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

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Date:   Fri, 20 Oct 2023 09:23:31 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Alexandru Matei <alexandru.matei@...ath.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 Fri, Oct 20, 2023 at 12:12:04AM +0300, Alexandru Matei wrote:
>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.

Oh, I see now, I thought virtio_transport_rx_work() returned early if 
'the_virtio_vsock' was not set.

>  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:

Yep, LGTM!

Thank you both for the fix, please send a v2 with this approach!

Stefano

>
>---
> 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);
>-- 
>