Re: [PATCH net-next] virtio-net: fix possible unsigned integer overflow

Michael S. Tsirkin
2023-01-31T07:21:50Z

On Tue, Jan 31, 2023 at 11:43:37AM +0800, Heng Qi wrote:
> When the single-buffer xdp is loaded and after xdp_linearize_page()
> is called, *num_buf becomes 0 and (*num_buf - 1) may overflow into
> a large integer in virtnet_build_xdp_buff_mrg(), resulting in
> unexpected packet dropping.
>
> Fixes: ef75cb51f139 ("virtio-net: build xdp_buff with multi buffers")
> Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>

Given the confusion, just make num_buf an int?

> ---
> drivers/net/virtio_net.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index aaa6fe9b214a..a8e9462903fa 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1007,6 +1007,9 @@ static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
> xdp_prepare_buff(xdp, buf - VIRTIO_XDP_HEADROOM,
> VIRTIO_XDP_HEADROOM + vi->hdr_len, len - vi->hdr_len, true);
>
> + if (!*num_buf)
> + return 0;
> +
> if (*num_buf > 1) {
> /* If we want to build multi-buffer xdp, we need
> * to specify that the flags of xdp_buff have the

This means truesize won't be set.

> @@ -1020,10 +1023,10 @@ static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
> shinfo->xdp_frags_size = 0;
> }
>
> - if ((*num_buf - 1) > MAX_SKB_FRAGS)
> + if (*num_buf > MAX_SKB_FRAGS + 1)
> return -EINVAL;

Admittedly this is cleaner.

>
> - while ((--*num_buf) >= 1) {
> + while (--*num_buf) {

A bit more fragile, > 0 would be better.

> buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx);
> if (unlikely(!buf)) {
> pr_debug("%s: rx error: %d buffers out of %d missing\n",
> --
> 2.19.1.6.gb485710b