s390/cio: use generic driver_override infrastructure

When a driver is probed through __driver_attach(), the bus' match()
callback is called without the device lock held, thus accessing the
driver_override field without a lock, which can cause a UAF.

Fix this by using the driver-core driver_override infrastructure taking
care of proper locking internally.

Note that calling match() from __driver_attach() without the device lock
held is intentional. [1]

Link: https://lore.kernel.org/driver-core/[e-mail]/ [1]
Reported-by: Gui-Dong Han <[e-mail]>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220789
Fixes: ebc3d1791503 ("s390/cio: introduce driver_override on the css bus")
Reviewed-by: Vineeth Vijayan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Danilo Krummrich <[e-mail]>

drivers/s390/cio/cio.h

···
 	struct work_struct todo_work;
 	struct schib_config config;
 	u64 dma_mask;
-	/*
-	 * Driver name to force a match.  Do not set directly, because core
-	 * frees it.  Use driver_set_override() to set or clear it.
-	 */
-	const char *driver_override;
 } __attribute__ ((aligned(8)));
 
 DECLARE_PER_CPU_ALIGNED(struct irb, cio_irb);

drivers/s390/cio/css.c

···
 
 	sch->config.intparm = 0;
 	cio_commit_config(sch);
-	kfree(sch->driver_override);
 	kfree(sch);
 }
 
···
 
 static DEVICE_ATTR_RO(modalias);
 
-static ssize_t driver_override_store(struct device *dev,
-				     struct device_attribute *attr,
-				     const char *buf, size_t count)
-{
-	struct subchannel *sch = to_subchannel(dev);
-	int ret;
-
-	ret = driver_set_override(dev, &sch->driver_override, buf, count);
-	if (ret)
-		return ret;
-
-	return count;
-}
-
-static ssize_t driver_override_show(struct device *dev,
-				    struct device_attribute *attr, char *buf)
-{
-	struct subchannel *sch = to_subchannel(dev);
-	ssize_t len;
-
-	device_lock(dev);
-	len = sysfs_emit(buf, "%s\n", sch->driver_override);
-	device_unlock(dev);
-	return len;
-}
-static DEVICE_ATTR_RW(driver_override);
-
 static struct attribute *subch_attrs[] = {
 	&dev_attr_type.attr,
 	&dev_attr_modalias.attr,
-	&dev_attr_driver_override.attr,
 	NULL,
 };
 
···
 	struct subchannel *sch = to_subchannel(dev);
 	const struct css_driver *driver = to_cssdriver(drv);
 	struct css_device_id *id;
+	int ret;
 
 	/* When driver_override is set, only bind to the matching driver */
-	if (sch->driver_override && strcmp(sch->driver_override, drv->name))
+	ret = device_match_driver_override(dev, drv);
+	if (ret == 0)
 		return 0;
 
 	for (id = driver->subchannel_type; id->match_flags; id++) {
···
 
 static const struct bus_type css_bus_type = {
 	.name     = "css",
+	.driver_override = true,
 	.match    = css_bus_match,
 	.probe    = css_probe,
 	.remove   = css_remove,