can: mcba_usb: fix memory leak in mcba_usb

Syzbot reported memory leak in SocketCAN driver for Microchip CAN BUS
Analyzer Tool. The problem was in unfreed usb_coherent.

In mcba_usb_start() 20 coherent buffers are allocated and there is
nothing, that frees them:

1) In callback function the urb is resubmitted and that's all
2) In disconnect function urbs are simply killed, but URB_FREE_BUFFER
   is not set (see mcba_usb_start) and this flag cannot be used with
   coherent buffers.

Fail log:
| [ 1354.053291][ T8413] mcba_usb 1-1:0.0 can0: device disconnected
| [ 1367.059384][ T8420] kmemleak: 20 new suspected memory leaks (see /sys/kernel/debug/kmem)

So, all allocated buffers should be freed with usb_free_coherent()
explicitly

NOTE:
The same pattern for allocating and freeing coherent buffers
is used in drivers/net/can/usb/kvaser_usb/kvaser_usb_core.c

Fixes: 51f3baad ("can: mcba_usb: Add support for Microchip CAN BUS Analyzer")
Link: https://lore.kernel.org/r/20210609215833.30393-1-paskripkin@gmail.com
Cc: linux-stable <stable@vger.kernel.org>
Reported-and-tested-by: syzbot+57281c762a3922e14dfe@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/usb/mcba_usb.c

@@ -82,6 +82,8 @@ struct mcba_priv {
 	bool can_ka_first_pass;
 	bool can_speed_check;
 	atomic_t free_ctx_cnt;
+	void *rxbuf[MCBA_MAX_RX_URBS];
+	dma_addr_t rxbuf_dma[MCBA_MAX_RX_URBS];
 };
 
 /* CAN frame */
@@ -633,6 +635,7 @@ static int mcba_usb_start(struct mcba_priv *priv)
 	for (i = 0; i < MCBA_MAX_RX_URBS; i++) {
 		struct urb *urb = NULL;
 		u8 *buf;
+		dma_addr_t buf_dma;
 
 		/* create a URB, and a buffer for it */
 		urb = usb_alloc_urb(0, GFP_KERNEL);
@@ -642,7 +645,7 @@ static int mcba_usb_start(struct mcba_priv *priv)
 		}
 
 		buf = usb_alloc_coherent(priv->udev, MCBA_USB_RX_BUFF_SIZE,
-					 GFP_KERNEL, &urb->transfer_dma);
+					 GFP_KERNEL, &buf_dma);
 		if (!buf) {
 			netdev_err(netdev, "No memory left for USB buffer\n");
 			usb_free_urb(urb);
@@ -661,11 +664,14 @@ static int mcba_usb_start(struct mcba_priv *priv)
 		if (err) {
 			usb_unanchor_urb(urb);
 			usb_free_coherent(priv->udev, MCBA_USB_RX_BUFF_SIZE,
-					  buf, urb->transfer_dma);
+					  buf, buf_dma);
 			usb_free_urb(urb);
 			break;
 		}
 
+		priv->rxbuf[i] = buf;
+		priv->rxbuf_dma[i] = buf_dma;
+
 		/* Drop reference, USB core will take care of freeing it */
 		usb_free_urb(urb);
 	}
@@ -708,7 +714,14 @@ static int mcba_usb_open(struct net_device *netdev)
 
 static void mcba_urb_unlink(struct mcba_priv *priv)
 {
+	int i;
+
 	usb_kill_anchored_urbs(&priv->rx_submitted);
+
+	for (i = 0; i < MCBA_MAX_RX_URBS; ++i)
+		usb_free_coherent(priv->udev, MCBA_USB_RX_BUFF_SIZE,
+				  priv->rxbuf[i], priv->rxbuf_dma[i]);
+
 	usb_kill_anchored_urbs(&priv->tx_submitted);
 }