[PATCH v3]cpufreq: add NULL check to the store method of cpufreq

[Shen Kai <shenkai8@huawei.com>]

From: Kai Shen <shenkai8@huawei.com>

Add NULL check in the store function here to avoid NULL callback invoking.
Though some interfaces of cpufreq are set as read-only, user can still get 
write permission using chmod which can lead to a kernel crash.

The following operations can lead to a kernel crash.

chmod +w /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq
echo 1 >  /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq

This bug was found on linux 4.19

Signed-off-by: Kai Shen <shenkai8@huawei.com>
Reported-by: Feilong Lin <linfeilong@huawei.com>
Reviewed-by: Feilong Lin <linfeilong@huawei.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
---
V2->V3
- use return value -EIO instead.
- do NULL check in show method too.

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

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 48a224a..bc19d6c 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -933,6 +933,9 @@ static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf)
 	struct freq_attr *fattr = to_attr(attr);
 	ssize_t ret;
 
+	if (!fattr->show)
+		return -EIO;
+
 	down_read(&policy->rwsem);
 	ret = fattr->show(policy, buf);
 	up_read(&policy->rwsem);
@@ -947,6 +950,9 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr,
 	struct freq_attr *fattr = to_attr(attr);
 	ssize_t ret = -EINVAL;
 
+	if (!fattr->store)
+		return -EIO;
+
 	/*
 	 * cpus_read_trylock() is used here to work around a circular lock
 	 * dependency problem with respect to the cpufreq_register_driver().
-- 
2.6.4.windows.1