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

[Jason Wang]

On 2017å11æ29æ 23:31, Michael S. Tsirkin wrote:
> On Wed, Nov 29, 2017 at 09:23:24AM -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 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<wexu@xxxxxxxxxx>
> > Reported-by: Matthew Rosato<mjrosato@xxxxxxxxxxxxxxxxxx>
> > ---
> >   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?

tun_recvmsg() has the following check:

static int tun_recvmsg(struct socket *sock, struct msghdr *m, size_t 
total_len,
ÂÂÂ ÂÂÂ ÂÂÂÂÂÂ int flags)
{
ÂÂÂ struct tun_file *tfile = container_of(sock, struct tun_file, socket);
ÂÂÂ struct tun_struct *tun = __tun_get(tfile);
ÂÂÂ int ret;

ÂÂÂ if (!tun)
ÂÂÂ ÂÂÂ return -EBADFD;

ÂÂÂ if (flags & ~(MSG_DONTWAIT|MSG_TRUNC|MSG_ERRQUEUE)) {
ÂÂÂ ÂÂÂ ret = -EINVAL;
ÂÂÂ ÂÂÂ goto out;
ÂÂÂ }

And tun_do_read() has:

ÂÂÂ if (!iov_iter_count(to))
ÂÂÂ ÂÂÂ return 0;

So I think we need free skb in those cases.

Thanks