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

[Eli Billauer <eli.billauer@gmail.com>]

On 13/11/2022 10:05, Hyunwoo Kim wrote:
> 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.
> 

You're correct. This submitted patch solves only one problem out of two. 
It prevents the content of @private_data to be freed, but you're right 
that @unit itself isn't protected well enough.

The problem you're pointing at is that @unit can be freed before its 
attempted use, because the mutex is released too early.

This is easily solved by moving down the mutex_unlock() call to after 
@unit has been used. Do you want the pleasure to submit this patch, or 
should I?

Thanks,
   Eli