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/
