Subject: Re: [PATCH net,stable v2] vhost: fix skb leak in handle_rx()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 29 Nov 2017 17:31:39 +0200

On Wed, Nov 29, 2017 at 09:23:24AM -0500, [email-masked] wrote:
> From: Wei Xu <[email-masked]>
> 
> Matthew found a roughly 40% tcp throughput regression with commit
> c67df11f(vhost_net: try batch dequing from skb array) as discussed
> in the following thread:
> https://www.mail-archive.com/[email-masked]/msg187936.html
> 
> Eventually we figured out that it was a skb leak in handle_rx()
> when sending packets to the VM. This usually happens when a guest
> can not drain out vq as fast as vhost fills in, afterwards it sets
> off the traffic jam and leaks skb(s) which occurs as no headcount
> to send on the vq from vhost side.
> 
> This can be avoided by making sure we have got enough headcount
> before actually consuming a skb from the batched rx array while
> transmitting, which is simply done by moving checking the zero
> headcount a bit ahead.
> 
> Also strengthen the small possibility of leak in case of recvmsg()
> fails by freeing the skb.
> 
> Signed-off-by: Wei Xu <[email-masked]>
> Reported-by: Matthew Rosato <[email-masked]>
> ---
>  drivers/vhost/net.c | 23 +++++++++++++----------
>  1 file changed, 13 insertions(+), 10 deletions(-)
> 
> v2:
> - add Matthew as the reporter, thanks matthew.
> - moving zero headcount check ahead instead of defer consuming skb
>   due to jason and mst's comment.
> - add freeing skb in favor of recvmsg() fails.
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 8d626d7..e302e08 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -778,16 +778,6 @@ static void handle_rx(struct vhost_net *net)
>  		/* On error, stop handling until the next kick. */
>  		if (unlikely(headcount < 0))
>  			goto out;
> -		if (nvq->rx_array)
> -			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
> -		/* On overrun, truncate and discard */
> -		if (unlikely(headcount > UIO_MAXIOV)) {
> -			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
> -			err = sock->ops->recvmsg(sock, &msg,
> -						 1, MSG_DONTWAIT | MSG_TRUNC);
> -			pr_debug("Discarded rx packet: len %zd\n", sock_len);
> -			continue;
> -		}
>  		/* OK, now we need to know about added descriptors. */
>  		if (!headcount) {
>  			if (unlikely(vhost_enable_notify(&net->dev, vq))) {
> @@ -800,6 +790,18 @@ static void handle_rx(struct vhost_net *net)
>  			 * they refilled. */
>  			goto out;
>  		}
> +		if (nvq->rx_array)
> +			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
> +		/* On overrun, truncate and discard */
> +		if (unlikely(headcount > UIO_MAXIOV)) {
> +			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);
> +			err = sock->ops->recvmsg(sock, &msg,
> +						 1, MSG_DONTWAIT | MSG_TRUNC);
> +			if (unlikely(err != 1))

Why 1? How is receiving 1 byte special or even possible?
Also, I wouldn't put an unlikely here. It's all error handling code anyway.

> +				kfree_skb((struct sk_buff *)msg.msg_control);

You do not need a cast here.
Also, is it really safe to refer to msg_control here?
I'd rather keep a copy of the skb pointer and use it than assume
caller did not change it. But also see below.

> +			pr_debug("Discarded rx packet: len %zd\n", sock_len);
> +			continue;
> +		}
>  		/* We don't need to be notified again. */
>  		iov_iter_init(&msg.msg_iter, READ, vq->iov, in, vhost_len);
>  		fixup = msg.msg_iter;
> @@ -818,6 +820,7 @@ static void handle_rx(struct vhost_net *net)
>  			pr_debug("Discarded rx packet: "
>  				 " len %d, expected %zd\n", err, sock_len);
>  			vhost_discard_vq_desc(vq, headcount);
> +			kfree_skb((struct sk_buff *)msg.msg_control);

You do not need a cast here.

Also, we have

        ret = tun_put_user(tun, tfile, skb, to);
        if (unlikely(ret < 0))
                kfree_skb(skb);
        else
                consume_skb(skb);

        return ret;

So it looks like recvmsg actually always consumes the skb.
So I was wrong when I said you need to kfree it after
recv msg, and your original patch was good.

Jason, what do you think?

>  			continue;
>  		}
>  		/* Supply virtio_net_hdr if VHOST_NET_F_VIRTIO_NET_HDR */
> -- 
> 1.8.3.1