[PATCH v2] i2c: core: fix debugfs UAF on adapter removal

[Vasileios Almpanis]

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

Its technically possible to create a client after i2c_deregister_clients
has run. That client will never be unregistered and making
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: 73febd775bdb ("i2c: create debugfs entry per adapter")
Reported-by: syzbot+23ad911c819b923238b7@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=23ad911c819b923238b7
Tested-by: syzbot+23ad911c819b923238b7@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Vasileios Almpanis <vasilisalmpanis@xxxxxxxxx>
---
 drivers/i2c/i2c-core-base.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
index 3ec04787a737..17cf3d225268 100644
--- a/drivers/i2c/i2c-core-base.c
+++ b/drivers/i2c/i2c-core-base.c
@@ -1815,6 +1815,12 @@ 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);

---
base-commit: 8ba098e6b6ff0db8edf28528d1552be261af30d4
change-id: 20260801-i2c-fb34477c0106

Best regards,
--  
Vasileios Almpanis <vasilisalmpanis@xxxxxxxxx>