USB: core: prevent malicious bNumInterfaces overflow

BugLink: http://bugs.launchpad.net/bugs/1745052

commit 48a4ff1c upstream.

A malicious USB device with crafted descriptors can cause the kernel
to access unallocated memory by setting the bNumInterfaces value too
high in a configuration descriptor.  Although the value is adjusted
during parsing, this adjustment is skipped in one of the error return
paths.

This patch prevents the problem by setting bNumInterfaces to 0
initially.  The existing code already sets it to the proper value
after parsing is complete.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

drivers/usb/core/config.c

@@ -550,6 +550,9 @@ static int usb_parse_configuration(struct usb_device *dev, int cfgidx,
 	unsigned iad_num = 0;
 
 	memcpy(&config->desc, buffer, USB_DT_CONFIG_SIZE);
+	nintf = nintf_orig = config->desc.bNumInterfaces;
+	config->desc.bNumInterfaces = 0;	// Adjusted later
+
 	if (config->desc.bDescriptorType != USB_DT_CONFIG ||
 	    config->desc.bLength < USB_DT_CONFIG_SIZE ||
 	    config->desc.bLength > size) {
@@ -563,7 +566,6 @@ static int usb_parse_configuration(struct usb_device *dev, int cfgidx,
 	buffer += config->desc.bLength;
 	size -= config->desc.bLength;
 
-	nintf = nintf_orig = config->desc.bNumInterfaces;
 	if (nintf > USB_MAXINTERFACES) {
 		dev_warn(ddev, "config %d has too many interfaces: %d, "
 		    "using maximum allowed: %d\n",