KVM: arm64: Only drop references on empty tables in stage2_free_walker

A subsequent change to the way KVM frees stage-2s will invoke the free
walker on sub-ranges of the VM's IPA space, meaning there's potential
for only partially visiting a table's PTEs.

Split the leaf and table visitors and only drop references on a table
when the page count reaches 1, implying there are no valid PTEs that
need to be visited. Invalidate the table PTE to avoid traversing the
stale reference.

Link: https://msgid.link/[e-mail]
Signed-off-by: Oliver Upton <[e-mail]>

arch/arm64/kvm/hyp/pgtable.c

···
 	return kvm_pgd_pages(ia_bits, start_level) * PAGE_SIZE;
 }
 
-static int stage2_free_walker(const struct kvm_pgtable_visit_ctx *ctx,
-			      enum kvm_pgtable_walk_flags visit)
+static int stage2_free_leaf(const struct kvm_pgtable_visit_ctx *ctx)
 {
 	struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
 
+	mm_ops->put_page(ctx->ptep);
+	return 0;
+}
+
+static int stage2_free_table_post(const struct kvm_pgtable_visit_ctx *ctx)
+{
+	struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
+	kvm_pte_t *childp = kvm_pte_follow(ctx->old, mm_ops);
+
+	if (mm_ops->page_count(childp) != 1)
+		return 0;
+
+	/*
+	 * Drop references and clear the now stale PTE to avoid rewalking the
+	 * freed page table.
+	 */
+	mm_ops->put_page(ctx->ptep);
+	mm_ops->put_page(childp);
+	kvm_clear_pte(ctx->ptep);
+	return 0;
+}
+
+static int stage2_free_walker(const struct kvm_pgtable_visit_ctx *ctx,
+			      enum kvm_pgtable_walk_flags visit)
+{
 	if (!stage2_pte_is_counted(ctx->old))
 		return 0;
 
-	mm_ops->put_page(ctx->ptep);
-
-	if (kvm_pte_table(ctx->old, ctx->level))
-		mm_ops->put_page(kvm_pte_follow(ctx->old, mm_ops));
-
-	return 0;
+	switch (visit) {
+	case KVM_PGTABLE_WALK_LEAF:
+		return stage2_free_leaf(ctx);
+	case KVM_PGTABLE_WALK_TABLE_POST:
+		return stage2_free_table_post(ctx);
+	default:
+		return -EINVAL;
+	}
 }
 
 void kvm_pgtable_stage2_destroy(struct kvm_pgtable *pgt)