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

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Date: Mon, 26 Feb 2024 10:56:13 -0600
From: John Allen <john.allen@....com>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: kvm@...r.kernel.org, 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 Thu, Nov 02, 2023 at 08:07:20PM +0200, Maxim Levitsky wrote:
> 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.

Hi Maxim,

I think you're right on this. My next version omits the patch and
testing seems to confirm that it's not needed.

> 
> 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?

>From what I gather, for a non-SEV-ES guest using the save area that is part
of the VMCB as opposed to the separate VMCB/VMSA for SEV-ES and SEV-SNP,
anything listed in that save area will effectively be swap type A. Does that
answer your question?

Thanks,
John

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