linux-kernel - Re: [PATCH v2] char: xillybus: Prevent use-after-free due to race condition

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Date:   Sun, 13 Nov 2022 00:05:58 -0800
From:   Hyunwoo Kim <imv4bel@...il.com>
To:     Eli Billauer <eli.billauer@...il.com>
Cc:     gregkh@...uxfoundation.org, arnd@...db.de,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        stern@...land.harvard.edu, imv4bel@...il.com
Subject: Re: [PATCH v2] char: xillybus: Prevent use-after-free due to race
 condition

Dear,

Sorry for the late review.

This patch cannot prevent the UAF scenario I presented:
```
                cpu0                                                cpu1
       1. xillyusb_open()
          mutex_lock(&kref_mutex);    // unaffected lock
          xillybus_find_inode()
          mutex_lock(&unit_mutex);
          unit = iter;
          mutex_unlock(&unit_mutex);
                                                             2. xillyusb_disconnect()
                                                                xillybus_cleanup_chrdev()
                                                                mutex_lock(&unit_mutex);
                                                                kfree(unit);
                                                                mutex_unlock(&unit_mutex);
       3. *private_data = unit->private_data;   // UAF

```

This is a UAF for 'unit', not a UAF for 'xdev'.
So, the added 'kref_mutex' has no effect.


Regards,
Hyunwoo Kim