Re: [PATCH 1/4 V2] Use get_online_cpus to avoid races involving CPU hotplug

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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
>