linux-kernel - Re: [PATCH v2] usb: gadget: fix race when gadget driver register via ioctl

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Date:   Sun, 8 May 2022 12:08:35 +0800
From:   Schspa Shi <schspa@...il.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Julia.Lawall@...ia.fr, andreyknvl@...il.com, balbi@...nel.org,
        gregkh@...uxfoundation.org, jannh@...gle.com,
        jj251510319013@...il.com, linux-kernel@...r.kernel.org,
        linux-usb@...r.kernel.org,
        syzbot+dc7c3ca638e773db07f6@...kaller.appspotmail.com
Subject: Re: [PATCH v2] usb: gadget: fix race when gadget driver register via ioctl

Alan Stern <stern@...land.harvard.edu> writes:

> On Sun, May 08, 2022 at 12:02:43AM +0800, Schspa Shi wrote:
>> The usb_gadget_register_driver can be called multi time by to
>> threads via USB_RAW_IOCTL_RUN ioctl syscall, which will lead
>> to multiple registrations.
>>
>> Call trace:
>>   driver_register+0x220/0x3a0 drivers/base/driver.c:171
>>   usb_gadget_register_driver_owner+0xfb/0x1e0
>>     drivers/usb/gadget/udc/core.c:1546
>>   raw_ioctl_run drivers/usb/gadget/legacy/raw_gadget.c:513 [inline]
>>   raw_ioctl+0x1883/0x2730 drivers/usb/gadget/legacy/raw_gadget.c:1220
>>   ioctl USB_RAW_IOCTL_RUN
>>
>> This routine allows two processes to register the same driver instance
>> via ioctl syscall. which lead to a race condition.
>>
>> We can fix it by adding a new STATE_DEV_REGISTERING device state to
>> avoid double register.
>
> Are you sure that this patch will fix the problem found by syzbot?  That
> is, are you sure that the problem really was caused by two threads
> registering the same driver concurrently?
>

Yes, from the console log from syzbot.
T8324 alloced driver_private was released by T8326.

> The fact that the error was "use after free" suggests that the problem
> might be something else.  It looks like one of the threads was trying to
> access the driver structure after the other thread had done something
> that caused it to be deallocated, which suggests an error in reference
> counting.
>

The direct cause of this place is because of the refcount error, but the
root cause is still caused by multiple registrations

Please refer to the following scenarios.

           T1                                  T2
------------------------------------------------------------------
usb_gadget_register_driver_owner
  driver_register                    driver_register
    driver_find                       driver_find
    bus_add_driver                    bus_add_driver
      priv alloced                     <context switch>
      drv->p = priv;
      <schedule out>
      kobject_init_and_add // refcount = 1;
   //couldn't find an available UDC or it's busy
   <context switch>
                                       priv alloced
                                       drv->priv = priv;
                                       kobject_init_and_add
                                         ---> refcount = 1 <------
                                       // register success
                                       <context switch>
===================== another ioctl/process ======================
                                      driver_register
                                       driver_find
                                        k = kset_find_obj()
                                         ---> refcount = 2 <------
                                        <context out>
   driver_unregister
   // drv->p become T2's priv
   ---> refcount = 1 <------
   <context switch>
                                        kobject_put(k)
                                         ---> refcount = 0 <------
                                        return priv->driver;
                                        --------UAF here----------

There will be UAF in this scenario.
And all the logs reported by syzbot can be matched to this scenario.

> Yes, this could be caused by two threads both registering the same
> driver.  But the evidence doesn't prove that this is what happened, as
> far as I can see.
>
> Alan Stern
>

BRs

Schspa Shi