[PATCH 1/1] virtio-net: fix bug 1451 aka "assert(!virtio_net_get_subqueue(nc)->async_tx.elem); "

[Alexey Dobriyan]

Reproducer from https://gitlab.com/qemu-project/qemu/-/issues/1451
creates small packet (1 segment, len=10 == n->guest_hdr_len),
destroys queue.

"if (n->host_hdr_len != n->guest_hdr_len)" is triggered. There is
nothing after guest header, if body creates zero length/zero segment packet.

qemu_sendv_packet_async() tries to send it.

Packet is discarded inside net_slirp_receive() which returns number of
sent bytes which is 0.

0 is propagated upwards and is interpreted as "zero bytes has been sent"
which is terrible because queue is being destroyed, nobody is waiting
for TX to complete, TX complete hook is _not_ called and
assert(async_tx.elem) is eventually triggered.

The fix is to discard such phantom packets in virtio_net_flush_tx().

Signed-off-by: Alexey Dobriyan <adobriyan@yandex-team.ru>
---
 hw/net/virtio-net.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 58014a92ad..e156f1d78c 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -2765,18 +2765,14 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
         out_sg = elem->out_sg;
         if (out_num < 1) {
             virtio_error(vdev, "virtio-net header not in first element");
-            virtqueue_detach_element(q->tx_vq, elem, 0);
-            g_free(elem);
-            return -EINVAL;
+            goto detach;
         }
 
         if (n->has_vnet_hdr) {
             if (iov_to_buf(out_sg, out_num, 0, &vhdr, n->guest_hdr_len) <
                 n->guest_hdr_len) {
                 virtio_error(vdev, "virtio-net header incorrect");
-                virtqueue_detach_element(q->tx_vq, elem, 0);
-                g_free(elem);
-                return -EINVAL;
+                goto detach;
             }
             if (n->needs_vnet_hdr_swap) {
                 virtio_net_hdr_swap(vdev, (void *) &vhdr);
@@ -2807,6 +2803,11 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
                              n->guest_hdr_len, -1);
             out_num = sg_num;
             out_sg = sg;
+
+            if (out_num < 1) {
+                virtio_error(vdev, "virtio-net nothing to send");
+                goto detach;
+            }
         }
 
         ret = qemu_sendv_packet_async(qemu_get_subqueue(n->nic, queue_index),
@@ -2827,6 +2828,11 @@ drop:
         }
     }
     return num_packets;
+
+detach:
+    virtqueue_detach_element(q->tx_vq, elem, 0);
+    g_free(elem);
+    return -EINVAL;
 }
 
 static void virtio_net_tx_timer(void *opaque);
-- 
2.43.0