linux-kernel - Re: [9/8] KVM: x86: limit MSR_IA32_SPEC_CTRL access based on CPUID availability

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Message-ID: <75049dca-3389-9cc7-44e3-a487a797c605@redhat.com>
Date:   Tue, 30 Jan 2018 18:11:28 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        David Woodhouse <dwmw2@...radead.org>
Cc:     Jim Mattson <jmattson@...gle.com>,
        Mihai Carabas <mihai.carabas@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Liran Alon <liran.alon@...cle.com>,
        Anthony Liguori <aliguori@...zon.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Borislav Petkov <bp@...en8.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [9/8] KVM: x86: limit MSR_IA32_SPEC_CTRL access based on CPUID
 availability

On 30/01/2018 12:45, Thomas Gleixner wrote:
> On Tue, 30 Jan 2018, David Woodhouse wrote:
> 
>> On Tue, 2018-01-30 at 08:57 -0800, Jim Mattson wrote:
>>> It's really hard to tell which patches are being proposed for which
>>> repositories, but assuming that everything else is correct, I don't
>>> think your condition is adequate. What if the physical CPU and the
>>> virtual CPU both have CPUID.(EAX=7H,ECX=0):EDX[26], but only the
>>> physical CPU has CPUID.(EAX=7H,ECX=0):EDX[27]? If the guest has write
>>> access to MSR_IA32_SPEC_CTRL, it can set MSR_IA32_SPEC_CTRL[1]
>>> (STIBP), even though setting that bit in the guest should raise #GP.
>>
>> Everything we're talking about here is for tip/x86/pti. Which I note
>> has just updated to be 4.15-based, although I thought it was going to
>> stay on 4.14 for now. So I've updated my tree at
>> http://git.infradead.org/linux-retpoline.git/shortlog/refs/heads/ibpb
>> accordingly.
> 
> Yes, we tried to stay on 4.14 base but this started to created nasty merge
> conflicts for no value. Merging in v4.15 turned out to resolve those issues
> while still serving as the feed branch for Gregs stable work. For the time
> being we try to make stable backporting at least for 4.14/15 as painless as
> possible.

Great, then the "per-VCPU MSR bitmaps" branch
(git://git.kernel.org/pub/scm/virt/kvm/kvm.git refs/heads/msr-bitmaps)
that I created last weekend can be pulled directly in tip/x86/pti.

It cherry-picks directly to both 4.14 so it will be fine for Greg too.

Paolo