[Qemu-devel] [PATCH 2/2] virtio-net: check that tx packet has positive size

[Oleinik, Alexander]

virtio_net_flush_tx does not check that the packet size is nonzero,
which causes q->aysnc_tx.elem to be set. Then, when the device is reset, there
is an assertion failure since q->aysnc_tx.elem must be flushed/cleared.
Zero-sized packets are unsupported - check packet size, prior to
sending.

Found while fuzzing virtio-net. The relevant stack-trace:
 #8 in __assert_fail (libc.so.6+0x30101)
 #9 in virtio_net_reset virtio-net.c:520:13
 #10 in virtio_reset virtio.c:1214:9
 #11 in virtio_pci_reset virtio-pci.c:1810:5
 #12 in qdev_reset_one qdev.c:259:5
 #13 in qdev_walk_children qdev.c:575:15
 #14 in qbus_walk_children bus.c:52:15
 #15 in qdev_walk_children qdev.c:567:15
 #16 in qbus_walk_children bus.c:52:15
 #17 in qemu_devices_reset reset.c:69:9
 #18 in pc_machine_reset pc.c:2628:5
 #19 in qemu_system_reset vl.c:1621:9
 #20 in LLVMFuzzerTestOneInput fuzz.c:184:4

Signed-off-by: Alexander Oleinik <address@hidden>
---
 hw/net/virtio-net.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index b9e1cd71cf..743f1c827c 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -2074,12 +2074,15 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
             out_sg = sg;
         }
 
-        ret = qemu_sendv_packet_async(qemu_get_subqueue(n->nic, queue_index),
-                                      out_sg, out_num, virtio_net_tx_complete);
-        if (ret == 0) {
-            virtio_queue_set_notification(q->tx_vq, 0);
-            q->async_tx.elem = elem;
-            return -EBUSY;
+        /* Do not try to send 0-sized packets */
+        if (iov_size(out_sg, out_num)) {
+            ret = qemu_sendv_packet_async(qemu_get_subqueue(n->nic,
+                        queue_index), out_sg, out_num, virtio_net_tx_complete);
+            if (ret == 0) {
+                virtio_queue_set_notification(q->tx_vq, 0);
+                q->async_tx.elem = elem;
+                return -EBUSY;
+            }
         }
 
 drop:
-- 
2.20.1