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

From: Felipe Balbi
Date: Sat Apr 24 2021 - 09:31:52 EST


Wesley Cheng <wcheng@xxxxxxxxxxxxxx> writes:
> On 4/24/2021 1:05 AM, Felipe Balbi wrote:
>> Hi,
>> Wesley Cheng <wcheng@xxxxxxxxxxxxxx> writes:
>>>>>> From: Hemant Kumar <hemantk@xxxxxxxxxxxxxx>
>>>>>> 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.
>> Could it be that you have a bug in your out-of-tree changes? Perhaps
>> there's some assumption which your changes aren't guaranteeing.
> Hi Felipe,
> Unless there is a limitation on how the USB gadget max speed can be
> used, i.e. the USB gadget max speed MUST stay the same throughout a
> device's boot period, then our out of tree changes may have the wrong

no, not throughout boot, but it can't change while the device is

> assumptions. However, I don't see that stated anywhere, and I still
> feel the current usb_assign_descriptors() and usb_free_all_descriptors()
> aren't aligned with one another. One API decides which descriptors to
> copy based on a parameter, whereas the other just frees all of them
> irrespective.

that's a valid point.


Attachment: signature.asc
Description: PGP signature