Re: [PATCH] KVM: x86: Fix the condition of #PF interception caused by MKTME

[Tao Su <tao1.su@linux.intel.com>]

On Tue, Apr 09, 2024 at 12:05:12PM -0700, Sean Christopherson wrote:
> On Tue, Mar 19, 2024, Tao Su wrote:
> > Intel MKTME repurposes several high bits of physical address as 'keyID',
> > so boot_cpu_data.x86_phys_bits doesn't hold physical address bits reported
> > by CPUID anymore.
> > 
> > If guest.MAXPHYADDR < host.MAXPHYADDR, the bit field of ‘keyID’ belongs
> > to reserved bits in guest’s view, so intercepting #PF to fix error code
> > is necessary, just replace boot_cpu_data.x86_phys_bits with
> > kvm_get_shadow_phys_bits() to fix.
> > 
> > Signed-off-by: Tao Su <tao1.su@linux.intel.com>
> > ---
> >  arch/x86/kvm/vmx/vmx.h | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> > index 65786dbe7d60..79b1757df74a 100644
> > --- a/arch/x86/kvm/vmx/vmx.h
> > +++ b/arch/x86/kvm/vmx/vmx.h
> > @@ -15,6 +15,7 @@
> >  #include "vmx_ops.h"
> >  #include "../cpuid.h"
> >  #include "run_flags.h"
> > +#include "../mmu.h"
> >  
> >  #define MSR_TYPE_R	1
> >  #define MSR_TYPE_W	2
> > @@ -719,7 +720,8 @@ static inline bool vmx_need_pf_intercept(struct kvm_vcpu *vcpu)
> >  	if (!enable_ept)
> >  		return true;
> >  
> > -	return allow_smaller_maxphyaddr && cpuid_maxphyaddr(vcpu) < boot_cpu_data.x86_phys_bits;
> > +	return allow_smaller_maxphyaddr &&
> > +		cpuid_maxphyaddr(vcpu) < kvm_get_shadow_phys_bits();
> 
> For posterity, because I had a brief moment where I thought we done messed up:
> 
> No change is needed in the reporting of MAXPHYADDR in KVM_GET_SUPPORTED_CPUID,
> as reporting boot_cpu_data.x86_phys_bits as MAXPHYADDR when TDP is disabled is ok
> because KVM always intercepts #PF when TDP is disabled, and KVM already reports
> the full/raw MAXPHYADDR when TDP is enabled.

You are right, but userspace can fully control guest.MAXPHYADDR when TDP is enabled.
Please see the unit-test[*], I think this issue could show up earlier if phys-bits
is set larger, such as 46-bits.

[*] https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/blob/master/x86/unittests.cfg?ref_type=heads#L156