linux-kernel - Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date: Fri, 17 May 2024 10:57:03 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Sean Christopherson <seanjc@...gle.com>, Weijiang Yang
 <weijiang.yang@...el.com>
Cc: rick.p.edgecombe@...el.com, pbonzini@...hat.com, dave.hansen@...el.com,
 x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 peterz@...radead.org, chao.gao@...el.com, mlevitsk@...hat.com,
 john.allen@....com
Subject: Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX
 and advertise to userspace

On Thu, May 16 2024 at 07:39, Sean Christopherson wrote:
> On Thu, May 16, 2024, Weijiang Yang wrote:
>> We synced the issue internally, and got conclusion that KVM should honor host
>> IBT config.  In this case IBT bit in boot_cpu_data should be honored.  With
>> this policy, it can avoid CPUID confusion to guest side due to host ibt=off
>> config.
>
> What was the reasoning?  CPUID confusion is a weak justification, e.g. it's not
> like the guest has visibility into the host kernel, and raw CPUID will still show
> IBT support in the host.
>
> On the other hand, I can definitely see folks wanting to expose IBT to guests
> when running non-complaint host kernels, especially when live migration is in
> play, i.e. when hiding IBT from the guest will actively cause problems.

I have to disagree here violently.

If the exposure of a CPUID bit to a guest requires host side support,
e.g. in xstate handling, then exposing it to a guest is simply not
possible.

Just because virtualization allows to do that does not mean that it's
correct in any way.

Thanks,

        tglx