[PATCH v2 5/5] usb: xhci: Skip only one TD on Ring Underrun/Overrun

[Michal Pecio <michal.pecio@gmail.com>]

If skipping is deferred to events other than Missed Service Error itsef,
it means we are running on an xHCI 1.0 host and don't know how many TDs
were missed until we reach some ordinary transfer completion event.

And in case of ring xrun, we can't know where the xrun happened either.

If we skip all pending TDs, we may prematurely give back TDs added after
the xrun had occurred, risking data loss or buffer UAF by the xHC.

If we skip none, a driver may become confused and stop working when all
its URBs are missed and appear to be "in flight" forever.

Skip exactly one TD on each xrun event - the first one that was missed,
as we can now be sure that the HC has finished processing it. Provided
that one more TD is queued before any subsequent doorbell ring, it will
become safe to skip another TD by the time we get an xrun again.

Signed-off-by: Michal Pecio <michal.pecio@gmail.com>
---
 drivers/usb/host/xhci-ring.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 167ae7dfca47..d78b7877f65f 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -2844,8 +2844,21 @@ static int handle_tx_event(struct xhci_hcd *xhci,
 
 			if (!ep_seg && usb_endpoint_xfer_isoc(&td->urb->ep->desc)) {
 				skip_isoc_td(xhci, td, ep, status);
-				if (!list_empty(&ep_ring->td_list))
+
+				if (!list_empty(&ep_ring->td_list)) {
+					if (ring_xrun_event) {
+						/*
+						 * If we are here, we are on xHCI 1.0 host with no
+						 * idea how many TDs were missed or where the xrun
+						 * occurred. New TDs may have been added after the
+						 * xrun, so skip only one TD to be safe.
+						 */
+						xhci_dbg(xhci, "Skipped one TD for slot %u ep %u",
+								slot_id, ep_index);
+						return 0;
+					}
 					continue;
+				}
 
 				xhci_dbg(xhci, "All TDs skipped for slot %u ep %u. Clear skip flag.\n",
 					 slot_id, ep_index);
-- 
2.48.1