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Date: Thu, 30 Nov 2017 15:25:32 +0200
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: wexu@...hat.com, virtualization@...ts.linux-foundation.org,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
mjrosato@...ux.vnet.ibm.com
Subject: Re: [PATCH net,stable v2] vhost: fix skb leak in handle_rx()
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,wexu@...hat.com wrote:
> > > From: Wei Xu<wexu@...hat.com>
> > >
> > > 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@vger.kernel.org/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@...hat.com>
> > > Reported-by: Matthew Rosato<mjrosato@...ux.vnet.ibm.com>
> > > ---
> > > 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
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