KVM: Restrict non-existing slot state transitions

The API documentation states:

	When changing an existing slot, it may be moved in the guest
	physical memory space, or its flags may be modified.

An "existing slot" requires a non-zero npages (memory_size).  The only
transition we should therefore allow for a non-existing slot should be
to create the slot, which includes setting a non-zero memory_size.  We
currently allow calls to modify non-existing slots, which is pointless,
confusing, and possibly wrong.

With this we know that the invalidation path of __kvm_set_memory_region
is always for a delete or move and never for adding a zero size slot.
Reviewed-by: NGleb Natapov <gleb@redhat.com>
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

virt/kvm/kvm_main.c

@@ -758,10 +758,15 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	new.npages = npages;
 	new.flags = mem->flags;
 
-	/* Disallow changing a memory slot's size. */
+	/*
+	 * Disallow changing a memory slot's size or changing anything about
+	 * zero sized slots that doesn't involve making them non-zero.
+	 */
 	r = -EINVAL;
 	if (npages && old.npages && npages != old.npages)
 		goto out_free;
+	if (!npages && !old.npages)
+		goto out_free;
 
 	/* Check for overlaps */
 	r = -EEXIST;
@@ -780,7 +785,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	r = -ENOMEM;
 
 	/* Allocate if a slot is being created */
-	if (npages && !old.npages) {
+	if (!old.npages) {
 		new.user_alloc = user_alloc;
 		new.userspace_addr = mem->userspace_addr;