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

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Date: Sun, 16 Apr 2023 06:40:45 +0000
From: Bobby Eshleman <bobbyeshleman@...il.com>
To: Stefan Hajnoczi <stefanha@...hat.com>
Cc: Cong Wang <xiyou.wangcong@...il.com>,
	Cong Wang <cong.wang@...edance.com>,
	Bobby Eshleman <bobby.eshleman@...edance.com>, kvm@...r.kernel.org,
	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [Patch net] vsock: improve tap delivery accuracy

On Wed, May 03, 2023 at 09:39:13AM -0400, Stefan Hajnoczi 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@...edance.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@...hat.com>
> > > > Cc: Stefano Garzarella <sgarzare@...hat.com>
> > > > Cc: Bobby Eshleman <bobby.eshleman@...edance.com>
> > > > Signed-off-by: Cong Wang <cong.wang@...edance.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.
> 

I see, thanks for the clarification.

Best,
Bobby