Use get_online_cpus to avoid races involving CPU hotplug

If arch/x86/kernel/cpuid.c is a module, a CPU might offline or online
between the for_each_online_cpu() loop and the call to
register_hotcpu_notifier in cpuid_init or the call to
unregister_hotcpu_notifier in cpuid_exit.  The potential races can
lead to leaks/duplicates, attempts to destroy non-existant devices, or
random pointer dereferences.

For example, in cpuid_exit if:

        for_each_online_cpu(cpu)
                cpuid_device_destroy(cpu);
        class_destroy(cpuid_class);
        __unregister_chrdev(CPUID_MAJOR, 0, NR_CPUS, "cpu/cpuid");
        <----- CPU onlines
        unregister_hotcpu_notifier(&cpuid_class_cpu_notifier);

the hotcpu notifier will attempt to create a device for the
cpuid_class, which the module already destroyed.

This fix surrounds for_each_online_cpu and register_hotcpu_notifier or
unregister_hotcpu_notifier with get_online_cpus+put_online_cpus.

Tested on a VM.

Signed-off-by: Silas Boyd-Wickizer <sbw@mit.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/arch/x86/kernel/cpuid.c b/arch/x86/kernel/cpuid.c
index 39472dd2323f31b030ed3d782b297794f8502b1b..60c78917190c1185abea1174258688a152b2e7f1 100644 (file)
--- a/arch/x86/kernel/cpuid.c
+++ b/arch/x86/kernel/cpuid.c
@@ -199,12 +199,14 @@ static int __init cpuid_init(void)
                goto out_chrdev;
        }
        cpuid_class->devnode = cpuid_devnode;
+       get_online_cpus();
        for_each_online_cpu(i) {
                err = cpuid_device_create(i);
                if (err != 0)
                        goto out_class;
        }
        register_hotcpu_notifier(&cpuid_class_cpu_notifier);
+       put_online_cpus();
 
        err = 0;
        goto out;
@@ -214,6 +216,7 @@ out_class:
        for_each_online_cpu(i) {
                cpuid_device_destroy(i);
        }
+       put_online_cpus();
        class_destroy(cpuid_class);
 out_chrdev:
        __unregister_chrdev(CPUID_MAJOR, 0, NR_CPUS, "cpu/cpuid");
@@ -225,11 +228,13 @@ static void __exit cpuid_exit(void)
 {
        int cpu = 0;
 
+       get_online_cpus();
        for_each_online_cpu(cpu)
                cpuid_device_destroy(cpu);
        class_destroy(cpuid_class);
        __unregister_chrdev(CPUID_MAJOR, 0, NR_CPUS, "cpu/cpuid");
        unregister_hotcpu_notifier(&cpuid_class_cpu_notifier);
+       put_online_cpus();
 }
 
 module_init(cpuid_init);