linux-kernel - Re: [PATCH 2/2] kvm/x86: don't expose MSR_IA32_UMWAIT

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Date:   Wed, 20 May 2020 18:33:40 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 2/2] kvm/x86: don't expose MSR_IA32_UMWAIT_CONTROL unconditionally

Maxim Levitsky <mlevitsk@...hat.com> writes:

> 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@...hat.com>
> ---
>  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;

I'm probably missing something but (if I understand correctly) the only
effect of dropping MSR_IA32_UMWAIT_CONTROL from msrs_to_save would be
that KVM userspace won't see it in e.g. KVM_GET_MSR_INDEX_LIST. But why
is this causing an issue? I see both vmx_get_msr()/vmx_set_msr() have
'host_initiated' check:

       case MSR_IA32_UMWAIT_CONTROL:
                if (!msr_info->host_initiated && !vmx_has_waitpkg(vmx))
                        return 1;

so KVM userspace should be able to read/write this MSR even when there's
no hardware support for it. Or who's trying to read/write it?

Also, kvm_cpu_cap_has() check is not equal to vmx_has_waitpkg() which
checks secondary execution controls.

>  		default:
>  			break;
>  		}

-- 
Vitaly