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

[Paolo Bonzini]

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,

Paolo