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    Subject[PATCH 4.9 127/151] cpufreq: Add NULL checks to show() and store() methods of cpufreq
    Date
    From: Kai Shen <shenkai8@huawei.com>

    commit e6e8df07268c1f75dd9215536e2ce4587b70f977 upstream.

    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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

    --- a/drivers/cpufreq/cpufreq.c
    +++ b/drivers/cpufreq/cpufreq.c
    @@ -875,6 +875,9 @@ static ssize_t show(struct kobject *kobj
    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);
    @@ -889,6 +892,9 @@ static ssize_t store(struct kobject *kob
    struct freq_attr *fattr = to_attr(attr);
    ssize_t ret = -EINVAL;

    + if (!fattr->store)
    + return -EIO;
    +
    get_online_cpus();

    if (cpu_online(policy->cpu)) {

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