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

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Message-ID: <4410f771de905b8df18f7fc87fe5a034d1a57a7b.camel@infradead.org>
Date: Thu, 06 Feb 2025 18:58:11 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
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, 2025-02-05 at 11:20 -0800, Sean Christopherson wrote:
> 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.

Helpfully, kvm_is_advertised_msr() doesn't actually return true for
MSR_IA32_XSS. Is that a bug?

And kvm_vcpu_reset() attempts to set MSR_IA32_XSS even if the guest
doesn't have X86_FEATURE_XSAVES. Is that a bug?



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