KVM: Register /dev/kvm as the _very_ last thing during initialization

Register /dev/kvm, i.e. expose KVM to userspace, only after all other
setup has completed.  Once /dev/kvm is exposed, userspace can start
invoking KVM ioctls, creating VMs, etc...  If userspace creates a VM
before KVM is done with its configuration, bad things may happen, e.g.
KVM will fail to properly migrate vCPU state if a VM is created before
KVM has registered preemption notifiers.

Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221130230934.1014142-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -5988,12 +5988,6 @@ int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
 
 	kvm_chardev_ops.owner = module;
 
-	r = misc_register(&kvm_dev);
-	if (r) {
-		pr_err("kvm: misc device register failed\n");
-		goto out_unreg;
-	}
-
 	register_syscore_ops(&kvm_syscore_ops);
 
 	kvm_preempt_ops.sched_in = kvm_sched_in;
@@ -6002,11 +5996,24 @@ int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
 	kvm_init_debug();
 
 	r = kvm_vfio_ops_init();
-	WARN_ON(r);
+	if (WARN_ON_ONCE(r))
+		goto err_vfio;
+
+	/*
+	 * Registration _must_ be the very last thing done, as this exposes
+	 * /dev/kvm to userspace, i.e. all infrastructure must be setup!
+	 */
+	r = misc_register(&kvm_dev);
+	if (r) {
+		pr_err("kvm: misc device register failed\n");
+		goto err_register;
+	}
 
 	return 0;
 
-out_unreg:
+err_register:
+	kvm_vfio_ops_exit();
+err_vfio:
 	kvm_async_pf_deinit();
 out_free_4:
 	for_each_possible_cpu(cpu)
@@ -6032,8 +6039,14 @@ void kvm_exit(void)
 {
 	int cpu;
 
-	debugfs_remove_recursive(kvm_debugfs_dir);
+	/*
+	 * Note, unregistering /dev/kvm doesn't strictly need to come first,
+	 * fops_get(), a.k.a. try_module_get(), prevents acquiring references
+	 * to KVM while the module is being stopped.
+	 */
 	misc_deregister(&kvm_dev);
+
+	debugfs_remove_recursive(kvm_debugfs_dir);
 	for_each_possible_cpu(cpu)
 		free_cpumask_var(per_cpu(cpu_kick_mask, cpu));
 	kmem_cache_destroy(kvm_vcpu_cache);