Re: [Qemu-devel] [PATCH 5/5] virtio-net: unbreak any layout

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 13, 2015 at 01:46:51PM +0800, Jason Wang wrote:
> Commit 032a74a1c0fcdd5fd1c69e56126b4c857ee36611
> ("virtio-net: byteswap virtio-net header") breaks any layout by
> requiring out_sg[0].iov_len >= n->guest_hdr_len. Fixing this by
> copying header to temporary buffer and copying it back after byteswap.
> 
> Fixes 032a74a1c0fcdd5fd1c69e56126b4c857ee36611
> ("virtio-net: byteswap virtio-net header")
> Cc: qemu-stable@nongnu.org
> Cc: clg@fr.ibm.com
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  hw/net/virtio-net.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index e3c2db3..b42af8f 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -1139,10 +1139,11 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
>      }
>  
>      while (virtqueue_pop(q->tx_vq, &elem)) {
> -        ssize_t ret, len;
> +        ssize_t ret, len, s;
>          unsigned int out_num = elem.out_num;
>          struct iovec *out_sg = &elem.out_sg[0];
>          struct iovec sg[VIRTQUEUE_MAX_SIZE];
> +        struct virtio_net_hdr hdr;
>  
>          if (out_num < 1) {
>              error_report("virtio-net header not in first element");
> @@ -1150,11 +1151,14 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
>          }
>  
>          if (n->has_vnet_hdr) {
> -            if (out_sg[0].iov_len < n->guest_hdr_len) {
> +            s = iov_to_buf(out_sg, out_num, 0, &hdr, sizeof(hdr));
> +            if (s != sizeof(hdr)) {
>                  error_report("virtio-net header incorrect");
>                  exit(1);
>              }
> -            virtio_net_hdr_swap(vdev, (void *) out_sg[0].iov_base);
> +            virtio_net_hdr_swap(vdev, (void *) &hdr);
> +            s = iov_from_buf(out_sg, out_num, 0, &hdr, sizeof(hdr));
> +            assert(s == sizeof(hdr));
>          }
>  
>          /*

Looks like this will write into out_sg - that's violating the virtio spec.

> -- 
> 2.1.4