linux-kernel - Re: [PATCH v3] usbip: vudc: Fix use after free bug in vudc

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Date:   Fri, 17 Mar 2023 16:53:07 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Zheng Wang <zyytlz.wz@....com>, valentina.manea.m@...il.com
Cc:     gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, hackerzheng666@...il.com,
        1395428693sheep@...il.com, alex000young@...il.com,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH v3] usbip: vudc: Fix use after free bug in vudc_remove due
 to race condition

On 3/17/23 04:09, Zheng Wang wrote:
> In vudc_probe, it calls init_vudc_hw, which bound &udc->timer with v_timer.
> 
> When it calls usbip_sockfd_store, it will call v_start_timer to start the
> timer work.
> 
> When we call vudc_remove to remove the driver, theremay be a sequence as
> follows:
> 
> Fix it by shutdown the timer work before cleanup in vudc_remove.
> 
> Note that removing a driver is a root-only operation, and should never
> happen. But the attacker can directly unplug the usb to trigger the remove
> function.
> 
> CPU0                  CPU1
> 
>                       |v_timer
> vudc_remove          |
> kfree(udc);          |
> //free shost         |
>                       |udc->gadget
>                       |//use
> 
> The udc might be removed before v_timer finished, and UAF happens.
> 
> This bug was found by Codeql static analysis and might by false positive.

This statement that this could be a false positive makes me hesitate
taking this patch.

What kind of testing have you done with this fix? Were you able to test
the scenario of unplugging usb?

thanks,
-- Shuah