backport: HID: wacom: Avoid using stale array indicies to read contac…

…t count

If we ever see a touch report with contact count data we initialize
several variables used to read the contact count in the pre-report
phase. These variables are never reset if we process a report which
doesn't contain a contact count, however. This can cause the pre-
report function to trigger a read of arbitrary memory (e.g. NULL
if we're lucky) and potentially crash the driver.

This commit restores resetting of the variables back to default
"none" values that were used prior to the commit mentioned
below.

Link: linuxwacom#276
Fixes: 003f50ab673c (HID: wacom: Update last_slot_field during pre_report phase)
CC: stable@vger.kernel.org
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
[jason.gerecke@wacom.com: Imported into input-wacom repository (20f3cf5f860f)]
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
[jason.gerecke@wacom.com: Backported from input-wacom repository (86c7513)]
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>

3.17/wacom_wac.c

@@ -2698,6 +2698,10 @@ static void wacom_wac_finger_pre_report(struct hid_device *hdev,
 
 	hid_data->confidence = true;
 
+	hid_data->cc_report = 0;
+	hid_data->cc_index = -1;
+	hid_data->cc_value_index = -1;
+
 	for (i = 0; i < report->maxfield; i++) {
 		struct hid_field *field = report->field[i];
 		int j;