On Mon, Sep 18, 2017 at 9:27 AM, Jason Gerecke <killertofu@xxxxxxxxx> wrote:
> 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: <stable@xxxxxxxxxxxxxxx> # v4.9+
> Signed-off-by: Jason Gerecke <jason.gerecke@xxxxxxxxx>
Reviewed-by: Ping Cheng <ping.cheng@xxxxxxxxx>
Cheers,
Ping
> ---
> drivers/hid/wacom_sys.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/hid/wacom_sys.c b/drivers/hid/wacom_sys.c
> index 735bfbbcaa82..906e654fb0ba 100644
> --- a/drivers/hid/wacom_sys.c
> +++ b/drivers/hid/wacom_sys.c
> @@ -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
>
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