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, 8 insertions, 0 deletions

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index a16c873b3232..6589091e8ce0 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2549,6 +2549,14 @@ static int snp_launch_finish(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	data->gctx_paddr = __psp_pa(sev->snp_context);
 	ret = sev_issue_cmd(kvm, SEV_CMD_SNP_LAUNCH_FINISH, data, &argp->error);
 
+	/*
+	 * Now that there will be no more SNP_LAUNCH_UPDATE ioctls, private pages
+	 * can be given to the guest simply by marking the RMP entry as private.
+	 * This can happen on first access and also with KVM_PRE_FAULT_MEMORY.
+	 */
+	if (!ret)
+		kvm->arch.pre_fault_allowed = true;
+
 	kfree(id_auth);
 
 e_free_id_block: