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SubjectRe: [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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