Re: [PATCH 2/2] kvm/x86: don't expose MSR_IA32_UMWAIT_CONTROL unconditionally

[Maxim Levitsky]

On Wed, 2020-05-20 at 23:05 +0200, Paolo Bonzini wrote:
> On 20/05/20 18:07, Maxim Levitsky wrote:
> > This msr is only available when the host supports WAITPKG feature.
> > 
> > This breaks a nested guest, if the L1 hypervisor is set to ignore
> > unknown msrs, because the only other safety check that the
> > kernel does is that it attempts to read the msr and
> > rejects it if it gets an exception.
> > 
> > Fixes: 6e3ba4abce KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL
> > 
> > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/x86.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index fe3a24fd6b263..9c507b32b1b77 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -5314,6 +5314,10 @@ static void kvm_init_msr_list(void)
> >  			if (msrs_to_save_all[i] - MSR_ARCH_PERFMON_EVENTSEL0 >=
> >  			    min(INTEL_PMC_MAX_GENERIC, x86_pmu.num_counters_gp))
> >  				continue;
> > +			break;
> > +		case MSR_IA32_UMWAIT_CONTROL:
> > +			if (!kvm_cpu_cap_has(X86_FEATURE_WAITPKG))
> > +				continue;
> >  		default:
> >  			break;
> >  		}
> 
> The patch is correct, and matches what is done for the other entries of
> msrs_to_save_all.  However, while looking at it I noticed that
> X86_FEATURE_WAITPKG is actually never added, and that is because it was
> also not added to the supported CPUID in commit e69e72faa3a0 ("KVM: x86:
> Add support for user wait instructions", 2019-09-24), which was before
> the kvm_cpu_cap mechanism was added.
> 
> So while at it you should also fix that.  The right way to do that is to
> add a
> 
>         if (vmx_waitpkg_supported())
>                 kvm_cpu_cap_check_and_set(X86_FEATURE_WAITPKG);
> 
> in vmx_set_cpu_caps.
> 
> Thanks,

Thank you very much for finding this. I didn't expect this to be broken.
I will send a new version with this fix as well tomorrow.

Best regards,
	Maxim Levitsky


> 
> Paolo
>