linux-kernel - Re: [PATCH] usb: dwc2: gadget: avoid null dereference on incomplete transfer

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Date:	Wed, 30 Mar 2016 15:45:27 -0700
From:	John Youn <John.Youn@...opsys.com>
To:	Felipe Balbi <felipe.balbi@...ux.intel.com>,
	John Keeping <john@...anate.com>,
	John Youn <John.Youn@...opsys.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: dwc2: gadget: avoid null dereference on incomplete
 transfer

On 3/30/2016 6:22 AM, Felipe Balbi wrote:
> 
> Hi,
> 
> John Keeping <john@...anate.com> writes:
>> Setting up a gadget with the uac2 function results in:
>>
>>   Unable to handle kernel NULL pointer dereference at virtual address 00000058
>>   ...
>>   PC is at dwc2_hsotg_irq+0x7f0/0x908
>>   LR is at dwc2_hsotg_irq+0x4c/0x908
>>   Backtrace:
>>   [<c03cd5fc>] (dwc2_hsotg_irq) from [<c00814fc>] (handle_irq_event_percpu+0x130/0x3ec)
>>   [<c00813cc>] (handle_irq_event_percpu) from [<c0081800>] (handle_irq_event+0x48/0x6c)
>>
>> In all other loops we already skip endpoints that are null, so do so
>> here as well.
>>
>> Signed-off-by: John Keeping <john@...anate.com>
>> ---
>>  drivers/usb/dwc2/gadget.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/usb/dwc2/gadget.c b/drivers/usb/dwc2/gadget.c
>> index 0abf73c..df43ec0 100644
>> --- a/drivers/usb/dwc2/gadget.c
>> +++ b/drivers/usb/dwc2/gadget.c
>> @@ -2606,7 +2606,9 @@ irq_retry:
>>  		for (idx = 1; idx < hsotg->num_of_eps; idx++) {
>>  			hs_ep = hsotg->eps_in[idx];
>>  
>> -			if (!hs_ep->isochronous || hs_ep->has_correct_parity)
>> +			if (!hs_ep ||
>> +			    !hs_ep->isochronous ||
>> +			    hs_ep->has_correct_parity)
> 
> this is fine (even though choice of where to break line is a bit odd),
> but I have a question about how the rest of the code works (a bit
> off-topic, sorry)
> 
>>  				continue;
>>  
>>  			epctl_reg = DIEPCTL(idx);
> 
> So, this means that the first ISO endpoint without correct parity will
> be used. Isn't this a bit fragile ? What happens when you use a device
> with several different interfaces using several different endpoints ?
> 
> Isn't there a register where we can check which physical endpoint
> generated the IRQ ? Seems like you really wanna check what:
> 

We discussed this back when the patch was first submitted and
determined it should work fine like this. I don't remember exactly why
though.

But this ISOC parity stuff is a workaround and we have a series of
patches to correctly set up ISOC allowing us to remove it. We're doing
some final tests before we send them.

Regards,
John