[PATCH 3.19 035/175] cpufreq: Set cpufreq_cpu_data to NULL before putting kobject

Greg Kroah-Hartman
2015-03-04T06:19:55Z

From: Viresh Kumar <viresh.kumar@linaro.org>

commit 6ffae8c06fab058d6c3f8ecb7f921327721034e7 upstream.

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: Ethan Zhao <ethan.zhao@oracle.com>
Reported-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/cpufreq/cpufreq.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1416,9 +1416,10 @@ static int __cpufreq_remove_dev_finish(s
unsigned long flags;
struct cpufreq_policy *policy;

- read_lock_irqsave(&cpufreq_driver_lock, flags);
+ write_lock_irqsave(&cpufreq_driver_lock, flags);
policy = per_cpu(cpufreq_cpu_data, cpu);
- read_unlock_irqrestore(&cpufreq_driver_lock, flags);
+ per_cpu(cpufreq_cpu_data, cpu) = NULL;
+ write_unlock_irqrestore(&cpufreq_driver_lock, flags);

if (!policy) {
pr_debug("%s: No cpu_data found\n", __func__);
@@ -1473,7 +1474,6 @@ static int __cpufreq_remove_dev_finish(s
}
}

- per_cpu(cpufreq_cpu_data, cpu) = NULL;
return 0;
}