Re: [PATCH v3 6/8] KVM:VMX: Load Guest CET via VMCS when CET is enabled in Guest

[Sean Christopherson]

On Thu, Feb 28, 2019 at 04:38:44PM +0800, Yang Weijiang wrote:
> On Thu, Feb 28, 2019 at 08:17:15AM -0800, Sean Christopherson wrote:
> > On Mon, Feb 25, 2019 at 09:27:14PM +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.
> > > 
> > > Signed-off-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > > ---
> > >  arch/x86/kvm/vmx.c | 32 ++++++++++++++++++++++++++++++++
> > >  1 file changed, 32 insertions(+)
> > > 
> > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > > index 89ee086e1729..d32cee9ee079 100644
> > > --- a/arch/x86/kvm/vmx.c
> > > +++ b/arch/x86/kvm/vmx.c
> > > @@ -55,6 +55,7 @@
> > >  #include <asm/mmu_context.h>
> > >  #include <asm/spec-ctrl.h>
> > >  #include <asm/mshyperv.h>
> > > +#include <asm/cet.h>
> > >  
> > >  #include "trace.h"
> > >  #include "pmu.h"
> > > @@ -4065,6 +4066,20 @@ static inline bool vmx_feature_control_msr_valid(struct kvm_vcpu *vcpu,
> > >  	return !(val & ~valid_bits);
> > >  }
> > >  
> > > +static int vmx_guest_cet_cap(struct kvm_vcpu *vcpu)
> > > +{
> > > +	u32 eax, ebx, ecx, edx;
> > > +
> > > +	/*
> > > +	 * Guest CET can work as long as HW supports the feature, independent
> > > +	 * to Host SW enabling status.
> > > +	 */
> > > +	cpuid_count(7, 0, &eax, &ebx, &ecx, &edx);
> > > +
> > > +	return ((ecx & bit(X86_FEATURE_SHSTK)) |
> > > +		(edx & bit(X86_FEATURE_IBT))) ? 1 : 0;
> > 
> > Given the holes in the (current) architecture/spec, I think KVM has to
> > require both features to be supported in the guest to allow CR4.CET to
> > be enabled.
> The reason why I use a "OR" here is to keep CET enabling control the
> same as that on host, right now on host, users can select to enable  SHSTK or IBT
> feature by disabling the unexpected one. It's free to select SHSTK & IBT
> or SHSTK | IBT.

Which is not the same as SHSTK != IBT in *hardware*, which is effectively
what this is allowing for the guest.  The problem is that the architecture
doesn't cleanly separate the two features, i.e. we'd have a virtualization
hole where the guest could touch state for a disabled feature.

Regardless, the guest would still be able to selectively enable each CET
feature, it would just never see a model where SHSTK != IBT.