Re: [RFC PATCH v2 4/6] KVM: SVM: Save shadow stack host state on VMRUN

[Sean Christopherson]

On Wed, May 24, 2023, John Allen wrote:
> When running as an SEV-ES guest, the PL0_SSP, PL1_SSP, PL2_SSP, PL3_SSP,
> and U_CET fields in the VMCB save area are type B, meaning the host
> state is automatically loaded on a VMEXIT, but is not saved on a VMRUN.
> The other shadow stack MSRs, S_CET, SSP, and ISST_ADDR are type A,
> meaning they are loaded on VMEXIT and saved on VMRUN. Manually save the
> type B host MSR values before VMRUN.
> 
> Signed-off-by: John Allen <john.allen@xxxxxxx>
> ---
>  arch/x86/kvm/svm/sev.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index c25aeb550cd9..03dd68bddd51 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3028,6 +3028,19 @@ void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa)
>  
>  	/* MSR_IA32_XSS is restored on VMEXIT, save the currnet host value */
>  	hostsa->xss = host_xss;
> +
> +	if (boot_cpu_has(X86_FEATURE_SHSTK)) {
> +		/*
> +		 * MSR_IA32_U_CET, MSR_IA32_PL0_SSP, MSR_IA32_PL1_SSP,
> +		 * MSR_IA32_PL2_SSP, and MSR_IA32_PL3_SSP are restored on
> +		 * VMEXIT, save the current host values.
> +		 */
> +		rdmsrl(MSR_IA32_U_CET, hostsa->u_cet);
> +		rdmsrl(MSR_IA32_PL0_SSP, hostsa->vmpl0_ssp);
> +		rdmsrl(MSR_IA32_PL1_SSP, hostsa->vmpl1_ssp);
> +		rdmsrl(MSR_IA32_PL2_SSP, hostsa->vmpl2_ssp);
> +		rdmsrl(MSR_IA32_PL3_SSP, hostsa->vmpl3_ssp);

Heh, can you send a patch to fix the names for the PLx_SSP fields?  They should
be ->plN_ssp, not ->vmplN_ssp.

As for the values themselves, the kernel doesn't support Supervisor Shadow Stacks
(SSS), so PL0-2_SSP are guaranteed to be zero.  And if/when SSS support is added,
I doubt the kernel will ever use PL1_SSP or PL2_SSP, so those can probably be
ignored entirely, and PL0_SSP might be constant per task?  In other words, I don't
see any reason to try and track the host values for support that doesn't exist,
just do what VMX does for BNDCFGS and yell if the MSRs are non-zero.  Though for
SSS it probably makes sense for KVM to refuse to load (KVM continues on for BNDCFGS
because it's a pretty safe assumption that the kernel won't regain MPX supported).

E.g. in rough pseudocode

	if (boot_cpu_has(X86_FEATURE_SHSTK)) {
		rdmsrl(MSR_IA32_PLx_SSP, host_plx_ssp);

		if (WARN_ON_ONCE(host_pl0_ssp || host_pl1_ssp || host_pl2_ssp))
			return -EIO;
	}