HID: wacom: Always increment hdev refcount within wacom_get_hdev_data

The wacom_get_hdev_data function is used to find and return a reference to the "other half" of a Wacom device (i.e., the touch device associated with a pen, or vice-versa). To ensure these references are properly accounted for, the function is supposed to automatically increment the refcount before returning. This was not done, however, for devices which have pen & touch on different interfaces of the same USB device. This can lead to a WARNING ("refcount_t: underflow; use-after-free") when removing the module or device as we call kref_put() more times than kref_get(). Triggering an "actual" use- after-free would be difficult since both devices will disappear nearly- simultaneously. To silence this warning and prevent the potential error, we need to increment the refcount for all cases within wacom_get_hdev_data.

Fixes 41372d5d40 ("HID: wacom: Augment 'oVid' and 'oPid' with heuristics for HID_GENERIC")

Cc <[email protected]> # v4.9+

Signed-off-by Jason Gerecke <[email protected]>

---
 drivers/hid/wacom_sys.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

drivers/hid/wacom_sys.c +3 -2

index 735bfbbcaa82..906e654fb0ba 100644
@@ -668,8 +668,10 @@ static struct wacom_hdev_data *wacom_get_hdev_data(struct hid_device *hdev)
 
        /* Try to find an already-probed interface from the same device */
        list_for_each_entry(data, &wacom_udev_list, list) {
-               if (compare_device_paths(hdev, data->dev, '/'))
+               if (compare_device_paths(hdev, data->dev, '/')) {
+                       kref_get(&data->kref);
                        return data;
+               }
        }
 
        /* Fallback to finding devices that appear to be "siblings" */
--

2.14.1

Haven't seen any activity on this patch, so sending a reminder.

The wacom_get_hdev_data function is used to find and return a reference to the "other half" of a Wacom device (i.e., the touch device associated with a pen, or vice-versa). To ensure these references are properly accounted for, the function is supposed to automatically increment the refcount before returning. This was not done, however, for devices which have pen & touch on different interfaces of the same USB device. This can lead to a WARNING ("refcount_t: underflow; use-after-free") when removing the module or device as we call kref_put() more times than kref_get(). Triggering an "actual" use- after-free would be difficult since both devices will disappear nearly- simultaneously. To silence this warning and prevent the potential error, we need to increment the refcount for all cases within wacom_get_hdev_data.

Fixes 41372d5d40 ("HID: wacom: Augment 'oVid' and 'oPid' with heuristics for HID_GENERIC")

Cc <[email protected]> # v4.9+

Signed-off-by Jason Gerecke <[email protected]>