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>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 13e8829..28a1a02 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/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);