Re: [PATCH net-next 2/2] mac80211: Resolve sk_refcnt/sk_wmem_alloc issue in wifi ack path

[Johannes Berg <johannes@sipsolutions.net>]

On Wed, 2014-09-10 at 18:05 -0400, Alexander Duyck wrote:
> There is a possible issue with the use, or lack thereof of sk_refcnt and
> sk_wmem_alloc in the wifi ack status functionality.
> 
> Specifically if a socket were to request acknowledgements, and the socket
> were to have sk_refcnt drop to 0 resulting in it waiting on sk_wmem_alloc
> to reach 0 it would be possible to have sock_queue_err_skb orphan the last
> buffer, resulting in __sk_free being called on the socket.  After this the
> buffer is enqueued on sk_error_queue, however the queue has already been
> flushed resulting in at least a memory leak, if not a data corruption.

Oh. Thanks :-)

> +	/* take a reference to prevent skb_orphan() from freeing the socket */
> +	sock_hold(sk);
> +
>  	err = sock_queue_err_skb(sk, skb);
>  	if (err)
>  		kfree_skb(skb);
> +
> +	sock_put(sk);
>  }
>  EXPORT_SYMBOL_GPL(skb_complete_wifi_ack);

Here I'm not sure it matters *for this function*? Wouldn't it be freed
then in sock_put(), which has the same net effect on this function
overall? It doesn't use it after sock_queue_err_skb().

Seems like maybe this should be in sock_queue_err_skb() itself, since it
does the orphaning first and then looks at the socket. Or the
documentation for that function should state that it has to be held, but
there are plenty of callers?

>  			spin_lock_irqsave(&local->ack_status_lock, flags);
> -			id = idr_alloc(&local->ack_status_frames, orig_skb,
> +			id = idr_alloc(&local->ack_status_frames, ack_skb,
>  				       1, 0x10000, GFP_ATOMIC);
>  			spin_unlock_irqrestore(&local->ack_status_lock, flags);
>  
>  			if (id >= 0) {
>  				info_id = id;
>  				info_flags |= IEEE80211_TX_CTL_REQ_TX_STATUS;
> -			} else if (skb_shared(skb)) {
> -				kfree_skb(orig_skb);
>  			} else {
> -				kfree_skb(skb);
> -				skb = orig_skb;
> +				kfree_skb(ack_skb);
>  			}

So you're removing this part, but can't we really not reuse the clone_sk
copy? The difference is that it's charged, but that's fine for the
purposes here, no? Or am I misunderstanding that?

johannes