Re: [RFC PATCH 06/12] drivers: hv: Add VSM boot driver and enable VTL1 at the partition level

[Wei Liu <wei.liu@kernel.org>]

On Tue, Sep 01, 2026 at 09:55:20AM -0700, Thara Gopinath wrote:
> Linux VBS (LVBS) uses Hyper-V's Virtual Secure Mode to run a small
> trusted kernel in VTL1 alongside the regular VTL0 kernel, so that
> security-sensitive state (e.g. hypervisor-enforced code integrity,
> credential isolation) can live behind a higher-privilege boundary
> that VTL0 compromise cannot cross. Bringing that up from Linux
> requires the VTL0 kernel to drive the VSM setup itself.
> 
> Add drivers/hv/hv_vsm_boot.c as the entry point for that sequence.
> This first step handles partition-level VTL1 enable only:
> 
>   - Probe VSM / VP-register privileges and SynIC availability before
>     doing anything.
>   - Pin init to the VTL0 boot CPU so VTL1 comes up on the same CPU
>     (later patches rely on this).
>   - Read HV_REGISTER_VSM_PARTITION_STATUS, and if VTL1 is not already
>     enabled, issue HVCALL_ENABLE_PARTITION_VTL with MBEC and confirm
>     by re-reading the register.
> 
> Signed-off-by: Thara Gopinath <tgopinath@linux.microsoft.com>
> ---
[...]
> +	cpumask_copy(mask, &current->cpus_mask);
> +	boot_cpu = cpumask_first(cpu_online_mask);
> +	set_cpus_allowed_ptr(current, cpumask_of(boot_cpu));
> +
> +	ret = hv_vsm_bootstrap_vtl();
> +	/*
> +	 * At this point VTL0 has already advertised VSM support to the
> +	 * bootloader/firmware via the Hyper-V OsLoaderIndications EFI
> +	 * variable (see the x86-stub change). That signals the platform
> +	 * that a trusted VTL1 will be brought up. If we fail to actually
> +	 * set VTL1 up here, the partition is left in a state where an
> +	 * attacker could race to configure VTL1 themselves and gain a
> +	 * higher-privilege foothold than VTL0. Panic rather than continue
> +	 * running with that exposure.
> +	 */

Okay, I think this answers my question in the previous patch -- if VTL1
is not used by us, that's a security problem.

It is unclear to me, if this code is enabled, how a generic kernel can
work without a secure kernel. There should be a way to configure the
system such that not enabling VTL1 is okay. Is there any pre-EFI command
line parsing we can do?

Wei


> +	if (ret)
> +		panic("VTL1 boot failure caused kernel panic; consult log for more details.\n");
> +
> +	set_cpus_allowed_ptr(current, mask);
> +	free_cpumask_var(mask);
> +	return ret;
> +}
> +device_initcall(hv_vsm_boot_init);
> -- 
> 2.34.1
> 
>