linux-kernel - Re: [PATCH v2] usb: gadget: Fix double free of device descriptor pointers

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Date:   Fri, 23 Apr 2021 21:16:26 -0700
From:   Wesley Cheng <wcheng@...eaurora.org>
To:     Felipe Balbi <balbi@...nel.org>, gregkh@...uxfoundation.org,
        peter.chen@...nel.org
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Hemant Kumar <hemantk@...eaurora.org>, stable@...r.kernel.org
Subject: Re: [PATCH v2] usb: gadget: Fix double free of device descriptor
 pointers



On 4/23/2021 12:10 PM, Wesley Cheng wrote:
> 
> 
> On 4/22/2021 4:01 AM, Felipe Balbi wrote:
>>
>> Hi,
>>
>> Wesley Cheng <wcheng@...eaurora.org> writes:
>>
>>> From: Hemant Kumar <hemantk@...eaurora.org>
>>>
>>> Upon driver unbind usb_free_all_descriptors() function frees all
>>> speed descriptor pointers without setting them to NULL. In case
>>> gadget speed changes (i.e from super speed plus to super speed)
>>> after driver unbind only upto super speed descriptor pointers get
>>> populated. Super speed plus desc still holds the stale (already
>>> freed) pointer. Fix this issue by setting all descriptor pointers
>>> to NULL after freeing them in usb_free_all_descriptors().
>>
>> could you describe this a little better? How can one trigger this case?
>> Is the speed demotion happening after unbinding? It's not clear how to
>> cause this bug.
>>
> Hi Felipe,
> 
> Internally, we have a mechanism to switch the DWC3 core maximum speed
> parameter dynamically for displayport use cases.  This issue happens
> whenever we have a maximum speed change occur on the USB gadget, which
> for DWC3 happens whenever we call gadget init.  When we switch in and
> out of host mode, gadget init is being executed, leading to the change
> in the USB gadget max speed parameter:
> 
> dwc->gadget->max_speed		= dwc->maximum_speed;
> 
> I know that configFS gadget has the max_speed sysfs file, which is a
> similar mechanism, but I haven't tried to see if we can reproduce the
> same issue with it.  Let me see if we can reproduce this with that
> configfs speed setting.
> 
> Thanks
> Wesley Cheng
> 

Hi Felipe,

So I tried with doing it through the configFS max_speed, but it doesn't
have the same effect, as the setting done in dwc3_gadget_init() will
still be assigning the composite/UDC device's maximum speed to SSP/SS.
This is what the usb_assign_descriptor() uses to determine whether or
not to copy the SSP and SS descriptors.

So in summary, at least for a DWC3 based subsystem, the only way to
reproduce it is if there is a way to dynamically switch the DWC3 core
max speed parameter.

Thanks
Wesley Cheng

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