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cpufreq: Add NULL checks to show() and store() methods of cpufreq
Add NULL checks to show() and store() in cpufreq.c to avoid attempts to invoke a NULL callback. Though some interfaces of cpufreq are set as read-only, users can still get write permission using chmod which can lead to a kernel crash, as follows: 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 in 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> [ rjw: Subject & changelog ] Cc: All applicable <stable@vger.kernel.org> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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@ -933,6 +933,9 @@ static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf)
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struct freq_attr *fattr = to_attr(attr);
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ssize_t ret;
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if (!fattr->show)
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return -EIO;
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down_read(&policy->rwsem);
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ret = fattr->show(policy, buf);
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up_read(&policy->rwsem);
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@ -947,6 +950,9 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr,
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struct freq_attr *fattr = to_attr(attr);
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ssize_t ret = -EINVAL;
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if (!fattr->store)
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return -EIO;
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/*
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* cpus_read_trylock() is used here to work around a circular lock
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* dependency problem with respect to the cpufreq_register_driver().
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