linux-kernel - Re: [PATCH 2/2] KVM: x86: Manually retrieve CPUID.0x1 when getting FMS for RESET/INIT

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Message-ID: <CALMp9eQZH80_vWEz26OGr8cwhLEP4yoSt2UdSC_75Fy9sMxhhQ@mail.gmail.com>
Date:   Wed, 29 Sep 2021 15:49:27 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        syzbot+f3985126b746b3d59c9d@...kaller.appspotmail.com,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH 2/2] KVM: x86: Manually retrieve CPUID.0x1 when getting
 FMS for RESET/INIT

On Wed, Sep 29, 2021 at 3:24 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Manually look for a CPUID.0x1 entry instead of bouncing through
> kvm_cpuid() when retrieving the Family-Model-Stepping information for
> vCPU RESET/INIT.  This fixes a potential undefined behavior bug due to
> kvm_cpuid() using the uninitialized "dummy" param as the ECX _input_,
> a.k.a. the index.
>
> A more minimal fix would be to simply zero "dummy", but the extra work in
> kvm_cpuid() is wasteful, and KVM should be treating the FMS retrieval as
> an out-of-band access, e.g. same as how KVM computes guest.MAXPHYADDR.
> Both Intel's SDM and AMD's APM describe the RDX value at RESET/INIT as
> holding the CPU's FMS information, not as holding CPUID.0x1.EAX.  KVM's
> usage of CPUID entries to get FMS is simply a pragmatic approach to avoid
> having yet another way for userspace to provide inconsistent data.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Reviewed-by: Jim Mattson <jmattson@...gle.com>