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>

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.
 

arch/x86/include/asm/kvm_host.h

@@ -1305,6 +1305,7 @@ struct kvm_arch {
 	u8 vm_type;
 	bool has_private_mem;
 	bool has_protected_state;
+	bool pre_fault_allowed;
 	struct hlist_head mmu_page_hash[KVM_NUM_MMU_PAGES];
 	struct list_head active_mmu_pages;
 	struct list_head zapped_obsolete_pages;

arch/x86/kvm/mmu/mmu.c

@@ -4743,6 +4743,9 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
 	u64 end;
 	int r;
 
+	if (!vcpu->kvm->arch.pre_fault_allowed)
+		return -EOPNOTSUPP;
+
 	/*
 	 * reload is efficient when called repeatedly, so we can do it on
 	 * every iteration.

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:

arch/x86/kvm/svm/svm.c

@@ -4949,6 +4949,7 @@ static int svm_vm_init(struct kvm *kvm)
 		to_kvm_sev_info(kvm)->need_init = true;
 
 		kvm->arch.has_private_mem = (type == KVM_X86_SNP_VM);
+		kvm->arch.pre_fault_allowed = !kvm->arch.has_private_mem;
 	}
 
 	if (!pause_filter_count || !pause_filter_thresh)

arch/x86/kvm/x86.c

@@ -12646,6 +12646,9 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	kvm->arch.vm_type = type;
 	kvm->arch.has_private_mem =
 		(type == KVM_X86_SW_PROTECTED_VM);
+	/* Decided by the vendor code for other VM types.  */
+	kvm->arch.pre_fault_allowed =
+		type == KVM_X86_DEFAULT_VM || type == KVM_X86_SW_PROTECTED_VM;
 
 	ret = kvm_page_track_init(kvm);
 	if (ret)