linux-kernel - Re: [PATCH 1/5] KVM: x86/xen: Restrict hypercall MSR to unofficial synthetic range

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Message-ID: <Z6O6Evrdl9pPM3hX@google.com>
Date: Wed, 5 Feb 2025 11:20:50 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Paul Durrant <paul@....org>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, 
	syzbot+cdeaeec70992eca2d920@...kaller.appspotmail.com, 
	Joao Martins <joao.m.martins@...cle.com>
Subject: Re: [PATCH 1/5] KVM: x86/xen: Restrict hypercall MSR to unofficial
 synthetic range

On Wed, Feb 05, 2025, David Woodhouse wrote:
> On Wed, 2025-02-05 at 16:18 +0000, David Woodhouse wrote:
> > 
> > > Oh!  It doesn't help KVM avoid breaking userspace, but a way for QEMU to avoid a
> > > future collision would be to have QEMU start at 0x40000200 when Hyper-V is enabled,
> > > but then use KVM_GET_MSR_INDEX_LIST to detect a collision with KVM Hyper-V, e.g.
> > > increment the index until an available index is found (with sanity checks and whatnot).
> > 
> > Makes sense. I think that's a third separate patch, yes?
> 
> To be clear, I think I mean a third patch which further restricts
> kvm_xen_hvm_config() to disallow indices for which
> kvm_is_advertised_msr() returns true?
> 
> We could roll that into your original patch instead, if you prefer.

Nah, I like the idea of separate patch.

> Q: Should kvm_is_advertised_msr() include the Xen hypercall MSR, if one
> is already configured? Life is easier if we answer 'no'...

No :-)

The idea with kvm_is_advertised_msr() is to ignore accesses to MSRs that don't
exist according the to vCPU model, but that KVM advertised to userspace (via
KVM_GET_MSR_INDEX_LIST) and so may be saved/restored by a naive/unoptimized
userspace.

For the Xen MSR, KVM never advertises the MSR, and IIUC, KVM will never treat
the MSR as non-existent because defining the MSR brings it into existence.