Subject: Re: [PATCH net v3 1/2] vsock/virtio: reset connection on receiving queue overflow

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 14 May 2026 13:44:53 -0400

On Thu, May 14, 2026 at 06:45:00PM +0200, Stefano Garzarella wrote:
> On Thu, 14 May 2026 at 17:16, Michael S. Tsirkin <[email-masked]> wrote:
> >
> > On Thu, May 14, 2026 at 04:57:16PM +0200, Stefano Garzarella wrote:
> > > On Wed, May 13, 2026 at 12:54:16PM +0200, Stefano Garzarella wrote:
> > > > From: Stefano Garzarella <[email-masked]>
> > > >
> > > > When there is no more space to queue an incoming packet, the packet is
> > > > silently dropped. This causes data loss without any notification to
> > > > either peer, since there is no retransmission.
> > > >
> > > > Under normal circumstances, this should never happen. However, it could
> > > > happen if the other peer doesn't respect the credit, or if the skb
> > > > overhead, which we recently began to take into account with commit
> > > > 059b7dbd20a6 ("vsock/virtio: fix potential unbounded skb queue"),
> > > > is too high.
> > > >
> > > > Fix this by resetting the connection and setting the local socket error
> > > > to ENOBUFS when virtio_transport_recv_enqueue() can no longer queue a
> > > > packet, so both peers are explicitly notified of the failure rather than
> > > > silently losing data.
> > > >
> > > > Fixes: ae6fcfbf5f03 ("vsock/virtio: discard packets if credit is not respected")
> > > > Signed-off-by: Stefano Garzarella <[email-masked]>
> > > > ---
> > > > net/vmw_vsock/virtio_transport_common.c | 19 ++++++++++++++-----
> > > > 1 file changed, 14 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> > > > index 989cc252d3d3..4a4ac69d1ad1 100644
> > > > --- a/net/vmw_vsock/virtio_transport_common.c
> > > > +++ b/net/vmw_vsock/virtio_transport_common.c
> > > > @@ -1350,7 +1350,7 @@ virtio_transport_recv_connecting(struct sock *sk,
> > > >     return err;
> > > > }
> > > >
> > > > -static void
> > > > +static bool
> > > > virtio_transport_recv_enqueue(struct vsock_sock *vsk,
> > > >                           struct sk_buff *skb)
> > > > {
> > > > @@ -1365,10 +1365,8 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,
> > > >     spin_lock_bh(&vvs->rx_lock);
> > > >
> > > >     can_enqueue = virtio_transport_inc_rx_pkt(vvs, len);
> > > > -   if (!can_enqueue) {
> > > > -           free_pkt = true;
> > > > +   if (!can_enqueue)
> > > >             goto out;
> > > > -   }
> > > >
> > > >     if (le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOM)
> > > >             vvs->msg_count++;
> > > > @@ -1408,6 +1406,8 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,
> > > >     spin_unlock_bh(&vvs->rx_lock);
> > > >     if (free_pkt)
> > > >             kfree_skb(skb);
> > > > +
> > > > +   return can_enqueue;
> > > > }
> > > >
> > > > static int
> > > > @@ -1420,7 +1420,16 @@ virtio_transport_recv_connected(struct sock *sk,
> > > >
> > > >     switch (le16_to_cpu(hdr->op)) {
> > > >     case VIRTIO_VSOCK_OP_RW:
> > > > -           virtio_transport_recv_enqueue(vsk, skb);
> > > > +           if (!virtio_transport_recv_enqueue(vsk, skb)) {
> > > > +                   /* There is no more space to queue the packet, so let's
> > > > +                    * close the connection; otherwise, we'll lose data.
> > > > +                    */
> > > > +                   (void)virtio_transport_reset(vsk, skb);
> > > > +                   sk->sk_state = TCP_CLOSE;
> > > > +                   sk->sk_err = ENOBUFS;
> > > > +                   sk_error_report(sk);
> > >
> > > sashiko reported some issues related to setting TCP_CLOSE state and not
> > > removing the socket from the connect table:
> > > https://sashiko.dev/#/patchset/20260513105417.56761-1-sgarzare%40redhat.com
> > >
> > > I'll change this by calling virtio_transport_do_close() and
> > > vsock_remove_sock() in the next version.
> > >
> > > Stefano
> > >
> > > > +                   break;
> > > > +           }
> > > >             vsock_data_ready(sk);
> > > >             return err;
> > > >     case VIRTIO_VSOCK_OP_CREDIT_REQUEST:
> > > > --
> > > > 2.54.0
> > > >
> >
> >
> > And so the bag of hacks grows. I feel this is energy not well spent.
> > Please, let us fix this properly *first*. And then worry about how to
> > backport.  Maybe it will not be so terrible to backport after all.
> >
> 
> TBH I don't think this is an hack, but an issue we should fix in any case.
> Regarding the second patch, I see your point, but it's a big change
> that worries me. I'd like some more time to fix it properly without
> rushing. Staying calm without realizing that userspace is broken like
> we are now without this series :-(
> 
> That said, evaluating further, I think we have a similar issue also
> with STREAM on the host side where the skb usually doesn't free space,
> so we need a merge strategy also there.
> 
> So, I'd like to have time to fix both definitely. If you have time and
> want to go ahead, please do.
> 
> Thanks,
> Stefano

Well my patch was a start, we just need a strategy how to avoid copying
everything, right?

-- 
MST