Re: [PATCH v10 0/9] KVM: mm: fd-based approach for supporting KVM

Sean Christopherson
2023-02-22T21:53:25Z

On Thu, Feb 16, 2023, David Hildenbrand wrote:
> On 16.02.23 06:13, Mike Rapoport wrote:
> > Hi,
> >
> > On Fri, Dec 02, 2022 at 02:13:38PM +0800, Chao Peng wrote:
> > > This patch series implements KVM guest private memory for confidential
> > > computing scenarios like Intel TDX[1]. If a TDX host accesses
> > > TDX-protected guest memory, machine check can happen which can further
> > > crash the running host system, this is terrible for multi-tenant
> > > configurations. The host accesses include those from KVM userspace like
> > > QEMU. This series addresses KVM userspace induced crash by introducing
> > > new mm and KVM interfaces so KVM userspace can still manage guest memory
> > > via a fd-based approach, but it can never access the guest memory
> > > content.
> >
> > Sorry for jumping late.
> >
> > Unless I'm missing something, hibernation will also cause an machine check
> > when there is TDX-protected memory in the system. When the hibernation
> > creates memory snapshot it essentially walks all physical pages and saves
> > their contents, so for TDX memory this will trigger machine check, right?

For hibernation specifically, I think that should be handled elsewhere as hibernation
is simply incompatible with TDX, SNP, pKVM, etc. without paravirtualizing the
guest, as none of those technologies support auto-export a la s390. I suspect
the right approach is to disallow hibernation if KVM is running any protected guests.

> I recall bringing that up in the past (also memory access due to kdump,
> /prov/kcore) and was told that the main focus for now is preventing
> unprivileged users from crashing the system, that is, not mapping such
> memory into user space (e.g., QEMU). In the long run, we'll want to handle
> such pages also properly in the other events where the kernel might access
> them.

Ya, unless someone strongly objects, the plan is to essentially treat "attacks"
from privileged users as out of to scope for initial support, and then iterate
as needed to fix/enable more features.

FWIW, read accesses, e.g. kdump, should be ok for TDX and SNP as they both play
nice with "bad" reads. pKVM is a different beast though as I believe any access
to guest private memory will fault. But my understanding is that this series
would be a big step forward for pKVM, which currently doesn't have any safeguards.