[PATCH] vhost/vsock: Refuse the connection immediately when guest isn't ready

[Polina Vishneva <polina.vishneva@virtuozzo.com>]

From: "Denis V. Lunev" <den@openvz.org>

When the host initiates an AF_VSOCK connect() to a guest that has not
yet loaded the virtio-vsock transport (i.e. still booting), the caller
blocks for VSOCK_DEFAULT_CONNECT_TIMEOUT (2 seconds), because
vhost_transport_do_send_pkt() silently exits when
vhost_vq_get_backend(vq) returns NULL.

If the guest doesn't start listening within this timeout, connect()
returns ETIMEDOUT.

This delay is usually pointless and it doesn't well align with our
behavior at other initialization stages: for example, if a connection is
attempted when the guest driver is already loaded, but when nothing is
listening yet, it returns ECONNRESET immediately without any wait.

Fix this by checking the RX virtqueue backend in
vhost_transport_send_pkt() before queuing. If the backend is NULL,
return -ECONNREFUSED immediately.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Co-developed-by: Polina Vishneva <polina.vishneva@virtuozzo.com>
Signed-off-by: Polina Vishneva <polina.vishneva@virtuozzo.com>
---
 drivers/vhost/vsock.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 1d8ec6bed53e..a3f218292c3a 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -302,6 +302,16 @@ vhost_transport_send_pkt(struct sk_buff *skb, struct net *net)
 		return -ENODEV;
 	}
 
+	/* Fast-fail if the guest hasn't enabled the RX vq yet. Reading
+	 * private_data without vq->mutex is deliberate: even if the backend becomes
+	 * NULL right after that check, do_send_pkt() checks it under the mutex.
+	 */
+	if (!data_race(READ_ONCE(vsock->vqs[VSOCK_VQ_RX].private_data))) {
+		rcu_read_unlock();
+		kfree_skb(skb);
+		return -ECONNREFUSED;
+	}
+
 	if (virtio_vsock_skb_reply(skb))
 		atomic_inc(&vsock->queued_replies);
 

base-commit: 8ab992f815d6736b5c7a6f5fd7bfe7bc106bb3dc
-- 
2.53.0