On Fri, Aug 03, 2012 at 12:34:50PM -0700, Silas Boyd-Wickizer wrote:
> 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 <s...@mit.edu>
Acked-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> ---
> arch/x86/kernel/cpuid.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/arch/x86/kernel/cpuid.c b/arch/x86/kernel/cpuid.c
> index 39472dd..60c7891 100644
> --- 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);
> --
> 1.7.10.4
>
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