Re: [PATCH v1 2/2] hamradio: defer 6pack kfree after unregister_netdev

[Jakub Kicinski]

On Mon,  8 Nov 2021 18:37:59 +0800 Lin Ma wrote:
> There is a possible race condition (use-after-free) like below
> 
>  (USE)                       |  (FREE)
>   dev_queue_xmit             |
>    __dev_queue_xmit          |
>     __dev_xmit_skb           |
>      sch_direct_xmit         | ...
>       xmit_one               |
>        netdev_start_xmit     | tty_ldisc_kill
>         __netdev_start_xmit  |  6pack_close
>          sp_xmit             |   kfree
>           sp_encaps          |
>                              |
> 
> According to the patch "defer ax25 kfree after unregister_netdev", this

This is the previous patch of the series? Maybe call it "previous
patch"?

> patch reorder the kfree after the unregister_netdev to avoid the possible
> UAF as the unregister_netdev() is well synchronized and won't return if
> there is a running routine.
> 
> Signed-off-by: Lin Ma <linma@xxxxxxxxxx>

> diff --git a/drivers/net/hamradio/6pack.c b/drivers/net/hamradio/6pack.c
> index 49f10053a794..bfdf89e54752 100644
> --- a/drivers/net/hamradio/6pack.c
> +++ b/drivers/net/hamradio/6pack.c
> @@ -672,11 +672,13 @@ static void sixpack_close(struct tty_struct *tty)
>  	del_timer_sync(&sp->tx_t);
>  	del_timer_sync(&sp->resync_t);
>  
> -	/* Free all 6pack frame buffers. */
> +	unregister_netdev(sp->dev);
> +
> +	/* Free all 6pack frame buffers after unreg. */
>  	kfree(sp->rbuff);
>  	kfree(sp->xbuff);
>  
> -	unregister_netdev(sp->dev);
> +	free_netdev(sp->dev);

You should mention in the commit message why you think it's safe to add
free_netdev() which wasn't here before...

This driver seems to be setting:

	dev->needs_free_netdev	= true;

so unregister_netdev() will free the netdev automatically.

That said I don't see a reason why this driver needs the automatic
cleanup.

You can either remove that setting and then you can call free_netdev()
like you do, or you need to move the cleanup to dev->priv_destructor.

>  }
>  
>  /* Perform I/O control on an active 6pack channel. */