Re: [PATCH v6 5/8] KVM: VMX: Load Guest CET via VMCS when CET is enabled in Guest

[Sean Christopherson]

On Thu, Jul 25, 2019 at 11:12:43AM +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 enabled on host and 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.
> 
> Co-developed-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> ---
>  arch/x86/kvm/vmx/vmx.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index ce5d1e45b7a5..fbf9c335cf7b 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -44,6 +44,7 @@
>  #include <asm/spec-ctrl.h>
>  #include <asm/virtext.h>
>  #include <asm/vmx.h>
> +#include <asm/cet.h>
>  
>  #include "capabilities.h"
>  #include "cpuid.h"
> @@ -2923,6 +2924,18 @@ int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>  		if (!nested_vmx_allowed(vcpu) || is_smm(vcpu))
>  			return 1;
>  	}
> +	if (cpu_x86_cet_enabled() &&

It'd probably be better to check a KVM function here, e.g. a wrapper of
kvm_supported_xss().  I don't think it will ever matter, but it'd be nice
to have a single kill switch given the variety of different enable bits
for CET.

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