Re: [PATCH 6/9] KVM: SVM: Add MSR_IA32_XSS to the GHCB for hypervisor kernel

[Maxim Levitsky]

On Tue, 2023-10-10 at 20:02 +0000, John Allen wrote:
> When a guest issues a cpuid instruction for Fn0000000D_x0B
> (CetUserOffset), KVM will intercept and need to access the guest
> MSR_IA32_XSS value. For SEV-ES, this is encrypted and needs to be
> included in the GHCB to be visible to the hypervisor.
> 
> Signed-off-by: John Allen <john.allen@xxxxxxx>
> ---
>  arch/x86/include/asm/svm.h |  1 +
>  arch/x86/kvm/svm/sev.c     | 12 ++++++++++--
>  arch/x86/kvm/svm/svm.c     |  1 +
>  arch/x86/kvm/svm/svm.h     |  3 ++-
>  4 files changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
> index 568d97084e44..5afc9e03379d 100644
> --- a/arch/x86/include/asm/svm.h
> +++ b/arch/x86/include/asm/svm.h
> @@ -678,5 +678,6 @@ DEFINE_GHCB_ACCESSORS(sw_exit_info_1)
>  DEFINE_GHCB_ACCESSORS(sw_exit_info_2)
>  DEFINE_GHCB_ACCESSORS(sw_scratch)
>  DEFINE_GHCB_ACCESSORS(xcr0)
> +DEFINE_GHCB_ACCESSORS(xss)

I don't see anywhere in the patch adding xss to ghcb_save_area.
What kernel version/commit these patches are based on?

>  
>  #endif
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index bb4b18baa6f7..94ab7203525f 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2445,8 +2445,13 @@ static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
>  
>  	svm->vmcb->save.cpl = kvm_ghcb_get_cpl_if_valid(svm, ghcb);
>  
> -	if (kvm_ghcb_xcr0_is_valid(svm)) {
> -		vcpu->arch.xcr0 = ghcb_get_xcr0(ghcb);
> +	if (kvm_ghcb_xcr0_is_valid(svm) || kvm_ghcb_xss_is_valid(svm)) {
> +		if (kvm_ghcb_xcr0_is_valid(svm))
> +			vcpu->arch.xcr0 = ghcb_get_xcr0(ghcb);
> +
> +		if (kvm_ghcb_xss_is_valid(svm))
> +			vcpu->arch.ia32_xss = ghcb_get_xss(ghcb);
> +
>  		kvm_update_cpuid_runtime(vcpu);
>  	}
>  
> @@ -3032,6 +3037,9 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
>  		if (guest_cpuid_has(&svm->vcpu, X86_FEATURE_RDTSCP))
>  			svm_clr_intercept(svm, INTERCEPT_RDTSCP);
>  	}
> +
> +	if (kvm_caps.supported_xss)
> +		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_XSS, 1, 1);

This is not just a virtualization hole. This allows the guest to set MSR_IA32_XSS
to whatever value it wants, and thus it might allow XSAVES to access some host msrs
that guest must not be able to access.

AMD might not yet have such msrs, but on Intel side I do see various components
like 'HDC State', 'HWP state' and such.

I understand that this is needed so that #VC handler could read this msr, and trying
to read it will cause another #VC which is probably not allowed (I don't know this detail of SEV-ES)

I guess #VC handler should instead use a kernel cached value of this msr instead, or at least
KVM should only allow reads and not writes to it.

In addition to that, if we decide to open the read access to the IA32_XSS from the guest,
this IMHO should be done in a separate patch.

Best regards,
	Maxim Levitsky


>  }
>  
>  void sev_init_vmcb(struct vcpu_svm *svm)
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 984e89d7a734..ee7c7d0a09ab 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -146,6 +146,7 @@ static const struct svm_direct_access_msrs {
>  	{ .index = MSR_IA32_PL1_SSP,                    .always = false },
>  	{ .index = MSR_IA32_PL2_SSP,                    .always = false },
>  	{ .index = MSR_IA32_PL3_SSP,                    .always = false },
> +	{ .index = MSR_IA32_XSS,                        .always = false },
>  	{ .index = MSR_INVALID,				.always = false },
>  };
>  
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index bdc39003b955..2011456d2e9f 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -30,7 +30,7 @@
>  #define	IOPM_SIZE PAGE_SIZE * 3
>  #define	MSRPM_SIZE PAGE_SIZE * 2
>  
> -#define MAX_DIRECT_ACCESS_MSRS	53
> +#define MAX_DIRECT_ACCESS_MSRS	54
>  #define MSRPM_OFFSETS	32
>  extern u32 msrpm_offsets[MSRPM_OFFSETS] __read_mostly;
>  extern bool npt_enabled;
> @@ -720,5 +720,6 @@ DEFINE_KVM_GHCB_ACCESSORS(sw_exit_info_1)
>  DEFINE_KVM_GHCB_ACCESSORS(sw_exit_info_2)
>  DEFINE_KVM_GHCB_ACCESSORS(sw_scratch)
>  DEFINE_KVM_GHCB_ACCESSORS(xcr0)
> +DEFINE_KVM_GHCB_ACCESSORS(xss)
>  
>  #endif