Re: [Patch net] vsock: improve tap delivery accuracy

Bobby Eshleman
2023-05-03T16:24:44Z

On Wed, May 03, 2023 at 09:38:50AM +0200, Stefano Garzarella wrote:
> On Sun, Apr 16, 2023 at 04:49:00AM +0000, Bobby Eshleman wrote:
> > On Tue, May 02, 2023 at 04:14:18PM -0400, Stefan Hajnoczi wrote:
> > > On Tue, May 02, 2023 at 10:44:04AM -0700, Cong Wang wrote:
> > > > From: Cong Wang <cong.wang@bytedance.com>
> > > >
> > > > When virtqueue_add_sgs() fails, the skb is put back to send queue,
> > > > we should not deliver the copy to tap device in this case. So we
> > > > need to move virtio_transport_deliver_tap_pkt() down after all
> > > > possible failures.
> > > >
> > > > Fixes: 82dfb540aeb2 ("VSOCK: Add virtio vsock vsockmon hooks")
> > > > Cc: Stefan Hajnoczi <stefanha@redhat.com>
> > > > Cc: Stefano Garzarella <sgarzare@redhat.com>
> > > > Cc: Bobby Eshleman <bobby.eshleman@bytedance.com>
> > > > Signed-off-by: Cong Wang <cong.wang@bytedance.com>
> > > > ---
> > > > net/vmw_vsock/virtio_transport.c | 5 ++---
> > > > 1 file changed, 2 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> > > > index e95df847176b..055678628c07 100644
> > > > --- a/net/vmw_vsock/virtio_transport.c
> > > > +++ b/net/vmw_vsock/virtio_transport.c
> > > > @@ -109,9 +109,6 @@ virtio_transport_send_pkt_work(struct work_struct *work)
> > > > if (!skb)
> > > > break;
> > > >
> > > > - virtio_transport_deliver_tap_pkt(skb);
> > > > - reply = virtio_vsock_skb_reply(skb);
> > > > -
> > > > sg_init_one(&hdr, virtio_vsock_hdr(skb), sizeof(*virtio_vsock_hdr(skb)));
> > > > sgs[out_sg++] = &hdr;
> > > > if (skb->len > 0) {
> > > > @@ -128,6 +125,8 @@ virtio_transport_send_pkt_work(struct work_struct *work)
> > > > break;
> > > > }
> > > >
> > > > + virtio_transport_deliver_tap_pkt(skb);
>
> I would move only the virtio_transport_deliver_tap_pkt(),
> virtio_vsock_skb_reply() is not related.
>
> > > > + reply = virtio_vsock_skb_reply(skb);
> > >
> > > I don't remember the reason for the ordering, but I'm pretty sure it was
> > > deliberate. Probably because the payload buffers could be freed as soon
> > > as virtqueue_add_sgs() is called.
> > >
> > > If that's no longer true with Bobby's skbuff code, then maybe it's safe
> > > to monitor packets after they have been sent.
> > >
> > > Stefan
> >
> > Hey Stefan,
> >
> > Unfortunately, skbuff doesn't change that behavior.
> >
> > If I understand correctly, the problem flow you are describing
> > would be something like this:
> >
> > Thread 0 Thread 1
> > guest:virtqueue_add_sgs()[@send_pkt_work]
> >
> > host:vhost_vq_get_desc()[@handle_tx_kick]
> > host:vhost_add_used()
> > host:vhost_signal()
> > guest:virtqueue_get_buf()[@tx_work]
> > guest:consume_skb()
> >
> > guest:deliver_tap_pkt()[@send_pkt_work]
> > ^ use-after-free
> >
> > Which I guess is possible because the receiver can consume the new
> > scatterlist during the processing kicked off for a previous batch?
> > (doesn't have to wait for the subsequent kick)
>
> This is true, but both `send_pkt_work` and `tx_work` hold `tx_lock`, so can
> they really go in parallel?
>

Oh good point, the tx_lock synchronizes it:

Thread 0 Thread 1
guest:virtqueue_add_sgs()[@send_pkt_work]

host:vhost_vq_get_desc()[@handle_tx_kick]
host:vhost_add_used()
host:vhost_signal()
guest:mutex_lock()[@tx_work]
guest:deliver_tap_pkt()[@send_pkt_work]
guest:mutex_unlock()
guest:virtqueue_get_buf()[@tx_work]
guest:consume_skb()

I'm pretty sure this should be safe.

Best,
Bobby