The only way for guest to control offloads (as enabled by
VIRTIO_NET_F_CTRL_GUEST_OFFLOADS) is by sending commands
through CTRL_VQ. So it does not make sense to
acknowledge VIRTIO_NET_F_CTRL_GUEST_OFFLOADS without
VIRTIO_NET_F_CTRL_VQ.

The spec does not outlaw devices with such a configuration, so we have
to support it. Simply clear VIRTIO_NET_F_CTRL_GUEST_OFFLOADS.
Note that Linux is still crashing if it tries to
change the offloads when there's no control vq.
That needs to be fixed by another patch.

Reported-by: Alistair Delva <redacted>
Reported-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Fixes: 3f93522ffab2 ("virtio-net: switch off offloads on demand if possible on XDP set")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

Same patch as v1 but update documentation so it's clear it's not
enough to fix the crash.

  drivers/net/virtio_net.c | 9 +++++++++
  1 file changed, 9 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 4d7d5434cc5d..7b8805b47f0d 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -2971,6 +2971,15 @@ static int virtnet_validate(struct virtio_device *vdev)
  	if (!virtnet_validate_features(vdev))
  		return -EINVAL;
  
+	/* VIRTIO_NET_F_CTRL_GUEST_OFFLOADS does not work without
+	 * VIRTIO_NET_F_CTRL_VQ. Unfortunately spec forgot to
+	 * specify that VIRTIO_NET_F_CTRL_GUEST_OFFLOADS depends
+	 * on VIRTIO_NET_F_CTRL_VQ so devices can set the later but
+	 * not the former.
+	 */
+	if (!virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
+			__virtio_clear_bit(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS);