On Thu, Feb 16, 2023, David Hildenbrand wrote:

On 16.02.23 06:13, Mike Rapoport wrote:

quoted

Hi,

On Fri, Dec 02, 2022 at 02:13:38PM +0800, Chao Peng wrote:

quoted

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.