On Tue, May 30, 2023, Rick P Edgecombe wrote: > On Fri, 2023-05-26 at 17:22 +0200, Micka�l Sala�n wrote: > > > > Can the guest kernel ask the host VMM's emulated devices to DMA into > > > > the protected data? It should go through the host userspace mappings I > > > > think, which don't care about EPT permissions. Or did I miss where you > > > > are protecting that another way? There are a lot of easy ways to ask > > > > the host to write to guest memory that don't involve the EPT. You > > > > probably need to protect the host userspace mappings, and also the > > > > places in KVM that kmap a GPA provided by the guest. > > > > > > Good point, I'll check this confused deputy attack. Extended KVM > > > protections should indeed handle all ways to map guests' memory. I'm > > > wondering if current VMMs would gracefully handle such new restrictions > > > though. > > > > I guess the host could map arbitrary data to the guest, so that need to be > > handled, but how could the VMM (not the host kernel) bypass/update EPT > > initially used for the guest (and potentially later mapped to the host)? > > Well traditionally both QEMU and KVM accessed guest memory via host > mappings instead of the EPT.�So I'm wondering what is stopping the > guest from passing a protected gfn when setting up the DMA, and QEMU > being enticed to write to it? The emulator as well would use these host > userspace mappings and not consult the EPT IIRC. > > I think Sean was suggesting host userspace should be more involved in > this process, so perhaps it could protect its own alias of the > protected memory, for example mprotect() it as read-only. Ya, though "suggesting" is really "demanding, unless someone provides super strong justification for handling this directly in KVM". It's basically the same argument that led to Linux Security Modules: I'm all for KVM providing the framework and plumbing, but I don't want KVM to get involved in defining policy, thread models, etc.
