Commit a2db672a authored Aug 03, 2012 by Silas Boyd-Wickizer Committed by Paul E. McKenney Sep 23, 2012

Use get_online_cpus to avoid races involving CPU hotplug



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

parent 5d180232

arch/x86/kernel/msr.c

+5 −0

Original line number	Diff line number	Diff line
		@@ -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);