Re: [PATCH] usbip: vudc: Prevent transfer timer rearm during teardown

[Shuah Khan <skhan@linuxfoundation.org>]

On 9/21/26 16:41, Myeonghun Pak wrote:
> Commit d96209626a29 ("usbip: vudc: Fix use after free bug in
> vudc_remove due to race condition") made v_stop_timer() delete the
> transfer timer synchronously before vudc_remove() frees its containing
> struct vudc.
> 
> That does not close the race with the receive thread, which is stopped
> later through usb_del_gadget_udc() and vudc_shutdown().  The thread
> calls v_kick_timer() for CMD_SUBMIT and CMD_UNLINK packets, and
> v_kick_timer() deliberately calls mod_timer() when the transfer state is
> VUDC_TR_STOPPED.  A packet received after v_stop_timer() returns can
> therefore enqueue the timer again before the vudc is freed.
> 
> timer_delete_sync() only drains the current timer instance and cannot
> prevent such a rearm.  Use timer_shutdown_sync(), which makes subsequent
> attempts to arm the timer no-ops.  The transfer state no longer needs to
> be updated, as it is freed along with the timer.
> 
> On a KASAN and DEBUG_OBJECTS enabled x86_64 QEMU guest, repeatedly
> binding the device, submitting a request and unbinding it produced a
> "free active timer_list" warning followed by a slab-use-after-free in
> v_timer().  With this change the same harness completed 4000 iterations
> cleanly.
> 
> This issue was identified during our ongoing static-analysis research
> while reviewing kernel code.

Were you able to reproduce this problem to test and verify that this patch
fixes it?

> 
> Fixes: d96209626a29 ("usbip: vudc: Fix use after free bug in vudc_remove due to race condition")
> Link: https://lore.kernel.org/all/20230316180940.1601515-1-zyytlz.wz@163.com/
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Co-developed-by: Ijae Kim <ae878000@gmail.com>
> Signed-off-by: Ijae Kim <ae878000@gmail.com>
> Signed-off-by: Myeonghun Pak <mhun512@gmail.com>
> ---
>   drivers/usb/usbip/vudc_transfer.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/usbip/vudc_transfer.c b/drivers/usb/usbip/vudc_transfer.c
> index d4ce85c4c6a2..4146b746a320 100644
> --- a/drivers/usb/usbip/vudc_transfer.c
> +++ b/drivers/usb/usbip/vudc_transfer.c
> @@ -441,7 +441,7 @@ static void v_timer(struct timer_list *t)
>   	spin_unlock_irqrestore(&udc->lock, flags);
>   }
>   
> -/* All timer functions are run with udc->lock held */
> +/* v_start_timer() and v_kick_timer() are called with udc->lock held. */
>   
>   void v_init_timer(struct vudc *udc)
>   {
> @@ -490,8 +490,6 @@ void v_stop_timer(struct vudc *udc)
>   {
>   	struct transfer_timer *t = &udc->tr_timer;
>   
> -	/* Delete the timer synchronously before teardown frees udc. */
>   	dev_dbg(&udc->pdev->dev, "timer stop");
> -	timer_delete_sync(&t->timer);
> -	t->state = VUDC_TR_STOPPED;
> +	timer_shutdown_sync(&t->timer);
>   }
> 
> base-commit: 238650ef6c7c7cca08e032527329424c9fbd70e5

thanks,
-- Shuah