Commit 429227bb 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/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>

parent a2db672a

arch/x86/kernel/cpuid.c

+5 −0

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

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