[PATCH] HID: wacom: Always increment hdev refcount within wacom_get_hdev_data

[PATCH] HID: wacom: Always increment hdev refcount within wacom_get_hdev_data

[Jason Gerecke]

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@vger.kernel.org> # v4.9+
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
---
 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

Re: [PATCH] HID: wacom: Always increment hdev refcount within wacom_get_hdev_data

[Ping Cheng]

On Mon, Sep 18, 2017 at 9:27 AM, Jason Gerecke <killertofu@gmail.com> 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@vger.kernel.org> # v4.9+
> Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>

Reviewed-by: Ping Cheng <ping.cheng@wacom.com>

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
>

Re: [PATCH] HID: wacom: Always increment hdev refcount within wacom_get_hdev_data

[Jiri Kosina]

On Tue, 26 Sep 2017, Jason Gerecke wrote:

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

Yeah, I have been travelling for past 3 weeks and acumulated quite some 
backlog, sorry. Everything pending should be processed by the end of this 
week.

-- 
Jiri Kosina
SUSE Labs

Re: [PATCH] HID: wacom: Always increment hdev refcount within wacom_get_hdev_data

[Jiri Kosina]

On Mon, 18 Sep 2017, Jason Gerecke 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@vger.kernel.org> # v4.9+
> Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>

Applied to for-4.14/upstream-fixes.

-- 
Jiri Kosina
SUSE Labs