HID: multitouch: validate indexes details

commit 8821f5dc upstream.

When working on report indexes, always validate that they are in bounds.
Without this, a HID device could report a malicious feature report that
could trick the driver into a heap overflow:

[  634.885003] usb 1-1: New USB device found, idVendor=0596, idProduct=0500
...
[  676.469629] BUG kmalloc-192 (Tainted: G        W   ): Redzone overwritten

Note that we need to change the indexes from s8 to s16 as they can
be between -1 and 255.

CVE-2013-2897
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
[bwh: Backported to 3.2: mt_device::{cc,cc_value,inputmode}_index do not
 exist and the corresponding indices do not need to be validated.
 mt_device::maxcontact_report_id does not exist either.  So all we need
 to do is to widen mt_device::inputmode.]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/hid/hid-multitouch.c

@@ -66,7 +66,7 @@ struct mt_device {
 	unsigned last_field_index;	/* last field index of the report */
 	unsigned last_slot_field;	/* the last field of a slot */
 	int last_mt_collection;	/* last known mt-related collection */
-	__s8 inputmode;		/* InputMode HID feature, -1 if non-existent */
+	__s16 inputmode;	/* InputMode HID feature, -1 if non-existent */
 	__u8 num_received;	/* how many contacts we received */
 	__u8 num_expected;	/* expected last contact index */
 	__u8 maxcontacts;