Re: [RFC V1 PATCH 0/5] selftests: KVM: selftests for fd-based approach of supporting private memory

[Chao Peng]

On Wed, Apr 13, 2022 at 08:42:00AM -0500, Michael Roth wrote:
> On Tue, Apr 12, 2022 at 05:16:22PM -0700, Andy Lutomirski wrote:
> > On Fri, Apr 8, 2022, at 2:05 PM, Vishal Annapurve wrote:
> > > This series implements selftests targeting the feature floated by Chao
> > > via:
> > > https://lore.kernel.org/linux-mm/20220310140911.50924-1-chao.p.peng@xxxxxxxxxxxxxxx/
> > >
> > > Below changes aim to test the fd based approach for guest private memory
> > > in context of normal (non-confidential) VMs executing on non-confidential
> > > platforms.
> > >
> > > Confidential platforms along with the confidentiality aware software
> > > stack support a notion of private/shared accesses from the confidential
> > > VMs.
> > > Generally, a bit in the GPA conveys the shared/private-ness of the
> > > access. Non-confidential platforms don't have a notion of private or
> > > shared accesses from the guest VMs. To support this notion,
> > > KVM_HC_MAP_GPA_RANGE
> > > is modified to allow marking an access from a VM within a GPA range as
> > > always shared or private. Any suggestions regarding implementing this ioctl
> > > alternatively/cleanly are appreciated.
> > 
> > This is fantastic.  I do think we need to decide how this should work in general.  We have a few platforms with somewhat different properties:
> > 
> > TDX: The guest decides, per memory access (using a GPA bit), whether an access is private or shared.  In principle, the same address could be *both* and be distinguished by only that bit, and the two addresses would refer to different pages.
> > 
> > SEV: The guest decides, per memory access (using a GPA bit), whether an access is private or shared.  At any given time, a physical address (with that bit masked off) can be private, shared, or invalid, but it can't be valid as private and shared at the same time.
> > 
> > pKVM (currently, as I understand it): the guest decides by hypercall, in advance of an access, which addresses are private and which are shared.
> > 
> > This series, if I understood it correctly, is like TDX except with no hardware security.
> > 
> > Sean or Chao, do you have a clear sense of whether the current fd-based private memory proposal can cleanly support SEV and pKVM?  What, if anything, needs to be done on the API side to get that working well?  I don't think we need to support SEV or pKVM right away to get this merged, but I do think we should understand how the API can map to them.
> 
> I've been looking at porting the SEV-SNP hypervisor patches over to
> using memfd, and I hit an issue that I think is generally applicable
> to SEV/SEV-ES as well. Namely at guest init time we have something
> like the following flow:
> 
>   VMM:
>     - allocate shared memory to back the guest and map it into guest
>       address space
>     - initialize shared memory with initialize memory contents (namely
>       the BIOS)
>     - ask KVM to encrypt these pages in-place and measure them to
>       generate the initial measured payload for attestation, via
>       KVM_SEV_LAUNCH_UPDATE with the GPA for each range of memory to
>       encrypt.
>   KVM:
>     - issue SEV_LAUNCH_UPDATE firmware command, which takes an HPA as
>       input and does an in-place encryption/measure of the page.
> 
> With current v5 of the memfd/UPM series, I think the expected flow is that
> we would fallocate() these ranges from the private fd backend in advance of
> calling KVM_SEV_LAUNCH_UPDATE (if VMM does it after we'd destroy the initial
> guest payload, since they'd be replaced by newly-allocated pages). But if
> VMM does it before, VMM has no way to initialize the guest memory contents,
> since mmap()/pwrite() are disallowed due to MFD_INACCESSIBLE.

OK, so for SEV, basically VMM puts vBIOS directly into guest memory and then
do in-place measurement.

TDX has no problem because TDX temporarily uses a VMM buffer (vs. guest memory)
to hold the vBIOS and then asks SEAM-MODULE to measure and copy that to guest
memory.

Maybe something like SHM_LOCK should be used instead of the aggressive
MFD_INACCESSIBLE. Before VMM calling SHM_LOCK on the memfd, the content
can be changed but after that it's not visible to userspace VMM. This
gives userspace a chance to modify the data in private page.

Chao
> 
> I think something similar to your proposal[1] here of making pread()/pwrite()
> possible for private-fd-backed memory that's been flagged as "shareable"
> would work for this case. Although here the "shareable" flag could be
> removed immediately upon successful completion of the SEV_LAUNCH_UPDATE
> firmware command.
> 
> I think with TDX this isn't an issue because their analagous TDH.MEM.PAGE.ADD
> seamcall takes a pair of source/dest HPA as input params, so the VMM
> wouldn't need write access to dest HPA at any point, just source HPA.
> 
> [1] https://lwn.net/ml/linux-kernel/eefc3c74-acca-419c-8947-726ce2458446@xxxxxxxxxxxxxxxx/