comedi: vmk80xx: fix bulk-buffer overflow

commit 78cdfd62bd upstream.

The driver is using endpoint-sized buffers but must not assume that the
tx and rx buffers are of equal size or a malicious device could overflow
the slab-allocated receive buffer when doing bulk transfers.

Fixes: 985cafccbf ("Staging: Comedi: vmk80xx: Add k8061 support")
Cc: stable@vger.kernel.org      # 2.6.31
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20211025114532.4599-5-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/comedi/drivers/vmk80xx.c

@@ -159,22 +159,20 @@ static void vmk80xx_do_bulk_msg(struct comedi_device *dev)
 	__u8 rx_addr;
 	unsigned int tx_pipe;
 	unsigned int rx_pipe;
-	size_t size;
+	size_t tx_size;
+	size_t rx_size;
 
 	tx_addr = devpriv->ep_tx->bEndpointAddress;
 	rx_addr = devpriv->ep_rx->bEndpointAddress;
 	tx_pipe = usb_sndbulkpipe(usb, tx_addr);
 	rx_pipe = usb_rcvbulkpipe(usb, rx_addr);
-
-	/*
-	 * The max packet size attributes of the K8061
-	 * input/output endpoints are identical
-	 */
-	size = usb_endpoint_maxp(devpriv->ep_tx);
+	tx_size = usb_endpoint_maxp(devpriv->ep_tx);
+	rx_size = usb_endpoint_maxp(devpriv->ep_rx);
 
 	usb_bulk_msg(usb, tx_pipe, devpriv->usb_tx_buf,
-		     size, NULL, devpriv->ep_tx->bInterval);
-	usb_bulk_msg(usb, rx_pipe, devpriv->usb_rx_buf, size, NULL, HZ * 10);
+		     tx_size, NULL, devpriv->ep_tx->bInterval);
+
+	usb_bulk_msg(usb, rx_pipe, devpriv->usb_rx_buf, rx_size, NULL, HZ * 10);
 }
 
 static int vmk80xx_read_packet(struct comedi_device *dev)