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

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Date:   Mon, 4 Mar 2019 10:43:07 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, rkrcmar@...hat.com, jmattson@...gle.com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org, mst@...hat.com,
        yu-cheng.yu@...el.com, Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Subject: Re: [PATCH v3 6/8] KVM:VMX: Load Guest CET via VMCS when CET is
 enabled in Guest

On Sun, Mar 03, 2019 at 08:26:08PM +0800, Yang Weijiang wrote:
> 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@...ux.intel.com>
> > > > > Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> > > > > ---
> > > > >  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?

In your approach, IA32_{S,U}_CET can be written if SHSTK or IBT is exposed
to the guest.  If only SHSTK is exposed, a devious guest can still use IBT
because it can set CR4.CET as well as the enable bits in IA32_{S,U}_CET.
Preventing the guest from using IBT in this scenario is infeasible as it
would require trapping and emulating the XSAVE as well as the relevent CET
MSRs.