KVM: x86: disallow pre-fault for SNP VMs before initialization

KVM_PRE_FAULT_MEMORY for an SNP guest can race with
sev_gmem_post_populate() in bad ways. The following sequence for
instance can potentially trigger an RMP fault:

  thread A, sev_gmem_post_populate: called
  thread B, sev_gmem_prepare: places below 'pfn' in a private state in RMP
  thread A, sev_gmem_post_populate: *vaddr = kmap_local_pfn(pfn + i);
  thread A, sev_gmem_post_populate: copy_from_user(vaddr, src + i * PAGE_SIZE, PAGE_SIZE);
  RMP #PF

Fix this by only allowing KVM_PRE_FAULT_MEMORY to run after a guest's
initial private memory contents have been finalized via
KVM_SEV_SNP_LAUNCH_FINISH.

Beyond fixing this issue, it just sort of makes sense to enforce this,
since the KVM_PRE_FAULT_MEMORY documentation states:

  "KVM maps memory as if the vCPU generated a stage-2 read page fault"

which sort of implies we should be acting on the same guest state that a
vCPU would see post-launch after the initial guest memory is all set up.

Co-developed-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index ec1cd8aa1d56..7b512286f8d2 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6402,6 +6402,12 @@ for the current vCPU state.  KVM maps memory as if the vCPU generated a
 stage-2 read page fault, e.g. faults in memory as needed, but doesn't break
 CoW.  However, KVM does not mark any newly created stage-2 PTE as Accessed.
 
+In the case of confidential VM types where there is an initial set up of
+private guest memory before the guest is 'finalized'/measured, this ioctl
+should only be issued after completing all the necessary setup to put the
+guest into a 'finalized' state so that the above semantics can be reliably
+ensured.
+
 In some cases, multiple vCPUs might share the page tables.  In this
 case, the ioctl can be called in parallel.