[PATCH v2] cpufreq: brcmstb-avs-cpufreq: avoid potential stuck and UAF risk

[qiwuchen55@gmail.com]

From: chenqiwu <chenqiwu@xiaomi.com>

brcm_avs_cpufreq_get() calls cpufreq_cpu_get() to get cpufreq policy,
meanwhile, it also increments the kobject reference count of policy to
mark it busy. However, a corresponding call of cpufreq_cpu_put() is
ignored to decrement the kobject reference count back, which may lead
to a potential stuck risk that percpu cpuhp thread deadly waits for
dropping of kobject refcount when percpu cpufreq policy free.

The call trace of stuck risk could be:
cpufreq_online()  //If cpufreq online failed, goto out_free_policy.
    ->cpufreq_policy_free()     //Do cpufreq_policy free.
        ->cpufreq_policy_put_kobj()
            ->kobject_put()       //Skip if policy kfref count is not 1.
                ->cpufreq_sysfs_release()
                    ->complete()  //Complete policy->kobj_unregister.
                ->wait_for_completion() //Wait for policy->kobj_unregister.

A simple way to avoid this stuck risk is use cpufreq_cpu_get_raw()
instead of cpufreq_cpu_get(), since this can be easily exercised by
attempting to force an unbind of the CPUfreq driver.

Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
---
 drivers/cpufreq/brcmstb-avs-cpufreq.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/cpufreq/brcmstb-avs-cpufreq.c b/drivers/cpufreq/brcmstb-avs-cpufreq.c
index 77b0e5d..6d2bf5c 100644
--- a/drivers/cpufreq/brcmstb-avs-cpufreq.c
+++ b/drivers/cpufreq/brcmstb-avs-cpufreq.c
@@ -452,8 +452,15 @@ static bool brcm_avs_is_firmware_loaded(struct private_data *priv)
 
 static unsigned int brcm_avs_cpufreq_get(unsigned int cpu)
 {
-	struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
-	struct private_data *priv = policy->driver_data;
+	struct cpufreq_policy *policy = cpufreq_cpu_get_raw(cpu);
+	struct private_data *priv;
+
+	if (!policy)
+		return 0;
+
+	priv = policy->driver_data;
+	if (!priv || !priv->base)
+		return 0;
 
 	return brcm_avs_get_frequency(priv->base);
 }
-- 
1.9.1