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SubjectRe: [PATCH 1/4 V2] Use get_online_cpus to avoid races involving CPU hotplug
On Fri, Aug 03, 2012 at 12:33:27PM -0700, Silas Boyd-Wickizer wrote:
> If arch/x86/kernel/msr.c is a module, a CPU might offline or online
> between the for_each_online_cpu(i) loop and the call to
> register_hotcpu_notifier in msr_init or the call to
> unregister_hotcpu_notifier in msr_exit. The potential races can lead
> to leaks/duplicates, attempts to destroy non-existant devices, or
> random pointer dereferences.
>
> For example, in msr_init if:
>
> for_each_online_cpu(i) {
> err = msr_device_create(i);
> if (err != 0)
> goto out_class;
> }
> <----- CPU offlines
> register_hotcpu_notifier(&msr_class_cpu_notifier);
>
> and the CPU never onlines before msr_exit, then the module will never
> call msr_device_destroy for the associated CPU.
>
> 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>

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> ---
> arch/x86/kernel/msr.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
> index eb11369..a7c5661 100644
> --- a/arch/x86/kernel/msr.c
> +++ b/arch/x86/kernel/msr.c
> @@ -257,12 +257,14 @@ static int __init msr_init(void)
> goto out_chrdev;
> }
> msr_class->devnode = msr_devnode;
> + get_online_cpus();
> for_each_online_cpu(i) {
> err = msr_device_create(i);
> if (err != 0)
> goto out_class;
> }
> register_hotcpu_notifier(&msr_class_cpu_notifier);
> + put_online_cpus();
>
> err = 0;
> goto out;
> @@ -271,6 +273,7 @@ out_class:
> i = 0;
> for_each_online_cpu(i)
> msr_device_destroy(i);
> + put_online_cpus();
> class_destroy(msr_class);
> out_chrdev:
> __unregister_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr");
> @@ -281,11 +284,13 @@ out:
> static void __exit msr_exit(void)
> {
> int cpu = 0;
> + get_online_cpus();
> for_each_online_cpu(cpu)
> msr_device_destroy(cpu);
> class_destroy(msr_class);
> __unregister_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr");
> unregister_hotcpu_notifier(&msr_class_cpu_notifier);
> + put_online_cpus();
> }
>
> module_init(msr_init);
> --
> 1.7.10.4
>



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