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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 30 Nov 2017 15:25:32 +0200

On Thu, Nov 30, 2017 at 10:46:17AM +0800, Jason Wang wrote:
> 
> 
> On 2017年11月29日 23:31, Michael S. Tsirkin wrote:
> > 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?
> > 
> 
> tun_recvmsg() has the following check:
> 
> static int tun_recvmsg(struct socket *sock, struct msghdr *m, size_t
> total_len,
>                int flags)
> {
>     struct tun_file *tfile = container_of(sock, struct tun_file, socket);
>     struct tun_struct *tun = __tun_get(tfile);
>     int ret;
> 
>     if (!tun)
>         return -EBADFD;
> 
>     if (flags & ~(MSG_DONTWAIT|MSG_TRUNC|MSG_ERRQUEUE)) {
>         ret = -EINVAL;
>         goto out;
>     }
>
> And tun_do_read() has:
> 
>     if (!iov_iter_count(to))
>         return 0;
> 
> So I think we need free skb in those cases.
> 
> Thanks

So it's a mess for callers. Let's free within tun then?

-- 
MST