Subject: Re: [PATCH] n_gsm: Add Mutex to avoid race when net destroy

From: Jiri Slaby <[email-masked]>
Date: Thu, 28 Feb 2013 10:53:01 +0100

On 02/28/2013 06:31 AM, channing wrote:
> 
> when gsm Net is enabled, data on dlci is transferrd by
> gsm_mux_net_start_xmit(), while userspace may trigger
> ioctrl to call gsm_destroy_network() during data was
> transferring, because there is no mutex protection between
> the two functions, following scenario may happen:
> 
> 1) gsm_mux_net_start_xmit() calls muxnet_get(mux_net);
> 2) gsm_destroy_network() is called from ioctrl, and it
> will not call net_free() to release net device because
> net device is still referred in step 1)
> 3) continue execute step 1), gsm_mux_net_start_xmit()
> calls muxnet_put(mux_net), and then calls net_free() to
> release net device.
> 4) if userspace triggers gsm_create_network() at same time
> with net_free() in step 3). it will hit race on dlci->net.
> 
> This patch is to add a mutex in tx function to avoid race
> between it and destroy function.
> 
> Signed-off-by: Chao Bi <[email-masked]>
> Signed-off-by: Pillet Vincent <[email-masked]>
> ---
>  drivers/tty/n_gsm.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index 4a43ef5..0ca810a 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
> @@ -2660,6 +2660,7 @@ static int gsm_mux_net_start_xmit(struct sk_buff *skb,
>  {
>  	struct gsm_mux_net *mux_net = (struct gsm_mux_net *)netdev_priv(net);
>  	struct gsm_dlci *dlci = mux_net->dlci;
> +	mutex_lock(&dlci->mutex);

Nack, start_xmit may be called in an atomic context -- you cannot call
mutex.

>  	muxnet_get(mux_net);
>  
>  	skb_queue_head(&dlci->skb_list, skb);
> @@ -2669,6 +2670,7 @@ static int gsm_mux_net_start_xmit(struct sk_buff *skb,
>  	/* And tell the kernel when the last transmit started. */
>  	net->trans_start = jiffies;
>  	muxnet_put(mux_net);

Instead the concept is broken. If this was the last reference (as
described in your steps above), it would blow up for the same reason I
refer to above, i.e. net_free here would call unregister_netdev which is
not atomic. Plus it will definitely deadlock because unregister_netdev
waits for start_xmit to finish.

It should stop the queue and schedule a workqueue to lock the mutex,
unregister the hetdev and reset dlci->net. (Or maybe just call
muxnet_put with the lock held.)

That will fix 4), but there is still a bug: what protects
gsm_create_network to be called twice or more in a sequence thus
re-setting dlci->net to a new and new pointer?

> +	mutex_unlock(&dlci->mutex);
>  	return NETDEV_TX_OK;
>  }

thanks,
-- 
js
suse labs