Preface: CVM restrictions

When OpenHCL detects that it is running as a Confidential VM it will restrict the diagnostics it sends to the VM host. This is done in order to prevent any guest secrets from being leaked to the host.

Info

Unless otherwise noted, all of the following restrictions only apply to release builds of OpenHCL for CVMs. The majority of these restrictions will not apply to debug builds of OpenHCL.

This is controlled by the enable_debug flag in the IGVM JSON definition.

Simulating CVM restrictions

Most of these restrictions can be simulated on a non-CVM OpenHCL VM by setting the OPENHCL_CONFIDENTIAL environment variable to 1, either in your IGVM JSON definition or by using the Set-VmFirmwareParameters cmdlet. This environment variable will cause OpenHCL to behave as if it is running in a CVM for the purpose of diagnostics.

Tracing

Tracing statements and spans will still be sent to the host, and therefore will still show up in ETW traces and Kusto. However, individual statements must opt in to being logged inside a CVM, as a way of affirming that they do not leak any guest secrets.

For Developers:

This is done by using the CVM_ALLOWED constant provided by the cvm_tracing crate. cvm_tracing also provides a CVM_CONFIDENTIAL constant, to mark statements that could contain secrets and should not be logged in a CVM.

Examples:

#![allow(unused)]
fn main() {
use cvm_tracing::{CVM_ALLOWED, CVM_CONFIDENTIAL};

tracing::info!(CVM_ALLOWED, foo, ?bar, "This statement will be logged in a CVM");
tracing::info!(baz, "This statement will not be logged in a CVM");
tracing::info!(CVM_CONFIDENTIAL, super_secret, "This statement will also not be logged in a CVM");

// This also works with spans.
let span = tracing::info_span!("a span", CVM_ALLOWED);
my_func.instrument(span).await;

// And the #[instrument] macro.
#[instrument(name = "foo", fields(CVM_ALLOWED))]
fn my_func() {
    // ...
}
}

Tip

Some of the tracing macros will not accept cvm_tracing::CVM_ALLOWED as an argument.

Instead, you will need to use cvm_tracing::CVM_ALLOWED, and then use just CVM_ALLOWED.

ohcldiag-dev

Most ohcldiag-dev commands will not work when connecting to a CVM.

One notable exception is the inspect command (albeit with restrictions).

inspect

The available inspect nodes for a CVM are restricted to prevent exposing guest data.

The vm/ top-level node is inaccessible, however most nodes containing information about the VTL2 processes are still available.

Crash information

Crash dumps can leak quite a bit of information, and as such, are heavily restricted in CVMs.

Dumps

Crash dumps will not be generated when a crash occurs in a CVM's VTL2.

Hyper-V MSRs

The Hyper-V crash MSRs will still be set when a crash occurs in a CVM's VTL2, but the data values will be sanitized to prevent leaking guest secrets. This will result in Hyper-V logging that a crash occurred, but there will be no debugging information available.

NOTE: This restriction also applies to debug builds of OpenHCL when running a CVM.

NOTE: This restriction cannot be simulated using OPENHCL_CONFIDENTIAL.

Saved state

Extracting the save state of a CVM is not supported. This applies both to the ohcldiag-dev save command, and to the save-on-crash registry key.

The OpenVMM Guide