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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 31 Jan 2023 02:20:49 -0500

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 <[email-masked]>
> Reviewed-by: Xuan Zhuo <[email-masked]>

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