linux-kernel - Re: [PATCH v2] usb: gadget: udc: fix race condition in usb_del

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Message-ID: <4c8e3538-23f9-4226-a6f6-18c84cc2f30d@rowland.harvard.edu>
Date: Tue, 14 Oct 2025 10:38:53 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Jimmy Hu <hhhuuu@...gle.com>
Cc: gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
	badhri@...gle.com, royluo@...gle.com, Thinh.Nguyen@...opsys.com,
	balbi@...com, linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] usb: gadget: udc: fix race condition in usb_del_gadget

On Tue, Oct 14, 2025 at 08:51:56AM +0000, Jimmy Hu wrote:
> A race condition during gadget teardown can lead to a use-after-free
> in usb_gadget_state_work(), as reported by KASAN:
> 
>   BUG: KASAN: invalid-access in sysfs_notify+0_x_2c/0_x_d0
>   Workqueue: events usb_gadget_state_work
> 
> The fundamental race occurs because a concurrent event (e.g., an
> interrupt) can call usb_gadget_set_state() and schedule gadget->work
> at any time during the cleanup process in usb_del_gadget().
> 
> Commit 399a45e5237c ("usb: gadget: core: flush gadget workqueue after
> device removal") attempted to fix this by moving flush_work() to after
> device_del(). However, this does not fully solve the race, as a new
> work item can still be scheduled *after* flush_work() completes but
> before the gadget's memory is freed, leading to the same use-after-free.
> 
> This patch fixes the race condition robustly by introducing a 'teardown'
> flag and a 'state_lock' spinlock to the usb_gadget struct. The flag is
> set during cleanup in usb_del_gadget() *before* calling flush_work() to
> prevent any new work from being scheduled once cleanup has commenced.
> The scheduling site, usb_gadget_set_state(), now checks this flag under
> the lock before queueing the work, thus safely closing the race window.
> 
> Changes in v2:
>   - Removed redundant inline comments as suggested by Alan Stern.
> 
> Fixes: 5702f75375aa9 ("usb: gadget: udc-core: move sysfs_notify() to a workqueue")
> Signed-off-by: Jimmy Hu <hhhuuu@...gle.com>
> Cc: stable@...r.kernel.org
> ---

Reviewed-by: Alan Stern <stern@...land.harvard.edu>

>  drivers/usb/gadget/udc/core.c | 17 ++++++++++++++++-
>  include/linux/usb/gadget.h    |  5 +++++
>  2 files changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/gadget/udc/core.c b/drivers/usb/gadget/udc/core.c
> index d709e24c1fd4..66d2428835da 100644
> --- a/drivers/usb/gadget/udc/core.c
> +++ b/drivers/usb/gadget/udc/core.c
> @@ -1123,8 +1123,13 @@ static void usb_gadget_state_work(struct work_struct *work)
>  void usb_gadget_set_state(struct usb_gadget *gadget,
>  		enum usb_device_state state)
>  {
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&gadget->state_lock, flags);
>  	gadget->state = state;
> -	schedule_work(&gadget->work);
> +	if (!gadget->teardown)
> +		schedule_work(&gadget->work);
> +	spin_unlock_irqrestore(&gadget->state_lock, flags);
>  }
>  EXPORT_SYMBOL_GPL(usb_gadget_set_state);
>  
> @@ -1357,6 +1362,8 @@ static void usb_udc_nop_release(struct device *dev)
>  void usb_initialize_gadget(struct device *parent, struct usb_gadget *gadget,
>  		void (*release)(struct device *dev))
>  {
> +	spin_lock_init(&gadget->state_lock);
> +	gadget->teardown = false;
>  	INIT_WORK(&gadget->work, usb_gadget_state_work);
>  	gadget->dev.parent = parent;
>  
> @@ -1531,6 +1538,7 @@ EXPORT_SYMBOL_GPL(usb_add_gadget_udc);
>  void usb_del_gadget(struct usb_gadget *gadget)
>  {
>  	struct usb_udc *udc = gadget->udc;
> +	unsigned long flags;
>  
>  	if (!udc)
>  		return;
> @@ -1544,6 +1552,13 @@ void usb_del_gadget(struct usb_gadget *gadget)
>  	kobject_uevent(&udc->dev.kobj, KOBJ_REMOVE);
>  	sysfs_remove_link(&udc->dev.kobj, "gadget");
>  	device_del(&gadget->dev);
> +	/*
> +	 * Set the teardown flag before flushing the work to prevent new work
> +	 * from being scheduled while we are cleaning up.
> +	 */
> +	spin_lock_irqsave(&gadget->state_lock, flags);
> +	gadget->teardown = true;
> +	spin_unlock_irqrestore(&gadget->state_lock, flags);
>  	flush_work(&gadget->work);
>  	ida_free(&gadget_id_numbers, gadget->id_number);
>  	cancel_work_sync(&udc->vbus_work);
> diff --git a/include/linux/usb/gadget.h b/include/linux/usb/gadget.h
> index 0f28c5512fcb..8b5e593f7966 100644
> --- a/include/linux/usb/gadget.h
> +++ b/include/linux/usb/gadget.h
> @@ -351,6 +351,9 @@ struct usb_gadget_ops {
>   *	can handle. The UDC must support this and all slower speeds and lower
>   *	number of lanes.
>   * @state: the state we are now (attached, suspended, configured, etc)
> + * @state_lock: Spinlock protecting the `state` and `teardown` members.
> + * @teardown: True if the device is undergoing teardown, used to prevent
> + *	new work from being scheduled during cleanup.
>   * @name: Identifies the controller hardware type.  Used in diagnostics
>   *	and sometimes configuration.
>   * @dev: Driver model state for this abstract device.
> @@ -426,6 +429,8 @@ struct usb_gadget {
>  	enum usb_ssp_rate		max_ssp_rate;
>  
>  	enum usb_device_state		state;
> +	spinlock_t			state_lock;
> +	bool				teardown;
>  	const char			*name;
>  	struct device			dev;
>  	unsigned			isoch_delay;
> -- 
> 2.51.0.760.g7b8bcc2412-goog
>