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

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

On Fri, Jul 19, 2019 at 03:21:15PM +0000, Oleinik, Alexander wrote:
> 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 <alxndr@bu.edu>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  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