net_util: fix ctrl_queue used_len to only count written bytes

The control queue handler passed the total length of all
descriptors (header + data + status) as used_len to add_used.
Per virtio spec section 2.6.8, used_len must only count bytes
written to device-writable descriptors. The device only writes
the 1-byte status/ack field.

Windows NetKVM >= 0.1.285 strictly checks this value and calls
NdisMRemoveMiniport when len != sizeof(virtio_net_ctrl_ack),
removing the network adapter immediately after activation.

Signed-off-by: CMGS <ilskdw@gmail.com>

Author: CMGS

net_util/src/ctrl_queue.rs

@@ -171,7 +171,9 @@ impl CtrlQueue {
                         .translate_gva(access_platform, status_desc.len() as usize),
                 )
                 .map_err(Error::GuestMemory)?;
-            let len = ctrl_desc.len() + data_desc.len() + status_desc.len();
+            // Per virtio spec 2.6.8, used_len is the number of bytes written
+            // to device-writable descriptors. Only the status byte is written.
+            let len = status_desc.len();
 
             queue
                 .add_used(desc_chain.memory(), desc_chain.head_index(), len)