[Qemu-devel] [PATCH v2] virtio-net: Always check for guest header length

[Oleinik, Alexander]

While fuzzing the virtio-net tx vq, I ran into an assertion failure due
to iov_copy offsets larger than the total iov size. Though there is
a check to cover this, it does not execute when !n->has_vnet_hdr. This
change always copies the guest header into the mhdr buffer and checks its
length, even if mhdr is not needed.

The call stack for the assertion failure:

 #8 in __assert_fail (libc.so.6+0x300f1)
 #9 in iov_copy iov.c:266:5
 #10 in virtio_net_flush_tx virtio-net.c:2073:23
 #11 in virtio_net_tx_bh virtio-net.c:2197:11
 #12 in aio_bh_poll async.c:118:13
 #13 in aio_dispatch aio-posix.c:460:5
 #14 in aio_ctx_dispatch async.c:261:5
 #15 in g_main_context_dispatch (libglib-2.0.so.0+0x4df2d)
 #16 in glib_pollfds_poll main-loop.c:213:9
 #17 in os_host_main_loop_wait main-loop.c:236
 #18 in main_loop_wait main-loop.c:512
 #19 in virtio_net_tx_fuzz virtio-net-fuzz.c:160:3

Signed-off-by: Alexander Oleinik <alx...@bu.edu>
---
 hw/net/virtio-net.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index b9e1cd71cf..003436b53c 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -2035,14 +2035,19 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
             return -EINVAL;
         }
 
-        if (n->has_vnet_hdr) {
-            if (iov_to_buf(out_sg, out_num, 0, &mhdr, n->guest_hdr_len) <
+        /*
+         * Even if !n->has_vnet_hdr and we dont need mhdr, we still need this
+         * to check that out_sg contains at least guest_hdr_len bytes
+         */
+        if (iov_to_buf(out_sg, out_num, 0, &mhdr, 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;
-            }
+            virtio_error(vdev, "virtio-net header incorrect");
+            virtqueue_detach_element(q->tx_vq, elem, 0);
+            g_free(elem);
+            return -EINVAL;
+        }
+
+        if (n->has_vnet_hdr) {
             if (n->needs_vnet_hdr_swap) {
                 virtio_net_hdr_swap(vdev, (void *) &mhdr);
                 sg2[0].iov_base = &mhdr;
-- 
2.20.1