Re: [PATCH] x86/entry/64: Disallow RDPID in paranoid entry if KVM is enabled

[Andy Lutomirski]

On Fri, Aug 21, 2020 at 3:07 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 21/08/20 11:48, Borislav Petkov wrote:
> >> It's not like we grab MSRs every day.  The user-return notifier restores
> >> 6 MSRs (7 on very old processors).  The last two that were added were
> >> MSR_TSC_AUX itself in 2009 (!) and MSR_IA32_TSX_CTRL last year.
> > What about "If it is a shared resource, there better be an agreement
> > about sharing it." is not clear?
> >
> > It doesn't matter how many or which resources - there needs to be a
> > contract for shared use so that shared use is possible. It is that
> > simple.
>
> Sure, and I'll make sure to have that discussion the next time we add a
> shared MSR in 2029.
>
> In the meanwhile:
>
> * for the syscall MSRs, patches to share them were reviewed by hpa and
> peterz so I guess there's no problem.
>
> * for MSR_TSC_AUX, which is the one that is causing problems, everybody
> seems to agree with just using LSL (in the lack specific numbers on
> performance improvements from RDPID).
>
> * for MSR_IA32_TSX_CTRL.RTM_DISABLE, which is the only one that was
> added in the last 10 years, I'm pretty sure there are no plans for using
> the Trusty Sidechannel eXtension in the kernel.  So that should be fine
> too.  (The CPUID_CLEAR bit of the MSR is not shared).
>

Regardless of how anyone feels about who owns what in arch/x86 vs
arch/x86/kvm, the x86 architecture is a mess that comes from Intel and
AMD, and we have to deal with it.  On VMX, when a VM exits, the VM's
value of MSR_TSC_AUX is live, and we can take an NMI, MCE, or
abominable new #SX, #VE, #VC, etc on the next instruction boundary.
And unless we use the atomic MSR switch mechanism, the result is that
we're going through the entry path with guest-controlled MSRs.

So I think we can chalk this one up to obnoxious hardware.