i2c: core: fix debugfs UAF on adapter removal

i2c_del_adapter() frees the adapter's debugfs directory before it
unregisters the adapter device, but the new_device sysfs attribute
stays writable until device_del(). A write racing with removal still
reaches i2c_device_probe(), which passes the freed adap->debugfs to
debugfs_create_dir() as the new client's parent:

  BUG: KASAN: slab-use-after-free in lookup_noperm_common+0x407/0x430
  Read of size 4 at addr ffff88803ef87810 by task syz.0.61/6090
   lookup_noperm_common+0x407/0x430
   simple_start_creating+0x9c/0x110
   debugfs_start_creating+0xdb/0x1a0
   debugfs_create_dir+0x24/0x350
   i2c_device_probe+0x814/0xbf0

It's technically possible to create a client after i2c_deregister_clients
has run. That client will never be unregistered and make
wait_for_completion hang.

Close the window by removing the new_device attribute at the start of
i2c_del_adapter(). device_remove_file() will drain any clients left.

Fixes: 73febd775b ("i2c: create debugfs entry per adapter")
Reported-by: syzbot+23ad911c819b923238b7@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=23ad911c819b923238b7
Signed-off-by: Vasileios Almpanis <vasilisalmpanis@gmail.com>
Cc: <stable@vger.kernel.org> # v6.8+
Tested-by: syzbot+23ad911c819b923238b7@syzkaller.appspotmail.com
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://patch.msgid.link/20260812-i2c-v2-1-5efaab4c3334@gmail.com

drivers/i2c/i2c-core-base.c

@@ -1816,6 +1816,12 @@ void i2c_del_adapter(struct i2c_adapter *adap)
 		return;
 	}
 
+	/*
+	 * This drains any in-flight writers, so all
+	 * clients will be caught by i2c_deregister_clients().
+	 */
+	device_remove_file(&adap->dev, &dev_attr_new_device);
+
 	i2c_acpi_remove_space_handler(adap);
 
 	i2c_deregister_clients(adap);