usb: gadget: u_audio: Free requests only after callback

As per the kernel doc for usb_ep_dequeue(), it states that "this
routine is asynchronous, that is, it may return before the completion
routine runs". And indeed since v5.0 the dwc3 gadget driver updated
its behavior to place dequeued requests on to a cancelled list to be
given back later after the endpoint is stopped.

The free_ep() was incorrectly assuming that a request was ready to
be freed after calling dequeue which results in a use-after-free
in dwc3 when it traverses its cancelled list. Fix this by moving
the usb_ep_free_request() call to the callback itself in case the
ep is disabled.

Fixes: eb9fecb9 ("usb: gadget: f_uac2: split out audio core")
Reported-and-tested-by: NFerry Toth <fntoth@gmail.com>
Reviewed-and-tested-by: NPeter Chen <peter.chen@nxp.com>
Acked-by: NFelipe Balbi <balbi@kernel.org>
Signed-off-by: NJack Pham <jackp@codeaurora.org>
Signed-off-by: NJerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20210118084642.322510-2-jbrunet@baylibre.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/u_audio.c

@@ -89,7 +89,12 @@ static void u_audio_iso_complete(struct usb_ep *ep, struct usb_request *req)
 	struct snd_uac_chip *uac = prm->uac;
 
 	/* i/f shutting down */
-	if (!prm->ep_enabled || req->status == -ESHUTDOWN)
+	if (!prm->ep_enabled) {
+		usb_ep_free_request(ep, req);
+		return;
+	}
+
+	if (req->status == -ESHUTDOWN)
 		return;
 
 	/*
@@ -336,8 +341,14 @@ static inline void free_ep(struct uac_rtd_params *prm, struct usb_ep *ep)
 
 	for (i = 0; i < params->req_number; i++) {
 		if (prm->ureq[i].req) {
-			usb_ep_dequeue(ep, prm->ureq[i].req);
-			usb_ep_free_request(ep, prm->ureq[i].req);
+			if (usb_ep_dequeue(ep, prm->ureq[i].req))
+				usb_ep_free_request(ep, prm->ureq[i].req);
+			/*
+			 * If usb_ep_dequeue() cannot successfully dequeue the
+			 * request, the request will be freed by the completion
+			 * callback.
+			 */
+
 			prm->ureq[i].req = NULL;
 		}
 	}