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

[Stefan Hajnoczi <stefanha@redhat.com>]

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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);
> > > +		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)

Yes, drivers must assume that the device completes request before
virtqueue_add_sgs() returns. For example, the device is allowed to poll
the virtqueue memory and may see the new descriptors immediately.

I haven't audited the current vsock code path to determine whether it's
possible to reach consume_skb() before deliver_tap_pkt() returns, so I
can't say whether it's safe or not.

Stefan

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