[PATCH] cpufreq: Fix race between suspend/resume and CPU hotplug

[Tianxiang Chen]

CPU hotplug operations can race with cpufreq_suspend()
and cpufreq_resume(), leading to null pointer dereferences
when accessing governor data. This occurs because there is
no synchronization between suspend/resume operations and
CPU hotplug, allowing concurrent access to
policy->governor_data while it is being freed or initialized.

Detailed race condition scenario:

1. Thread A (cpufreq_suspend) starts execution:
   - Iterates through active policies
   - Calls cpufreq_stop_governor(policy) for each policy
   - Sets cpufreq_suspended = true

2. Thread B (CPU hotplug) executes concurrently:
   - Calls cpu_down(cpu)
   - Calls cpuhp_cpufreq_offline(cpu)
   - Calls cpufreq_offline(cpu)
   - Inside cpufreq_offline():
     * Stops governor: policy->governor->stop(policy)
     * Exits governor: policy->governor->exit(policy)
     * Frees governor_data: kfree(policy->governor_data)
     * Sets policy->governor_data = NULL

3. Race window between step 1 and step 2:
   - Thread A is iterating policies and stopping governors
   - Thread B is concurrently executing CPU offline
   - Both threads may access the same policy->governor_data
   - Thread B frees governor_data while Thread A is still using it
   - Thread A accesses freed governor_data → null pointer dereference

Similarly, cpufreq_resume() can race with CPU hotplug where governor_data
is being initialized while hotplug is trying to access it, leading to
accessing uninitialized data.

Signed-off-by: Tianxiang Chen <nanmu@xxxxxxxxxx>
---
 drivers/cpufreq/cpufreq.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 1f794524a1d9..8b03785764fa 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1979,6 +1979,7 @@ void cpufreq_suspend(void)
        if (!cpufreq_driver)
                return;

+       cpus_read_lock();
        if (!has_target() && !cpufreq_driver->suspend)
                goto suspend;

@@ -1998,6 +1999,7 @@ void cpufreq_suspend(void)

 suspend:
        cpufreq_suspended = true;
+       cpus_read_unlock();
 }

 /**
@@ -2017,10 +2019,11 @@ void cpufreq_resume(void)
        if (unlikely(!cpufreq_suspended))
                return;

+       cpus_read_lock();
        cpufreq_suspended = false;

        if (!has_target() && !cpufreq_driver->resume)
-               return;
+               goto out;

        pr_debug("%s: Resuming Governors\n", __func__);

@@ -2038,6 +2041,9 @@ void cpufreq_resume(void)
                                       __func__, policy->cpu);
                }
        }
+
+out:
+       cpus_read_unlock();
 }

 /**
--
2.34.1

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