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

[Wei Xu]

On Tue, Nov 28, 2017 at 07:50:58PM +0200, Michael S. Tsirkin wrote:
> On Tue, Nov 28, 2017 at 12:17:16PM -0500, wexu@xxxxxxxxxx wrote:
> > From: Wei Xu <wexu@xxxxxxxxxx>
> > 
> > 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/netdev@xxxxxxxxxxxxxxx/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 deferring it a moment later
> > in this patch.
> > 
> > Signed-off-by: Wei Xu <wexu@xxxxxxxxxx>
> 
> Looks like a good way to fix it, but it will still leak if recvmsg
> returns a wrong length (I'm not sure this can happen in practice, but
> best to keep it simple).

Right, it is better to defend this case, will include it in v2.

> 
> Also, we need to add this before each recvmsg, including overrun,
> and discard on error.

OK.

Wei

> 
> > ---
> >  drivers/vhost/net.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 8d626d7..e76535e 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -778,8 +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);
> > @@ -809,6 +807,8 @@ static void handle_rx(struct vhost_net *net)
> >  			 */
> >  			iov_iter_advance(&msg.msg_iter, vhost_hlen);
> >  		}
> > +		if (nvq->rx_array)
> > +			msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
> >  		err = sock->ops->recvmsg(sock, &msg,
> >  					 sock_len, MSG_DONTWAIT | MSG_TRUNC);
> >  		/* Userspace might have consumed the packet meanwhile:
> > -- 
> > 1.8.3.1