Re: [PATCH] usb: gadget: udc: fix race condition in usb_del_gadget

[Alan Stern <stern@rowland.harvard.edu>]

On Mon, Oct 13, 2025 at 07:57:56AM +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.

Good analysis.

> 
> Fixes: 5702f75375aa9 ("usb: gadget: udc-core: move sysfs_notify() to a workqueue")
> Signed-off-by: Jimmy Hu <hhhuuu@google.com>
> Cc: stable@vger.kernel.org
> ---
>  drivers/usb/gadget/udc/core.c | 18 +++++++++++++++++-
>  include/linux/usb/gadget.h    |  6 ++++++
>  2 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/gadget/udc/core.c b/drivers/usb/gadget/udc/core.c
> index d709e24c1fd4..c4268b76d747 100644
> --- a/drivers/usb/gadget/udc/core.c
> +++ b/drivers/usb/gadget/udc/core.c

> @@ -1357,6 +1362,9 @@ 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))
>  {
> +	/* For race-free teardown */
> +	spin_lock_init(&gadget->state_lock);
> +	gadget->teardown = false;
>  	INIT_WORK(&gadget->work, usb_gadget_state_work);
>  	gadget->dev.parent = parent;

> diff --git a/include/linux/usb/gadget.h b/include/linux/usb/gadget.h
> index 0f28c5512fcb..8302aeaea82e 100644
> --- a/include/linux/usb/gadget.h
> +++ b/include/linux/usb/gadget.h

> @@ -426,6 +429,9 @@ struct usb_gadget {
>  	enum usb_ssp_rate		max_ssp_rate;
>  
>  	enum usb_device_state		state;
> +	/* For race-free teardown and state management */

The comments here and above merely repeat information that's already 
given in the kerneldoc.  They aren't needed.

Alan Stern