linux-kernel - Re: [PATCH] kvm/x86/vmx: switch MSR_MISC_FEATURES

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Date:   Thu, 14 Mar 2019 12:30:54 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Xiaoyao Li <xiaoyao.li@...ux.intel.com>,
        Kyle Huey <me@...ehuey.com>
Cc:     Kyle Huey <khuey@...ehuey.com>, Chao Gao <chao.gao@...el.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        kvm list <kvm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kvm/x86/vmx: switch MSR_MISC_FEATURES_ENABLES between
 host and guest

On 14/03/19 11:43, Xiaoyao Li wrote:
> Yes, you are right. With your patches, only when we enable cpuid faulting for
> the QEMU or other VMM userspace processes via arch_prctl, does it "leak" into
> the KVM guest. 
> 
> But arch_prctl is not the only way to enable it. We can enable cpuid faulting
> via module "msr.ko". And I think it's indeed an issue, we should not ignore it
> as it needs special cases and purpose to make it happen so far. 

Whatever is done with "msr.ko" is a non-issue.  msr.ko is at best a
debugging tool if you use it for MSRs that the kernel can write.  That
said, even the arch_prctl is an issue, as well as Peter's patch you
quote below.

The patch is a good idea even though it should be implemented in a more
efficient way.

Paolo

> Besides, Peter's this patch https://patchwork.kernel.org/patch/10850143/ is much
> likely to cause the cpuid fauting enabled in host by default during kernel
> startup. If that one is going to be applied, it's better to fix the cpuid
> faulting leak issue before that.