linux-kernel - Re: [PATCH] KVM: VMX: expose the host's ARCH

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Date:   Wed, 7 Mar 2018 16:39:42 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        David Woodhouse <dwmw@...zon.co.uk>,
        KarimAllah Ahmed <karahmed@...zon.de>
Subject: Re: [PATCH] KVM: VMX: expose the host's ARCH_CAPABILITIES MSR to
 userspace

2018-03-07 16:10+0100, Paolo Bonzini:
> On 07/03/2018 15:56, Radim Krčmář wrote:
> > The MSR_F10H_DECFG default is questionable -- MSR_F10H_DECFG is an
> > architectural MSR, so we'd be changing the guest under the sight of
> > existing userspaces.
> > A potential security risk if they migrate the guest to a CPU that
> > doesn't serialize LFENCE.  ARCH_CAPABILITIES are at least hidden by a
> > new CPUID bit.
> 
> Good point.  Perhaps we should add a KVM-specific CPUID bit for
> serializing LFENCE.

I reckon it wouldn't help much in the wild: we'd need userspace changes
(at least for QEMU) and at that point, userspace can as well just
implement MSR_F10H_DECFG and use an unmodified guest.

We can't easily intercept LFENCE to emulate the feature either, so it
seems like a waste of effort to me.