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?
> Hence fixing it.
>
> Signed-off-by: Venu Byravarasu <vbyravarasu@xxxxxxxxxx>
> ---
> By mistake sent incorrect patch set number as v2 earlier.
> Hence fixing it.
>
> drivers/usb/host/ehci-sched.c | 16 ++++++++++------
> 1 files changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/usb/host/ehci-sched.c b/drivers/usb/host/ehci-sched.c
> index 33182c6..20d0c38 100644
> --- a/drivers/usb/host/ehci-sched.c
> +++ b/drivers/usb/host/ehci-sched.c
> @@ -1715,6 +1715,7 @@ itd_complete (
> struct ehci_iso_stream *stream = itd->stream;
> struct usb_device *dev;
> unsigned retval = false;
> + u32 stream_ref_count = 0;
>
> /* 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.
> /* If iso_stream_put() were called here, stream
> * would be freed. Instead, just prevent reuse.
> */
Alan Stern
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