power: supply: bq25890: Fix external_power_changed race

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

power: supply: bq25890: Fix external_power_changed race

bq25890_charger_external_power_changed() dereferences bq->charger, which gets sets in bq25890_power_supply_init() like this:

bq->charger = devm_power_supply_register(bq->dev, &bq->desc, &psy_cfg);

As soon as devm_power_supply_register() has called device_add() the external_power_changed callback can get called. So there is a window where bq25890_charger_external_power_changed() may get called while bq->charger has not been set yet leading to a NULL pointer dereference.

This race hits during boot sometimes on a Lenovo Yoga Book 1 yb1-x90f when the cht_wcove_pwrsrc (extcon) power_supply is done with detecting the connected charger-type which happens to exactly hit the small window:

BUG: kernel NULL pointer dereference, address: 0000000000000018

RIP: 0010:__power_supply_is_supplied_by+0xb/0xb0

Call Trace:

__power_supply_get_supplier_property+0x19/0x50 class_for_each_device+0xb1/0xe0 power_supply_get_property_from_supplier+0x2e/0x50 bq25890_charger_external_power_changed+0x38/0x1b0 [bq25890_charger] __power_supply_changed_work+0x30/0x40 class_for_each_device+0xb1/0xe0 power_supply_changed_work+0x5f/0xe0

Fixing this is easy. The external_power_changed callback gets passed the power_supply which will eventually get stored in bq->charger, so bq25890_charger_external_power_changed() can simply directly use the passed in psy argument which is always valid.