driver core: class: properly reference count class_dev_iter()

When class_dev_iter is initialized, the reference count for the subsys
private structure is incremented, but never decremented, causing a
memory leak over time.  To resolve this, save off a pointer to the
internal structure into the class_dev_iter structure and then when the
iterator is finished, drop the reference count.

Reported-and-tested-by: syzbot+e7afd76ad060fa0d2605@syzkaller.appspotmail.com
Fixes: 7b884b7f ("driver core: class.c: convert to only use class_to_subsys")
Reported-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Cc: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Tested-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Link: https://lore.kernel.org/r/2023051610-stove-condense-9a77@gregkhSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/class.c

@@ -320,6 +320,7 @@ void class_dev_iter_init(struct class_dev_iter *iter, const struct class *class,
 		start_knode = &start->p->knode_class;
 	klist_iter_init_node(&sp->klist_devices, &iter->ki, start_knode);
 	iter->type = type;
+	iter->sp = sp;
 }
 EXPORT_SYMBOL_GPL(class_dev_iter_init);
 
@@ -361,6 +362,7 @@ EXPORT_SYMBOL_GPL(class_dev_iter_next);
 void class_dev_iter_exit(struct class_dev_iter *iter)
 {
 	klist_iter_exit(&iter->ki);
+	subsys_put(iter->sp);
 }
 EXPORT_SYMBOL_GPL(class_dev_iter_exit);
 

include/linux/device/class.h

@@ -74,6 +74,7 @@ struct class {
 struct class_dev_iter {
 	struct klist_iter		ki;
 	const struct device_type	*type;
+	struct subsys_private		*sp;
 };
 
 int __must_check class_register(const struct class *class);