[ARC] Risk in dind Mode: Workflow Containers Can Modify Node System Time via Privileged Docker-in-Docker

[be-hase]

Why are you starting this discussion?

Product Feedback

What GitHub Actions topic or product is this about?

ARC (Actions Runner Controller)

Discussion Details

Description

Summary

We are operating ARC using AutoscalingRunnerSet with a dind-based configuration (i.e., a docker:dind sidecar).

In the default Helm template, the docker:dind container (defined under initContainers with restartPolicy: Always, effectively acting as a sidecar) is configured as follows:

initContainers:
  - name: dind
    image: docker:dind
    restartPolicy: Always
    securityContext:
      privileged: true

Under this configuration, when a workflow executes docker run and the launched container runs:

date -u -s "..."

the system time of the Kubernetes node itself (CLOCK_REALTIME) is modified.

As a result:

• Because the system time is out of sync, GitHub token validation ends up failing
  • The ARC that I actually manage has become inoperable due to token expiry

Technical Background

The behavior appears consistent with Linux semantics:

• The dind sidecar runs with privileged: true
• Workflow jobs use docker run via that dind daemon
• If a workflow runs a container with --privileged or --cap-add=SYS_TIME, the container can successfully execute date -s
• Linux containers share the host kernel clock
• Therefore, modifying system time inside the container modifies the node’s clock

This does not appear to be a bug in ARC itself, but rather a consequence of the privileged dind configuration combined with Linux kernel behavior.

Questions

• Is the current privileged configuration in dind mode an intentional design choice?
• Is it expected that workflows can execute docker run --privileged in this setup?
• Is this risk documented anywhere as a known limitation of dind mode?

Thank you for your work on ARC.

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