A crash found while fuzzing device virtio-net-socket-check-used.
Assertion "offset == 0" in iov_copy() fails if less than guest_hdr_len bytes
were transmited.

Signed-off-by: Dmitry Frolov <frolov@swemel.ru>
---
v1: https://patchew.org/QEMU/20240527133140.218300-2-frolov@swemel.ru/
v2: replaced repeating code with goto
---
 hw/net/virtio-net.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 24e5e7d347..424bada8cf 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -2749,18 +2749,14 @@ 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 error;
         }
 
         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 error;
             }
             if (n->needs_vnet_hdr_swap) {
                 virtio_net_hdr_swap(vdev, (void *) &vhdr);
@@ -2783,6 +2779,10 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
          */
         assert(n->host_hdr_len <= n->guest_hdr_len);
         if (n->host_hdr_len != n->guest_hdr_len) {
+            if (iov_size(out_sg, out_num) < n->guest_hdr_len) {
+                virtio_error(vdev, "virtio-net header is invalid");
+                goto error;
+            }
             unsigned sg_num = iov_copy(sg, ARRAY_SIZE(sg),
                                        out_sg, out_num,
                                        0, n->host_hdr_len);
@@ -2811,6 +2811,11 @@ drop:
         }
     }
     return num_packets;
+
+error:
+    virtqueue_detach_element(q->tx_vq, elem, 0);
+    g_free(elem);
+    return -EINVAL;
 }
 
 static void virtio_net_tx_timer(void *opaque);
-- 
2.43.0