linux-kernel - Re: [PATCH v1] usb: host: Fix possible kernel crash

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Date:	Tue, 10 Jul 2012 09:56:39 +0530
From:	Venu Byravarasu <vbyravarasu@...dia.com>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] usb: host: Fix possible kernel crash

Thanks Alan for your comments.

On Monday 09 July 2012 08:04 PM, Alan Stern wrote:
> On Mon, 9 Jul 2012, Venu Byravarasu wrote:
>
>> In functions itd_complete &  sitd_complete, a pointer
>> by name stream may get dereferenced after freeing it, when
>> iso_stream_put is called with stream->refcount = 2.
> I don't understand the problem.  Did you actually see this happen or is
> it only theoretical?

Yes it is a theoretical problem, as complained by Coverity.

>   	/* for each uframe with a packet */
>   	for (uframe = 0; uframe < 8; uframe++) {
> @@ -1783,7 +1784,8 @@ itd_complete (
>   			dev->devpath, stream->bEndpointAddress & 0x0f,
>   			(stream->bEndpointAddress & USB_DIR_IN) ? "in" : "out");
>   	}
> -	iso_stream_put (ehci, stream);
> +	stream_ref_count = stream->refcount;
> +	iso_stream_put(ehci, stream);
> This iso_stream_put removes the reference held by the URB.  Before it
> is called, stream->refcount must be >= 3:
>
> 	refcount is set to 1 when the stream is created;
>
> 	each active URB holds a reference;
>
> 	each itd holds a reference.
>
> So after the call, the refcount value must be >= 2 and the stream could
> not have been deallocated.
>
>>   done:
>>   	itd->urb = NULL;
>> @@ -1797,7 +1799,7 @@ done:
>>   		 * Move it to a safe place until a new frame starts.
>>   		 */
>>   		list_move(&itd->itd_list, &ehci->cached_itd_list);
>> -		if (stream->refcount == 2) {
>> +		if (stream_ref_count == 3) {
> Therefore this seems unnecessary.

As per the logic you explained above, this change is not needed.
However coverity was complaining as below:

/kernel/drivers/usb/host/ehci-sched.c 1777 USE_AFTER_FREE Dereferencing 
freed pointer "stream"

Hence to pacify coverity, this change is done.
Please let me know if you see any other better way to handle it.

>>   			/* If iso_stream_put() were called here, stream
>>   			 * would be freed.  Instead, just prevent reuse.
>>   			 */
> Alan Stern
>

Thanks,
Venu
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