USB: serial: ftdi_sio: fix latency-timer error handling

commit e3e574ad85a208cb179f33720bb5f12b453de33c upstream.

Make sure to detect short responses when reading the latency timer to
avoid using stale buffer data.

Note that no heap data would currently leak through sysfs as
ASYNC_LOW_LATENCY is set by default.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: jhovold

drivers/usb/serial/ftdi_sio.c

@@ -1439,10 +1439,13 @@ static int read_latency_timer(struct usb_serial_port *port)
 			     FTDI_SIO_GET_LATENCY_TIMER_REQUEST_TYPE,
 			     0, priv->interface,
 			     buf, 1, WDR_TIMEOUT);
-	if (rv < 0)
+	if (rv < 1) {
 		dev_err(&port->dev, "Unable to read latency timer: %i\n", rv);
-	else
+		if (rv >= 0)
+			rv = -EIO;
+	} else {
 		priv->latency = buf[0];
+	}
 
 	kfree(buf);