linux-kernel - Re: [PATCH v5] usb: gadget: udc: renesas_usb3: Fix use after free bug in renesas_usb3

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Date:   Thu, 16 Mar 2023 17:23:06 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Zheng Wang <zyytlz.wz@....com>
Cc:     p.zabel@...gutronix.de, biju.das.jz@...renesas.com,
        phil.edworthy@...esas.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, hackerzheng666@...il.com,
        1395428693sheep@...il.com, alex000young@...il.com,
        yoshihiro.shimoda.uh@...esas.com
Subject: Re: [PATCH v5] usb: gadget: udc: renesas_usb3: Fix use after free
 bug in  renesas_usb3_remove due to race condition

On Thu, Mar 16, 2023 at 11:56:45PM +0800, Zheng Wang wrote:
> In renesas_usb3_probe, &usb3->role_work is bound with
> renesas_usb3_role_work. renesas_usb3_start will be called
> to start the work.

You have a full 72 columns to write in, please use them :)

> 
> If we remove the driver which will call usbhs_remove, there may be
> an unfinished work. The possible sequence is as follows:
> 
> Fix it by canceling the work before cleanup in the renesas_usb3_remove.
> 
> CPU0                  CPU1
> 
>                     |renesas_usb3_role_work
> renesas_usb3_remove |
> usb_role_switch_unregister  |
> device_unregister   |
> kfree(sw)  	     |
> free usb3->role_sw  |
>                     |   usb_role_switch_set_role
>                     |   //use usb3->role_sw

Why can't this be written so it looks better:

 CPU0                         CPU1

                            | renesas_usb3_role_work
 renesas_usb3_remove        |
 usb_role_switch_unregister |
 device_unregister          |
 kfree(sw)  	            |
 free usb3->role_sw         |
                            |  usb_role_switch_set_role
                            |  // use usb3->role_sw

Note that removing a driver is a root-only operation, and should never
happen so please also say that.

thanks,

greg k-h