[PATCH 4.14 34/44] usb: dwc3: gadget: Prevent core from processing stale TRBs

Greg Kroah-Hartman
2022-02-14T09:33:56Z

From: Udipto Goswami <quic_ugoswami@quicinc.com>

commit 117b4e96c7f362eb6459543883fc07f77662472c upstream.

With CPU re-ordering on write instructions, there might
be a chance that the HWO is set before the TRB is updated
with the new mapped buffer address.
And in the case where core is processing a list of TRBs
it is possible that it fetched the TRBs when the HWO is set
but before the buffer address is updated.
Prevent this by adding a memory barrier before the HWO
is updated to ensure that the core always process the
updated TRBs.

Fixes: f6bafc6a1c9d ("usb: dwc3: convert TRBs into bitshifts")
Cc: stable <stable@vger.kernel.org>
Reviewed-by: Pavankumar Kondeti <quic_pkondeti@quicinc.com>
Signed-off-by: Udipto Goswami <quic_ugoswami@quicinc.com>
Link: https://lore.kernel.org/r/1644207958-18287-1-git-send-email-quic_ugoswami@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/usb/dwc3/gadget.c | 13 +++++++++++++
1 file changed, 13 insertions(+)

--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c
@@ -1004,6 +1004,19 @@ static void __dwc3_prepare_one_trb(struc
if (usb_endpoint_xfer_bulk(dep->endpoint.desc) && dep->stream_capable)
trb->ctrl |= DWC3_TRB_CTRL_SID_SOFN(stream_id);

+ /*
+ * As per data book 4.2.3.2TRB Control Bit Rules section
+ *
+ * The controller autonomously checks the HWO field of a TRB to determine if the
+ * entire TRB is valid. Therefore, software must ensure that the rest of the TRB
+ * is valid before setting the HWO field to '1'. In most systems, this means that
+ * software must update the fourth DWORD of a TRB last.
+ *
+ * However there is a possibility of CPU re-ordering here which can cause
+ * controller to observe the HWO bit set prematurely.
+ * Add a write memory barrier to prevent CPU re-ordering.
+ */
+ wmb();
trb->ctrl |= DWC3_TRB_CTRL_HWO;

dwc3_ep_inc_enq(dep);