usb: chipidea: debug: avoid out of bound read

A string written by the user may not be zero terminated.

sscanf may read memory beyond the buffer if no zero byte
is found.

For testing build with CONFIG_USB_CHIPIDEA=y, CONFIG_USB_CHIPIDEA_DEBUG=y.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Peter Chen <peter.chen@freescale.com>

drivers/usb/chipidea/debug.c

@@ -88,9 +88,13 @@ static ssize_t ci_port_test_write(struct file *file, const char __user *ubuf,
 	char buf[32];
 	int ret;
 
-	if (copy_from_user(buf, ubuf, min_t(size_t, sizeof(buf) - 1, count)))
+	count = min_t(size_t, sizeof(buf) - 1, count);
+	if (copy_from_user(buf, ubuf, count))
 		return -EFAULT;
 
+	/* sscanf requires a zero terminated string */
+	buf[count] = '\0';
+
 	if (sscanf(buf, "%u", &mode) != 1)
 		return -EINVAL;