linux-kernel - [PATCH 5.4 57/71] usb: dwc3: gadget: Prevent core from processing stale TRBs

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Date:   Mon, 14 Feb 2022 10:26:25 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Pavankumar Kondeti <quic_pkondeti@...cinc.com>,
        Udipto Goswami <quic_ugoswami@...cinc.com>
Subject: [PATCH 5.4 57/71] usb: dwc3: gadget: Prevent core from processing stale TRBs

From: Udipto Goswami <quic_ugoswami@...cinc.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@...r.kernel.org>
Reviewed-by: Pavankumar Kondeti <quic_pkondeti@...cinc.com>
Signed-off-by: Udipto Goswami <quic_ugoswami@...cinc.com>
Link: https://lore.kernel.org/r/1644207958-18287-1-git-send-email-quic_ugoswami@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/usb/dwc3/gadget.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c
@@ -1020,6 +1020,19 @@ 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);