linux-kernel - Re: [PATCH] vsock: initialize the_virtio

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Date:   Fri, 20 Oct 2023 00:12:04 +0300
From:   Alexandru Matei <alexandru.matei@...ath.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Mihai Petrisor <mihai.petrisor@...ath.com>,
        Viorel Canja <viorel.canja@...ath.com>
Subject: Re: [PATCH] vsock: initialize the_virtio_vsock before using VQs

On 10/19/2023 11:54 AM, Stefano Garzarella wrote:
> On Wed, Oct 18, 2023 at 09:32:47PM +0300, Alexandru Matei wrote:
>> Once VQs are filled with empty buffers and we kick the host, it can send
>> connection requests. If 'the_virtio_vsock' is not initialized before,
>> replies are silently dropped and do not reach the host.
> 
> Are replies really dropped or we just miss the notification?
> 
> Could the reverse now happen, i.e., the guest wants to send a connection request, finds the pointer assigned but can't use virtqueues because they haven't been initialized yet?
> 
> Perhaps to avoid your problem, we could just queue vsock->rx_work at the bottom of the probe to see if anything was queued in the meantime.
> 
> Nit: please use "vsock/virtio" to point out that this problem is of the virtio transport.
> 
> Thanks,
> Stefano

The replies are dropped , the scenario goes like this:

  Once rx_run is set to true and rx queue is filled with empty buffers, the host sends a connection request.
  The request is processed in virtio_transport_recv_pkt(), and since there is no bound socket, it calls virtio_transport_reset_no_sock() which tries to send a reset packet. 
  In virtio_transport_send_pkt() it checks 'the_virtio_vsock' and because it is null it exits with -ENODEV, basically dropping the packet.

I looked on your scenario and there is an issue from the moment we set the_virtio_vsock (in this patch) up until vsock->tx_run is set to TRUE. 
virtio_transport_send_pkt() will queue the packet, but virtio_transport_send_pkt_work() will exit because tx_run is FALSE. This could be fixed by moving rcu_assign_pointer() after tx_run is set to TRUE.
virtio_transport_cancel_pkt() uses the rx virtqueue once the_virtio_vsock is set, so rcu_assign_pointer() should be moved after virtio_find_vqs() is called.

I think the way to go is to split virtio_vsock_vqs_init() in two: virtio_vsock_vqs_init() and virtio_vsock_vqs_fill(), as Vadim suggested. This should fix all the cases:

---
 net/vmw_vsock/virtio_transport.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index ad64f403536a..1f95f98ddd3f 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -594,6 +594,11 @@ static int virtio_vsock_vqs_init(struct virtio_vsock *vsock)
 	vsock->tx_run = true;
 	mutex_unlock(&vsock->tx_lock);
 
+	return 0;
+}
+
+static void virtio_vsock_vqs_fill(struct virtio_vsock *vsock)
+{
 	mutex_lock(&vsock->rx_lock);
 	virtio_vsock_rx_fill(vsock);
 	vsock->rx_run = true;
@@ -603,8 +608,6 @@ static int virtio_vsock_vqs_init(struct virtio_vsock *vsock)
 	virtio_vsock_event_fill(vsock);
 	vsock->event_run = true;
 	mutex_unlock(&vsock->event_lock);
-
-	return 0;
 }
 
 static void virtio_vsock_vqs_del(struct virtio_vsock *vsock)
@@ -707,6 +710,7 @@ static int virtio_vsock_probe(struct virtio_device *vdev)
 		goto out;
 
 	rcu_assign_pointer(the_virtio_vsock, vsock);
+	virtio_vsock_vqs_fill(vsock);
 
 	mutex_unlock(&the_virtio_vsock_mutex);
 
@@ -779,6 +783,7 @@ static int virtio_vsock_restore(struct virtio_device *vdev)
 		goto out;
 
 	rcu_assign_pointer(the_virtio_vsock, vsock);
+	virtio_vsock_vqs_fill(vsock);
 
 out:
 	mutex_unlock(&the_virtio_vsock_mutex);
--