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().

Fixes: d38a87d7c064 ("s390/ap: Support driver_override for AP queue devices")
Tested-by: Holger Dengler <[e-mail]>
Reviewed-by: Holger Dengler <[e-mail]>
Reviewed-by: Harald Freudenberger <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Danilo Krummrich <[e-mail]>

drivers/s390/crypto/ap_bus.c

···
 
 static int __ap_revise_reserved(struct device *dev, void *dummy)
 {
-	int rc, card, queue, devres, drvres;
+	int rc, card, queue, devres, drvres, ovrd;
 
 	if (is_queue_dev(dev)) {
 		struct ap_driver *ap_drv = to_ap_drv(dev->driver);
 		struct ap_queue *aq = to_ap_queue(dev);
-		struct ap_device *ap_dev = &aq->ap_dev;
 
 		card = AP_QID_CARD(aq->qid);
 		queue = AP_QID_QUEUE(aq->qid);
 
-		if (ap_dev->driver_override) {
-			if (strcmp(ap_dev->driver_override,
-				   ap_drv->driver.name)) {
-				pr_debug("reprobing queue=%02x.%04x\n", card, queue);
-				rc = device_reprobe(dev);
-				if (rc) {
-					AP_DBF_WARN("%s reprobing queue=%02x.%04x failed\n",
-						    __func__, card, queue);
-				}
+		ovrd = device_match_driver_override(dev, &ap_drv->driver);
+		if (ovrd > 0) {
+			/* override set and matches, nothing to do */
+		} else if (ovrd == 0) {
+			pr_debug("reprobing queue=%02x.%04x\n", card, queue);
+			rc = device_reprobe(dev);
+			if (rc) {
+				AP_DBF_WARN("%s reprobing queue=%02x.%04x failed\n",
+					    __func__, card, queue);
 			}
 		} else {
 			mutex_lock(&ap_attr_mutex);
···
 	if (aq) {
 		const struct device_driver *drv = aq->ap_dev.device.driver;
 		const struct ap_driver *ap_drv = to_ap_drv(drv);
-		bool override = !!aq->ap_dev.driver_override;
+		bool override = device_has_driver_override(&aq->ap_dev.device);
 
 		if (override && drv && ap_drv->flags & AP_DRIVER_FLAG_DEFAULT)
 			rc = 1;
···
 {
 	struct ap_device *ap_dev = to_ap_dev(dev);
 	struct ap_driver *ap_drv = to_ap_drv(dev->driver);
-	int card, queue, devres, drvres, rc = -ENODEV;
+	int card, queue, devres, drvres, rc = -ENODEV, ovrd;
 
 	if (!get_device(dev))
 		return rc;
···
 		 */
 		card = AP_QID_CARD(to_ap_queue(dev)->qid);
 		queue = AP_QID_QUEUE(to_ap_queue(dev)->qid);
-		if (ap_dev->driver_override) {
-			if (strcmp(ap_dev->driver_override,
-				   ap_drv->driver.name))
-				goto out;
+		ovrd = device_match_driver_override(dev, &ap_drv->driver);
+		if (ovrd > 0) {
+			/* override set and matches, nothing to do */
+		} else if (ovrd == 0) {
+			goto out;
 		} else {
 			mutex_lock(&ap_attr_mutex);
 			devres = test_bit_inv(card, ap_perms.apm) &&

drivers/s390/crypto/ap_bus.h

···
 struct ap_device {
 	struct device device;
 	int device_type;		/* AP device type. */
-	const char *driver_override;
 };
 
 #define to_ap_dev(x) container_of((x), struct ap_device, device)

drivers/s390/crypto/ap_queue.c

···
 				    struct device_attribute *attr,
 				    char *buf)
 {
-	struct ap_queue *aq = to_ap_queue(dev);
-	struct ap_device *ap_dev = &aq->ap_dev;
-	int rc;
-
-	device_lock(dev);
-	if (ap_dev->driver_override)
-		rc = sysfs_emit(buf, "%s\n", ap_dev->driver_override);
-	else
-		rc = sysfs_emit(buf, "\n");
-	device_unlock(dev);
-
-	return rc;
+	guard(spinlock)(&dev->driver_override.lock);
+	return sysfs_emit(buf, "%s\n", dev->driver_override.name ?: "");
 }
 
 static ssize_t driver_override_store(struct device *dev,
 				     struct device_attribute *attr,
 				     const char *buf, size_t count)
 {
-	struct ap_queue *aq = to_ap_queue(dev);
-	struct ap_device *ap_dev = &aq->ap_dev;
 	int rc = -EINVAL;
 	bool old_value;
 
···
 	if (ap_apmask_aqmask_in_use)
 		goto out;
 
-	old_value = ap_dev->driver_override ? true : false;
-	rc = driver_set_override(dev, &ap_dev->driver_override, buf, count);
+	old_value = device_has_driver_override(dev);
+	rc = __device_set_driver_override(dev, buf, count);
 	if (rc)
 		goto out;
-	if (old_value && !ap_dev->driver_override)
+	if (old_value && !device_has_driver_override(dev))
 		--ap_driver_override_ctr;
-	else if (!old_value && ap_dev->driver_override)
+	else if (!old_value && device_has_driver_override(dev))
 		++ap_driver_override_ctr;
 
 	rc = count;