linux-kernel - Re: [PATCH v5 5/8] KVM: VMX: Load Guest CET via VMCS when CET is enabled in Guest

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Date:   Tue, 4 Jun 2019 13:03:36 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, mst@...hat.com, rkrcmar@...hat.com,
        jmattson@...gle.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, yu-cheng.yu@...el.com
Subject: Re: [PATCH v5 5/8] KVM: VMX: Load Guest CET via VMCS when CET is
 enabled in Guest

On Wed, May 22, 2019 at 03:00:58PM +0800, Yang Weijiang wrote:
> "Load Guest CET state" bit controls whether Guest CET states
> will be loaded at Guest entry. Before doing that, KVM needs
> to check if CPU CET feature is available to Guest.
> 
> Note: SHSTK and IBT features share one control MSR:
> MSR_IA32_{U,S}_CET, which means it's difficult to hide
> one feature from another in the case of SHSTK != IBT,
> after discussed in community, it's agreed to allow Guest
> control two features independently as it won't introduce
> security hole.
> 
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> Co-developed-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 9321da538f65..1c0d487a4037 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -47,6 +47,7 @@
>  #include <asm/spec-ctrl.h>
>  #include <asm/virtext.h>
>  #include <asm/vmx.h>
> +#include <asm/cet.h>

Is this include actually needed?  I haven't attempted to compile, but a
glance everything should be in cpufeatures.h or vmx.h.

>  #include "capabilities.h"
>  #include "cpuid.h"
> @@ -2929,6 +2930,17 @@ int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>  		if (!nested_vmx_allowed(vcpu) || is_smm(vcpu))
>  			return 1;
>  	}
> +	if (guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) ||
> +	    guest_cpuid_has(vcpu, X86_FEATURE_IBT)) {
> +		if (cr4 & X86_CR4_CET)
> +			vmcs_set_bits(VM_ENTRY_CONTROLS,
> +				      VM_ENTRY_LOAD_GUEST_CET_STATE);
> +		else
> +			vmcs_clear_bits(VM_ENTRY_CONTROLS,
> +					VM_ENTRY_LOAD_GUEST_CET_STATE);
> +	} else if (cr4 & X86_CR4_CET) {
> +		return 1;
> +	}

Don't we also need to check for host CET support prior to toggling
VM_ENTRY_LOAD_GUEST_CET_STATE?

>  
>  	if (to_vmx(vcpu)->nested.vmxon && !nested_cr4_valid(vcpu, cr4))
>  		return 1;
> -- 
> 2.17.2
>