software node: Prevent link creation failure from causing kobj reference count imbalance

syzbot reported a uaf in software_node_notify_remove. [1]

When any of the two sysfs_create_link() in software_node_notify() fails,
the swnode->kobj reference count will not increase normally, which will
cause swnode to be released incorrectly due to the imbalance of kobj reference
count when executing software_node_notify_remove().

Increase the reference count of kobj before creating the link to avoid uaf.

[1]
BUG: KASAN: slab-use-after-free in software_node_notify_remove+0x1bc/0x1c0 drivers/base/swnode.c:1108
Read of size 1 at addr ffff888033c08908 by task syz-executor105/5844
Freed by task 5844:
 software_node_notify_remove+0x159/0x1c0 drivers/base/swnode.c:1106
 device_platform_notify_remove drivers/base/core.c:2387 [inline]

Fixes: 9eb59204 ("iommufd/selftest: Add set_dev_pasid in mock iommu")
Reported-by:  <syzbot+2ff22910687ee0dfd48e@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=2ff22910687ee0dfd48e


Tested-by:  <syzbot+2ff22910687ee0dfd48e@syzkaller.appspotmail.com>
Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20250414071123.1228331-1-lizhi.xu@windriver.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>