• Viresh Kumar's avatar
    cpufreq: Set cpufreq_cpu_data to NULL before putting kobject · 6ffae8c0
    Viresh Kumar authored
    In __cpufreq_remove_dev_finish(), per-cpu 'cpufreq_cpu_data' needs
    to be cleared before calling kobject_put(&policy->kobj) and under
    cpufreq_driver_lock. Otherwise, if someone else calls cpufreq_cpu_get()
    in parallel with it, they can obtain a non-NULL policy from that after
    kobject_put(&policy->kobj) was executed.
    
    Consider this case:
    
    Thread A				Thread B
    cpufreq_cpu_get()
      acquire cpufreq_driver_lock
      read-per-cpu cpufreq_cpu_data
    					kobject_put(&policy->kobj);
      kobject_get(&policy->kobj);
    					...
    					per_cpu(&cpufreq_cpu_data, cpu) = NULL
    
    And this will result in a warning like this one:
    
     ------------[ cut here ]------------
     WARNING: CPU: 0 PID: 4 at include/linux/kref.h:47
     kobject_get+0x41/0x50()
     Modules linked in: acpi_cpufreq(+) nfsd auth_rpcgss nfs_acl
     lockd grace sunrpc xfs libcrc32c sd_mod ixgbe igb mdio ahci hwmon
     ...
     Call Trace:
      [<ffffffff81661b14>] dump_stack+0x46/0x58
      [<ffffffff81072b61>] warn_slowpath_common+0x81/0xa0
      [<ffffffff81072c7a>] warn_slowpath_null+0x1a/0x20
      [<ffffffff812e16d1>] kobject_get+0x41/0x50
      [<ffffffff815262a5>] cpufreq_cpu_get+0x75/0xc0
      [<ffffffff81527c3e>] cpufreq_update_policy+0x2e/0x1f0
      [<ffffffff810b8cb2>] ? up+0x32/0x50
      [<ffffffff81381aa9>] ? acpi_ns_get_node+0xcb/0xf2
      [<ffffffff81381efd>] ? acpi_evaluate_object+0x22c/0x252
      [<ffffffff813824f6>] ? acpi_get_handle+0x95/0xc0
      [<ffffffff81360967>] ? acpi_has_method+0x25/0x40
      [<ffffffff81391e08>] acpi_processor_ppc_has_changed+0x77/0x82
      [<ffffffff81089566>] ? move_linked_works+0x66/0x90
      [<ffffffff8138e8ed>] acpi_processor_notify+0x58/0xe7
      [<ffffffff8137410c>] acpi_ev_notify_dispatch+0x44/0x5c
      [<ffffffff8135f293>] acpi_os_execute_deferred+0x15/0x22
      [<ffffffff8108c910>] process_one_work+0x160/0x410
      [<ffffffff8108d05b>] worker_thread+0x11b/0x520
      [<ffffffff8108cf40>] ? rescuer_thread+0x380/0x380
      [<ffffffff81092421>] kthread+0xe1/0x100
      [<ffffffff81092340>] ? kthread_create_on_node+0x1b0/0x1b0
      [<ffffffff81669ebc>] ret_from_fork+0x7c/0xb0
      [<ffffffff81092340>] ? kthread_create_on_node+0x1b0/0x1b0
     ---[ end trace 89e66eb9795efdf7 ]---
    
    The actual code flow is as follows:
    
     Thread A: Workqueue: kacpi_notify
    
     acpi_processor_notify()
       acpi_processor_ppc_has_changed()
             cpufreq_update_policy()
               cpufreq_cpu_get()
                 kobject_get()
    
     Thread B: xenbus_thread()
    
     xenbus_thread()
       msg->u.watch.handle->callback()
         handle_vcpu_hotplug_event()
           vcpu_hotplug()
             cpu_down()
               __cpu_notify(CPU_POST_DEAD..)
                 cpufreq_cpu_callback()
                   __cpufreq_remove_dev_finish()
                     cpufreq_policy_put_kobj()
                       kobject_put()
    
    cpufreq_cpu_get() gets the policy from per-cpu variable cpufreq_cpu_data
    under cpufreq_driver_lock, and once it gets a valid policy it expects it
    to not be freed until cpufreq_cpu_put() is called.
    
    But the race happens when another thread puts the kobject first and updates
    cpufreq_cpu_data before or later. And so the first thread gets a valid policy
    structure and before it does kobject_get() on it, the second one has already
    done kobject_put().
    
    Fix this by setting cpufreq_cpu_data to NULL before putting the kobject and that
    too under locks.
    Reported-by: default avatarEthan Zhao <ethan.zhao@oracle.com>
    Reported-by: default avatarSantosh Shilimkar <santosh.shilimkar@oracle.com>
    Signed-off-by: default avatarViresh Kumar <viresh.kumar@linaro.org>
    Cc: 3.12+ <stable@vger.kernel.org> # 3.12+
    Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
    6ffae8c0
cpufreq.c 64.6 KB