Appendix: Google Cloud Debug and Hardened Images

The security properties the debug image relaxes relative to the hardened image.

Overview

A node boots a host image on the physical machine and a guest image in each of the two confidential VMs that the host launches: the Workload VM and the Key Protection Service VM.

Each image ships in two variants with separate measurements:

A production-configured node boots the hardened variant, which serves user requests.
A debug variant adds an SSH server, operator login, and debugging and crash-collection tooling for development, hardware bring-up, performance work, and security research.

Apple publishes production measurements for the hardened images only, and a client releases request data to a node only when the node’s measurements appear in the transparency log, so a node booted from a debug image can’t decrypt user requests.

Debug-image policies

The behavior of the debug image differs from the hardened image in the following ways.

Layer	Property	Hardened	Debug
Host	Operator shell access	OpenSSH is not installed, so the image exposes no interactive login.	Permits root SSH from the Workload VM to the host across the internal interface.
Host	VM serial consoles	The VMs’ virtual serial consoles are not attached.	Multiplexes both VMs’ serial consoles onto the host serial output for triage.
Host	Host serial output	Carries the host kernel console and a filtered feed of host script, crash-trace, out-of-memory, KVM, and libvirt logs.	Carries the same, plus the full serial output of both VMs.
Host	Runtime customization	The host uses the static configuration packaged on its boot disk.	Accepts changes to VM memory and disk sizes, and to the VM image reference, at runtime.
Host	GPU confidential mode	Locked to confidential-computing mode, with encrypted traffic on the NVLink interconnect between the GPUs.	Offers a profiling mode that relaxes the GPU’s confidential-computing protections.
Host	Boot breakpoints	None; the host boots through to launching the VMs.	Can halt before launching the VMs, so a researcher can examine the environment first.
Host	VM termination	The host powers off as soon as the Workload VM terminates, for any reason, making user data inaccessible.	Can be configured to pause instead, so a researcher can inspect the machine.
Workload VM, Key Protection Service VM	Shell access	OpenSSH is not installed on either VM.	Installs OpenSSH on both VMs. The host provisions an inbound credential into the Key Protection Service VM and hands the Workload VM the matching private key.
Workload VM, Key Protection Service VM	Logging and telemetry	Forwards only allowed log lines from the workload’s own components, plus the unfiltered journals of the guest platform services. Neither VM has a serial console attached, so neither emits serial logs.	Emits standard system logs, over both the serial console and the forwarding path.
Workload VM	Container log redirection	The launcher refuses to start when instance metadata asks it to redirect the container’s output streams.	Redirects the container’s output streams to the guest journal, the serial console, or both.