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

[Yang Weijiang]

On Fri, Mar 01, 2019 at 06:58:19AM -0800, Sean Christopherson wrote:
> 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.
Hi, Sean,
I'd like to understand your concerns. From my point of view, e.g., 
when only IBT is enabled, PL3_SSP MSR would be unnecessrily exposed,
this is the design "limitation", but PL3_SSP keeps 0 if SHSTK is not
configured. Could you detail your concerns?