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

[Jim Mattson]

On Tue, Jan 30, 2018 at 9:14 AM, David Woodhouse <dwmw2@xxxxxxxxxxxxx> 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.
>
> You can always write to the STIBP bit without a #GP even when it's not
> advertised/available.

Oops. Yes, you're right. It's writing the IBRS bit when only STIBP is
available that results in a #GP.

> There's a possibility that we'll want to always trap and *prevent*
> that, instead of passing through â because doing so will also have an
> effect on the HT siblings. But as discussed, I wanted to get the basics
> working before exploring the complex IBRS/STIBP interactions. This much
> should be OK to start with.