On 10/15/21 2:50 PM, Sean Christopherson wrote:

On Fri, Aug 20, 2021, Brijesh Singh wrote:

quoted

From: Tom Lendacky <thomas.lendacky@amd.com>

Add support for the SEV-SNP AP Creation NAE event. This allows SEV-SNP
guests to alter the register state of the APs on their own. This allows
the guest a way of simulating INIT-SIPI.

A new event, KVM_REQ_UPDATE_PROTECTED_GUEST_STATE, is created and used
so as to avoid updating the VMSA pointer while the vCPU is running.

For CREATE
  The guest supplies the GPA of the VMSA to be used for the vCPU with the
  specified APIC ID. The GPA is saved in the svm struct of the target
  vCPU, the KVM_REQ_UPDATE_PROTECTED_GUEST_STATE event is added to the
  vCPU and then the vCPU is kicked.

For CREATE_ON_INIT:
  The guest supplies the GPA of the VMSA to be used for the vCPU with the
  specified APIC ID the next time an INIT is performed. The GPA is saved
  in the svm struct of the target vCPU.

For DESTROY:
  The guest indicates it wishes to stop the vCPU. The GPA is cleared from
  the svm struct, the KVM_REQ_UPDATE_PROTECTED_GUEST_STATE event is added
  to vCPU and then the vCPU is kicked.


The KVM_REQ_UPDATE_PROTECTED_GUEST_STATE event handler will be invoked as
a result of the event or as a result of an INIT. The handler sets the vCPU
to the KVM_MP_STATE_UNINITIALIZED state, so that any errors will leave the
vCPU as not runnable. Any previous VMSA pages that were installed as
part of an SEV-SNP AP Creation NAE event are un-pinned. If a new VMSA is
to be installed, the VMSA guest page is pinned and set as the VMSA in the
vCPU VMCB and the vCPU state is set to KVM_MP_STATE_RUNNABLE. If a new
VMSA is not to be installed, the VMSA is cleared in the vCPU VMCB and the
vCPU state is left as KVM_MP_STATE_UNINITIALIZED to prevent it from being
run.

LOL, this part of the GHCB is debatable, though I guess it does say "may"...

  Using VMGEXIT SW_EXITCODE 0x8000_0013, an SEV-SNP guest can create or update the
  vCPU state of an AP, which may allow for a simpler and more secure method of
                                             ^^^^^^^
  booting an AP.

quoted

+	if (VALID_PAGE(svm->snp_vmsa_pfn)) {

KVM's VMSA page should be freed on a successful "switch", because AFAICT it's
incorrect for KVM to ever go back to the original VMSA.

quoted

+		/*
+		 * The snp_vmsa_pfn fields holds the hypervisor physical address
+		 * of the about to be replaced VMSA which will no longer be used
+		 * or referenced, so un-pin it.
+		 */
+		kvm_release_pfn_dirty(svm->snp_vmsa_pfn);
+		svm->snp_vmsa_pfn = INVALID_PAGE;
+	}
+
+	if (VALID_PAGE(svm->snp_vmsa_gpa)) {
+		/*
+		 * The VMSA is referenced by the hypervisor physical address,
+		 * so retrieve the PFN and pin it.
+		 */
+		pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(svm->snp_vmsa_gpa));

Oh yay, a gfn.  That means that the page is subject to memslot movement.  I don't
think the code will break per se, but it's a wrinkle that's not handled.

I'm also pretty sure the page will effectively be leaked, I don't see a

	kvm_release_pfn_dirty(svm->snp_vmsa_pfn);

in vCPU teardown.

Furthermore, letting the guest specify the page would open up to exploits of the
erratum where a spurious RMP violation is signaled if an in-use page, a.k.a. VMSA
page, is 2mb aligned.  That also means the _guest_ needs to be somehow be aware
of the erratum.

Good point Sean, a guest could exploit the IN_USE erratum in this case.
We need to somehow communicate this to guest so that it does not
allocate the VMSA at 2MB boundary. It would be nice if GHCB spec can add
a requirement that VMSA should not be a 2MB aligned. I will see what we
can do to address this.

And digging through the guest patches, this gives the guest _full_ control over
the VMSA contents.  That is bonkers.  At _best_ it gives the guest the ability to
fuzz VMRUN ucode by stuffing garbage into the VMSA.

If guest puts garbage in VMSA then VMRUN will fail. I am sure ucode is
doing all kind of sanity checks to ensure that VMSA does not contain
invalid value before the run.

Honestly, why should KVM even support guest-provided VMSAs?  It's far, far simpler
to handle this fully in the guest with a BIOS<=>kernel mailbox; see the MP wakeup
protocol being added for TDX.  That would allow improving the security for SEV-ES
as well, though I'm guessing no one actually cares about that in practice.
IIUC, the use case for VMPLs is that VMPL0 would be fully trusted by both the host
and guest, i.e. attacks via the VMSA are out-of-scope.  That is very much not the
case here.