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Bounding Self-Labeling Kubelets

Table of Contents

Motivation

Today the node client has total authority over its own Node labels. This ability is incredibly useful for the node auto-registration flow. The kubelet reports a set of well-known labels, as well as additional labels specified on the command line with --node-labels.

While this distributed method of registration is convenient and expedient, it has two problems that a centralized approach would not have. Minorly, it makes management difficult. Instead of configuring labels in a centralized place, we must configure N kubelet command lines. More significantly, the approach greatly compromises security. Below are two straightforward escalations on an initially compromised node that exhibit the attack vector.

Capturing Dedicated Workloads

Suppose company foo needs to run an application that deals with PII on dedicated nodes to comply with government regulation. A common mechanism for implementing dedicated nodes in Kubernetes today is to set a label or taint (e.g. foo/dedicated=customer-info-app) on the node and to select these dedicated nodes in the workload controller running customer-info-app.

Since the nodes self reports labels upon registration, an intruder can easily register a compromised node with label foo/dedicated=customer-info-app. The scheduler will then bind customer-info-app to the compromised node potentially giving the intruder easy access to the PII.

This attack also extends to secrets. Suppose company foo runs their outward facing nginx on dedicated nodes to reduce exposure to the company's publicly trusted server certificates. They use the secret mechanism to distribute the serving certificate key. An intruder captures the dedicated nginx workload in the same way and can now use the node certificate to read the company's serving certificate key.

Proposal

  1. Modify the NodeRestriction admission plugin to prevent Kubelets from self-setting labels within the k8s.io and kubernetes.io namespaces except for these specifically allowed labels/prefixes:

    kubernetes.io/hostname
kubernetes.io/instance-type
kubernetes.io/os
kubernetes.io/arch

beta.kubernetes.io/instance-type
beta.kubernetes.io/os
beta.kubernetes.io/arch

failure-domain.beta.kubernetes.io/zone
failure-domain.beta.kubernetes.io/region

failure-domain.kubernetes.io/zone
failure-domain.kubernetes.io/region

[*.]kubelet.kubernetes.io/*
[*.]node.kubernetes.io/*

  2. Reserve and document the node-restriction.kubernetes.io/* label prefix for cluster administrators that want to label their Node objects centrally for isolation purposes.

    The node-restriction.kubernetes.io/* label prefix is reserved for cluster administrators to isolate nodes. These labels cannot be self-set by kubelets when the NodeRestriction admission plugin is enabled.

This accomplishes the following goals:

  • continues allowing people to use arbitrary labels under their own namespaces any way they wish
  • supports legacy labels kubelets are already adding
  • provides a place under the kubernetes.io label namespace for node isolation labeling
  • provide a place under the kubernetes.io label namespace for kubelets to self-label with kubelet and node-specific labels

Implementation Timeline

v1.13:

  • Kubelet deprecates setting kubernetes.io or k8s.io labels via --node-labels, other than the specifically allowed labels/prefixes described above, and warns when invoked with kubernetes.io or k8s.io labels outside that set.
  • NodeRestriction admission prevents kubelets from adding/removing/modifying [*.]node-restriction.kubernetes.io/* labels on Node create and update
  • NodeRestriction admission prevents kubelets from adding/removing/modifying kubernetes.io or k8s.io labels other than the specifically allowed labels/prefixes described above on Node update only

v1.14:

  • Begin migration/removal of in-tree --node-labels use outside of the allowed set by addons:
    • beta.kubernetes.io/fluentd-ds-ready
      • addon: remove from the nodeSelector
      • kube-up: remove from the default --node-labels flag
    • beta.kubernetes.io/metadata-proxy-ready
      • addon: announce the nodeSelector will switch to cloud.google.com/metadata-proxy-ready in 1.15
      • kube-up: add cloud.google.com/metadata-proxy-ready=true along with the existing label to --node-labels
      • kube-up: add cloud.google.com/metadata-proxy-ready=true to existing nodes with the beta.kubernetes.io/metadata-proxy-ready=true label
    • beta.kubernetes.io/kube-proxy-ds-ready
      • addon: announce the nodeSelector will switch to node.kubernetes.io/kube-proxy-ds-ready in 1.15
      • kube-up: add node.kubernetes.io/kube-proxy-ds-ready=true along with the existing label to --node-labels
      • kube-up: add node.kubernetes.io/kube-proxy-ds-ready=true to existing nodes with the beta.kubernetes.io/kube-proxy-ds-ready=true label
    • beta.kubernetes.io/masq-agent-ds-ready
      • addon: announce the nodeSelector will switch to node.kubernetes.io/masq-agent-ds-ready in 1.16
      • kube-up: add node.kubernetes.io/masq-agent-ds-ready=true to existing nodes with the beta.kubernetes.io/masq-agent-ds-ready=true label

v1.16:

  • Complete migration/removal of in-tree --node-labels use outside of the allowed set by addons:
    • beta.kubernetes.io/metadata-proxy-ready
      • addon: change the nodeSelector to cloud.google.com/metadata-proxy-ready
      • kube-up: stop setting beta.kubernetes.io/metadata-proxy-ready
    • beta.kubernetes.io/kube-proxy-ds-ready
      • addon: change the nodeSelector to node.kubernetes.io/kube-proxy-ds-ready
      • kube-up: stop setting beta.kubernetes.io/kube-proxy-ds-ready
    • beta.kubernetes.io/masq-agent-ds-ready
      • addon: change the nodeSelector to node.kubernetes.io/masq-agent-ds-ready
  • Kubelet removes the ability to set kubernetes.io or k8s.io labels via --node-labels other than the specifically allowed labels/prefixes described above (deprecation period of 6 months for CLI elements of admin-facing components is complete)

v1.19:

  • NodeRestriction admission prevents kubelets from adding/removing/modifying kubernetes.io or k8s.io labels other than the specifically allowed labels/prefixes described above on Node update and create (oldest supported kubelet running against a v1.19 apiserver is v1.17)

Alternatives Considered

File or flag-based configuration of the apiserver to allow specifying allowed labels

  • A fixed set of labels and label prefixes is simpler to reason about, and makes every cluster behave consistently
  • File-based config isn't easily inspectable to be able to verify enforced labels
  • File-based config isn't easily kept in sync in HA apiserver setups

API-based configuration of the apiserver to allow specifying allowed labels

  • A fixed set of labels and label prefixes is simpler to reason about, and makes every cluster behave consistently
  • An API object that controls the allowed labels is a potential escalation path for a compromised node

Allow kubelets to add any labels they wish, and add NoSchedule taints if disallowed labels are added

  • To be robust, this approach would also likely involve a controller to automatically inspect labels and remove the NoSchedule taint. This seemed overly complex. Additionally, it was difficult to come up with a tainting scheme that preserved information about which labels were the cause.

Forbid all labels regardless of namespace except for a specifically allowed set

  • This was much more disruptive to existing usage of --node-labels.
  • This was much more difficult to integrate with other systems allowing arbitrary topology labels like CSI.
  • This placed restrictions on how labels outside the kubernetes.io and k8s.io label namespaces could be used, which didn't seem proper.