hwmon: (hp-wmi-sensors) Fix use-after-free in fungible_show()

In the Linux kernel, the following vulnerability has been resolved:

hwmon: (hp-wmi-sensors) Fix use-after-free in fungible_show()

nsensor->current_state is dynamically replaced as the sensor's state
changes. update_numeric_sensor_from_wobj() does this by freeing the
old string and installing a new one:

if (strcmp(trimmed, nsensor->current_state)) {
new_string = hp_wmi_strdup(dev, trimmed);
if (new_string) {
devm_kfree(dev, nsensor->current_state);
nsensor->current_state = new_string;
}
}

This function is only ever called from hp_wmi_update_info() while
state->lock is held, so the free-and-replace itself is properly
serialized against concurrent updates.

fungible_show(), however, reads the same pointer after the lock has
already been dropped:

err = hp_wmi_update_info(state, info);
if (err)
return err;

switch (prop) {
...
case HP_WMI_PROPERTY_CURRENT_STATE:
seq_printf(seqf, "%s\n", nsensor->current_state);
break;

hp_wmi_update_info() takes state->lock internally and releases it
before returning, so by the time fungible_show() dereferences
nsensor->current_state in seq_printf(), no lock is held. Two
processes reading a sensor's current_state debugfs entry at
overlapping times (or one reading it while another read of the same
sensor triggers a refresh) can race: one thread's seq_printf() can
be part-way through printing the string at the moment another
thread's call into update_numeric_sensor_from_wobj() frees it with
devm_kfree() and installs a new pointer, causing a use-after-free
read.

Take state->lock around the read in fungible_show() as well, so it
can never run concurrently with the free-and-replace in
update_numeric_sensor_from_wobj().