[PATCH] cpufreq: brcmstb-avs-cpufreq: avoid a stuck risk and UAF issue in brcm_avs_cpufreq_get()

[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 to cpufreq_cpu_put() is
ignored to decrement the kobject reference count back, which may lead
to a potential stuck risk that cpuhp thread deadly wait for dropping
of refcount when cpufreq policy free.

The call trace of stuck risk could be:
cpufreq_online()	//cpufreq initialization 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 brcmstb-avs driver just want to get cpufreq
policy in cpufreq_notify_transition().

What's more, there is a potential UAF issue in cpufreq_notify_transition()
that the cpufreq policy of current cpu has been released before using it.
So we should make a judgement to avoid it.

Thanks!
Qiwu

Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
---
 drivers/cpufreq/brcmstb-avs-cpufreq.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/cpufreq/brcmstb-avs-cpufreq.c b/drivers/cpufreq/brcmstb-avs-cpufreq.c
index 77b0e5d..31aa76f 100644
--- a/drivers/cpufreq/brcmstb-avs-cpufreq.c
+++ b/drivers/cpufreq/brcmstb-avs-cpufreq.c
@@ -452,8 +452,17 @@ 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) {
+		dev_warn(dev, "cpu %d: CPUFreq policy not found\n", cpu);
+		return NULL;
+	}
+
+	priv = policy->driver_data;
+	if (!priv || !priv->base)
+		return NULL;
 
 	return brcm_avs_get_frequency(priv->base);
 }
-- 
1.9.1


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