Re: [PATCH net] nfc: pn533: prevent division by zero in the listen mode timer

[Simon Horman <horms@kernel.org>]

On Mon, Jun 15, 2026 at 03:35:47AM -0700, Yinhao Hu wrote:
> The listen-mode timer handler advances the polling state machine through
> pn533_poll_next_mod(), which computes:
> 
> dev->poll_mod_curr = (dev->poll_mod_curr + 1) % dev->poll_mod_count;
> 
> pn533_poll_reset_mod_list() clears dev->poll_mod_count without first
> stopping that timer: pn533_dep_link_down() deletes no timer at all, and
> pn533_stop_poll() uses timer_delete(), which does not wait for a handler
> already running on another CPU. When the handler runs after the count
> has been zeroed, it divides by zero:
> 
> Oops: divide error: 0000 [#1] SMP
> RIP: 0010:pn533_listen_mode_timer+0x9b/0x110
> 
> Delete the timer synchronously in pn533_poll_reset_mod_list(), the single
> place that clears the list, so the handler can no longer run past a reset.
> Also return early when poll_mod_count is already zero, covering the window
> where pn533_wq_poll() re-arms the timer just before a reset.
> 
> Fixes: 6fbbdc16be38 ("NFC: Implement pn533 polling loop")
> Signed-off-by: Yinhao Hu <dddddd@hust.edu.cn>
> ---
>  drivers/nfc/pn533/pn533.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/nfc/pn533/pn533.c b/drivers/nfc/pn533/pn533.c
> index d7bdbc82e2ba..88df99001b4a 100644
> --- a/drivers/nfc/pn533/pn533.c
> +++ b/drivers/nfc/pn533/pn533.c
> @@ -951,6 +951,7 @@ static inline void pn533_poll_next_mod(struct pn533 *dev)
>  
>  static void pn533_poll_reset_mod_list(struct pn533 *dev)
>  {
> +	timer_delete_sync(&dev->listen_timer);
>  	dev->poll_mod_count = 0;
>  }
>  
> @@ -1235,6 +1236,10 @@ static void pn533_listen_mode_timer(struct timer_list *t)
>  {
>  	struct pn533 *dev = timer_container_of(dev, t, listen_timer);
>  
> +	/* Polling may have been stopped while the timer was pending. */
> +	if (!dev->poll_mod_count)
> +		return;
> +

I am concerned that access to poll_mod_count is not synchronised and thus
this may not work as intended.

>  	dev->cancel_listen = 1;
>  
>  	pn533_poll_next_mod(dev);
> -- 
> 2.43.0
>