Subject: Re: [PATCH net v2] ax25: fix use-after-free bugs caused by ax25_ds_del_timer

From: Simon Horman <[email-masked]>
Date: Sun, 31 Mar 2024 10:32:41 +0100

On Fri, Mar 29, 2024 at 09:50:23AM +0800, Duoming Zhou wrote:
> When the ax25 device is detaching, the ax25_dev_device_down()
> calls ax25_ds_del_timer() to cleanup the slave_timer. When
> the timer handler is running, the ax25_ds_del_timer() that
> calls del_timer() in it will return directly. As a result,
> the use-after-free bugs could happen, one of the scenarios
> is shown below:
> 
>       (Thread 1)          |      (Thread 2)
>                           | ax25_ds_timeout()
> ax25_dev_device_down()    |
>   ax25_ds_del_timer()     |
>     del_timer()           |
>   ax25_dev_put() //FREE   |
>                           |  ax25_dev-> //USE
> 
> In order to mitigate bugs, when the device is detaching, use
> timer_shutdown_sync() to stop the timer.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Duoming Zhou <[email-masked]>
> ---
> Changes in v2:
>   - Call timer_shutdown_sync() in ax25_dev_device_down().

Thanks,

as per my review of v1, I do think this is a correct approach to addressing
a valid concern.  But I would also value another set of eyes on the problem.

Reviewed-by: Simon Horman <[email-masked]>

>  net/ax25/ax25_dev.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ax25/ax25_dev.c b/net/ax25/ax25_dev.c
> index c5462486dbc..282ec581c07 100644
> --- a/net/ax25/ax25_dev.c
> +++ b/net/ax25/ax25_dev.c
> @@ -105,7 +105,7 @@ void ax25_dev_device_down(struct net_device *dev)
>  	spin_lock_bh(&ax25_dev_lock);
>  
>  #ifdef CONFIG_AX25_DAMA_SLAVE
> -	ax25_ds_del_timer(ax25_dev);
> +	timer_shutdown_sync(&ax25_dev->dama.slave_timer);
>  #endif
>  
>  	/*
> -- 
> 2.17.1
>