linux-kernel - Re: [PATCH v2 1/5] KVM: x86/xen: Restrict hypercall MSR to unofficial synthetic range

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Message-ID: <3A169373-3F1D-4938-8505-F75D1F5CA726@infradead.org>
Date: Thu, 20 Feb 2025 20:04:36 +0100
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,
 Joao Martins <joao.m.martins@...cle.com>,
 David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v2 1/5] KVM: x86/xen: Restrict hypercall MSR to unofficial synthetic range

On 20 February 2025 19:36:41 CET, Sean Christopherson <seanjc@...gle.com> wrote:
>On Sat, Feb 15, 2025, David Woodhouse wrote:
>> On 15 February 2025 02:14:33 CET, Sean Christopherson <seanjc@...gle.com> wrote:
>> >Reject userspace attempts to set the Xen hypercall page MSR to an index
>> >outside of the "standard" virtualization range [0x40000000, 0x4fffffff],
>> >as KVM is not equipped to handle collisions with real MSRs, e.g. KVM
>> >doesn't update MSR interception, conflicts with VMCS/VMCB fields, special
>> >case writes in KVM, etc.
>> >
>> >While the MSR index isn't strictly ABI, i.e. can theoretically float to
>> >any value, in practice no known VMM sets the MSR index to anything other
>> >than 0x40000000 or 0x40000200.
>
>...
>
>> This patch should probably have a docs update too.
>
>To avoid sending an entirely new version only to discover I suck at writing docs,
>how does this look?
>
>diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>index 2b52eb77e29c..5fe84f2427b5 100644
>--- a/Documentation/virt/kvm/api.rst
>+++ b/Documentation/virt/kvm/api.rst
>@@ -1000,6 +1000,10 @@ blobs in userspace.  When the guest writes the MSR, kvm copies one
> page of a blob (32- or 64-bit, depending on the vcpu mode) to guest
> memory.
> 
>+The MSR index must be in the range [0x40000000, 0x4fffffff], i.e. must reside
>+in the range that is unofficially reserved for use by hypervisors.  The min/max
>+values are enumerated via KVM_XEN_MSR_MIN_INDEX and KVM_XEN_MSR_MAX_INDEX.
>+
> ::
> 
>   struct kvm_xen_hvm_config {

LGTM, thanks