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>

1 files changed, 5 insertions, 0 deletions

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index eb113693f043..a7c5661f8496 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);