linux-kernel - [PATCH v3] usb: dwc3: gadget: Move null pinter check after window closed

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Message-Id: <20220513065709.1606634-1-albertccwang@google.com>
Date:   Fri, 13 May 2022 14:57:09 +0800
From:   Albert Wang <albertccwang@...gle.com>
To:     balbi@...nel.org, gregkh@...uxfoundation.org,
        quic_jackp@...cinc.com
Cc:     badhri@...gle.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, Albert Wang <albertccwang@...gle.com>
Subject: [PATCH v3] usb: dwc3: gadget: Move null pinter check after window closed

After inspecting further, we do see the locking is implicit, with the
main gotcha being the unlock/re-lock. That creates a window for a race
to happen. This change moves the NULL check to be adjacent to where
to it's used and after the window is "closed".

Fixes: 26288448120b ("usb: dwc3: gadget: Fix null pointer exception")
Signed-off-by: Albert Wang <albertccwang@...gle.com>
---
 v3: Add change log to be compliant with the canonical patch format
 v2: Remove redundant 'else' and add additional comments and more
     descriptive commit text

 drivers/usb/dwc3/gadget.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
index 19477f4bbf54..fda58951cf27 100644
--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c
@@ -3366,14 +3366,19 @@ static bool dwc3_gadget_endpoint_trbs_complete(struct dwc3_ep *dep,
 	struct dwc3		*dwc = dep->dwc;
 	bool			no_started_trb = true;
 
-	if (!dep->endpoint.desc)
-		return no_started_trb;
-
+	/*
+	 * This function eventually leads to dwc3_giveback() which unlocks
+	 * the dwc->lock and relocks afterwards. This actually creates a
+	 * a window for a race to happen.
+	 */
 	dwc3_gadget_ep_cleanup_completed_requests(dep, event, status);
 
 	if (dep->flags & DWC3_EP_END_TRANSFER_PENDING)
 		goto out;
 
+	if (!dep->endpoint.desc)
+		return no_started_trb;
+
 	if (usb_endpoint_xfer_isoc(dep->endpoint.desc) &&
 		list_empty(&dep->started_list) &&
 		(list_empty(&dep->pending_list) || status == -EXDEV))
-- 
2.36.0.550.gb090851708-goog