linux-kernel - Re: [PATCH 5/9] KVM: SVM: Save shadow stack host state on VMRUN

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Date:   Thu, 02 Nov 2023 20:07:20 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     John Allen <john.allen@....com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        weijiang.yang@...el.com, rick.p.edgecombe@...el.com,
        seanjc@...gle.com, x86@...nel.org, thomas.lendacky@....com,
        bp@...en8.de
Subject: Re: [PATCH 5/9] KVM: SVM: Save shadow stack host state on VMRUN

On Tue, 2023-10-10 at 20:02 +0000, 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. PL0_SSP, PL1_SSP,
> and PL2_SSP are currently unused. Manually save the other type B host
> MSR values before VMRUN.
> 
> Signed-off-by: John Allen <john.allen@....com>
> ---
>  arch/x86/kvm/svm/sev.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index b9a0a939d59f..bb4b18baa6f7 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3098,6 +3098,15 @@ void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa)
>  		hostsa->dr2_addr_mask = amd_get_dr_addr_mask(2);
>  		hostsa->dr3_addr_mask = amd_get_dr_addr_mask(3);
>  	}
> +
> +	if (boot_cpu_has(X86_FEATURE_SHSTK)) {
> +		/*
> +		 * MSR_IA32_U_CET 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_PL3_SSP, hostsa->pl3_ssp);
> +	}
>  }


Do we actually need this patch?

Host FPU state is not encrypted, and as far as I understood from the common CET patch series,
that on return to userspace these msrs will be restored.

Best regards,
	Maxim Levitsky


PS: AMD's APM is silent on how 'S_CET, SSP, and ISST_ADDR' are saved/restored for non encrypted guests.
Are they also type A?

Can the VMSA table be trusted in general to provide the same swap type as for the non encrypted guests?


>  
>  void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)