Confer Worker Image

Confidential GCP worker image for document and code generation.

Overview

This repository uses Nix and mkosi to build a minimal, reproducible Ubuntu 24.04 image. It includes:

• a unified kernel image (UKI) and dm-verity-protected SquashFS root;
• a standard-library Python listener that reserves exactly one connection;
• OpenSSH in inetd mode for the reserved, unprivileged worker session;
• Python document libraries, PDF tools, and fonts; and
• an ephemeral tmpfs workspace with measured network and process restrictions.

The VM has no administrative SSH service or external IP. It stops accepting new work after a randomly selected 18–20-hour window so workers rotate gradually.

Architecture

1. The worker advertises health while its one-shot listener is available.
2. The controller reserves it and relays a challenge and the proxy's ephemeral SSH client key.
3. The worker returns a hardware attestation quote, its ephemeral SSH host key, and its exact Sigstore manifest and inclusion bundle from instance metadata.
4. The proxy verifies the quote, signed measurements, signing identity, and inclusion proof before beginning SSH on the same connection.
5. OpenSSH accepts only the attested client key and disables passwords, PTYs, forwarding, user startup files, and concurrent sessions.

The quote binds the challenge and both SSH keys. Any change to the measured boot chain changes the attestation measurements and is rejected by the proxy.

Building

Prerequisite: Nix with flakes enabled on Linux.

nix develop
make test
make build
make gcp-archive
make prepare-release

The build produces mkosi.output/confer-worker-image_<version>.raw and a Compute Engine archive named confer-worker-image_<version>.tar.gz.

The release workflow additionally runs sudo make test-linux-policy on Linux.

Measurements and releases

The release workflow derives every signed measurement from checked-in inputs:

• MRTD and RTMR0 use the reviewed GCP firmware value and normalized CCEL event fixture in the checked-in platform profile;
• RTMR1 is calculated from the disk's GPT, fallback UKI, embedded Linux EFI application, and EDK2 boot transcript; and
• RTMR2 is empty for this direct-UKI boot configuration.

scripts/measure-release-image validates the disk and emits the four measurements. A reviewer can reproduce them with:

nix develop --command scripts/measure-release-image \
  mkosi.output/confer-worker-image_<version>.raw