Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs in mind (for KVM to isolate its guests per CPU)

[David Woodhouse <dwmw2@infradead.org>]

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On Tue, 2018-08-21 at 17:01 +0300, Liran Alon wrote:
> 
> > On 21 Aug 2018, at 12:57, David Woodhouse <dwmw2@infradead.org>
> wrote:
> > 
> > Another alternative... I'm told POWER8 does an interesting thing
> with
> > hyperthreading and gang scheduling for KVM. The host kernel doesn't
> > actually *see* the hyperthreads at all, and KVM just launches the
> full
> > set of siblings when it enters a guest, and gathers them again when
> any
> > of them exits. That's definitely worth investigating as an option
> for
> > x86, too.
> 
> I actually think that such scheduling mechanism which prevents
> leaking cache entries to sibling hyperthreads should co-exist
> together with the KVM address space isolation to fully mitigate L1TF
> and other similar vulnerabilities. The address space isolation should
> prevent VMExit handlers code gadgets from loading arbitrary host
> memory to the cache. Once VMExit code path switches to full host
> address space, then we should also make sure that no other sibling
> hyprethread is running in the guest.

The KVM POWER8 solution (see arch/powerpc/kvm/book3s_hv.c) does that.
The siblings are *never* running host kernel code; they're all torn
down when any of them exits the guest. And it's always the *same*
guest.

> Focusing on the scheduling mechanism, we must make sure that when a
> logical processor runs guest code, all siblings logical processors
> must run code which do not populate L1D cache with information
> unrelated to this VM. This includes forbidding one logical processor
> to run guest code while sibling is running a host task such as a NIC
> interrupt handler.
> Thus, when a vCPU thread exits the guest into the host and VMExit
> handler reaches code flow which could populate L1D cache with this
> information, we should force an exit from the guest of the siblings
> logical processors, such that they will be allowed to resume only on
> a core which we can promise that the L1D cache is free from
> information unrelated to this VM.
> 
> At first, I have created a patch series which attempts to implement
> such mechanism in KVM. However, it became clear to me that this may
> need to be implemented in the scheduler itself. This is because:
> 1. It is difficult to handle all new scheduling contrains only in
> KVM.
> 2. This mechanism should be relevant for any Type-2 hypervisor which
> runs inside Linux besides KVM (Such as VMware Workstation or
> VirtualBox).
> 3. This mechanism could also be used to prevent future “core-cache-
> leaking” vulnerabilities to be exploited between processes of
> different security domains which run as siblings on the same core.

I'm not sure I agree. If KVM is handling "only let siblings run the
*same* guest" and the siblings aren't visible to the host at all,
that's quite simple. Any other hypervisor can also do it.

Now, the down-side of this is that the siblings aren't visible to the
host. They can't be used to run multiple threads of the same userspace
processes; only multiple threads of the same KVM guest. A truly generic
core scheduler would cope with userspace threads too.

BUT I strongly suspect there's a huge correlation between the set of
people who care enough about the KVM/L1TF issue to enable a costly
XFPO-like solution, and the set of people who mostly don't give a shit
about having sibling CPUs available to run the host's userspace anyway.

This is not the "I happen to run a Windows VM on my Linux desktop" use
case...

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