Re: [PATCH 3/4] HID: wacom: Fix Use-After-Free in wacom_bamboo_pad

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Lockless and unprotected dereference of RCU pointers `shared->pen` and `shared->touch` in `wacom_mode_change_work()` leading to Use-After-Free.
--

--- Patch [3]: [PATCH 3/4] HID: wacom: Fix Use-After-Free in wacom_bamboo_pad ---
commit 767b68ab7301c7003f61a85491f11aed0373e519
Author: Lee Jones <lee@kernel.org>

HID: wacom: Fix Use-After-Free in wacom_bamboo_pad

This commit resolves a lockless access to wacom->shared->pen by introducing
RCU protection for pen and touch pointers. It also updates writers to use
rcu_assign_pointer() and updates mode change workers to use
rcu_access_pointer().

> diff --git a/drivers/hid/wacom_sys.c b/drivers/hid/wacom_sys.c
> index cf9c12f3cff53..05d27fcd311f8 100644
> --- a/drivers/hid/wacom_sys.c
> +++ b/drivers/hid/wacom_sys.c

[ ... ]

> @@ -2810,16 +2821,19 @@ static void wacom_mode_change_work(struct work_struct *work)
>  	bool is_direct = wacom->wacom_wac.is_direct_mode;
>  	int error = 0;
>  
> -	if (shared->pen) {
> -		wacom1 = hid_get_drvdata(shared->pen);
> +	struct hid_device *pen = rcu_access_pointer(shared->pen);
> +	struct hid_device *touch = rcu_access_pointer(shared->touch);
> +
> +	if (pen) {
> +		wacom1 = hid_get_drvdata(pen);
>  		wacom_release_resources(wacom1);
>  		hid_hw_stop(wacom1->hdev);

[Severity: High]
This is a pre-existing issue, but could this lockless access to the RCU
pointers still lead to a use-after-free if the sibling device is removed
concurrently?

Because pen and touch are fetched using rcu_access_pointer() and
dereferenced without rcu_read_lock(), synchronize_rcu() in
wacom_remove_shared_data() won't block this worker thread.

If the sibling device is removed and freed right after rcu_access_pointer()
returns a non-NULL pointer, won't hid_get_drvdata(pen), hid_hw_stop(), and
wacom_release_resources() operate on freed memory?

>  		wacom1->wacom_wac.has_mode_change = true;
>  		wacom1->wacom_wac.is_direct_mode = is_direct;
>  	}
>  
> -	if (shared->touch) {
> -		wacom2 = hid_get_drvdata(shared->touch);
> +	if (touch) {
> +		wacom2 = hid_get_drvdata(touch);
>  		wacom_release_resources(wacom2);
>  		hid_hw_stop(wacom2->hdev);

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260804103030.1491356-1-lee@kernel.org?part=3