s390/ap: use generic driver_override infrastructure

Overview

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

s390/ap: use generic driver_override infrastructure

When the AP masks are updated via apmask_store() or aqmask_store(), ap_bus_revise_bindings() is called after ap_attr_mutex has been released.

This calls __ap_revise_reserved(), which accesses the driver_override field without holding any lock, racing against a concurrent driver_override_store() that may free the old string, resulting in a potential UAF.

Fix this by using the driver-core driver_override infrastructure, which protects all accesses with an internal spinlock.

Note that unlike most other buses, the AP bus does not check driver_override in its match() callback; the override is checked in ap_device_probe() and __ap_revise_reserved() instead.

Also note that we do not enable the driver_override feature of struct bus_type, as AP - in contrast to most other buses - passes "" to sysfs_emit() when the driver_override pointer is NULL. Thus, printing "\n" instead of "(null)\n".

Additionally, AP has a custom counter that is modified in the corresponding custom driver_override_store().

Affected

• Linux >= d38a87d7c0643db61e7a3bfc3ebeea2dc2568f7e < 8f2eca0570438b94602da1297353eb7b10dcb6cb
• Linux >= d38a87d7c0643db61e7a3bfc3ebeea2dc2568f7e < 81d6f7c3a70b10ff757ee8b5f8114a190871cf1e
• Linux 6.19

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.