
+Add this feature to the existing `UserNamespacesSupport` feature gate:
+
+  * This was ruled out because the `UserNamespacesSupport` feature is approaching GA, and its functionality should be stable.
+Adding a new, externally-dependent, and immature behavior to a nearly-GA feature would introduce unnecessary risk and delays. Keeping the two feature gates separate is cleaner and safer.
+
+Do not implement this feature:
+  * Users can use `hostPort` as an alternative to `hostNetwork`, but this may cause some disruption to the existing user environment, as certain privileged containers require direct interaction with the host network stack.
+
+## Infrastructure Needed (Optional)
+
+No new infrastructure needed.