Subject: Re: [PATCH net-next v2] virtio-net: fix possible unsigned integer overflow

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 2 Feb 2023 03:07:49 -0500

On Tue, Jan 31, 2023 at 04:50:04PM +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]>
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---
> v1->v2:
> - Change the type of num_buf from unsigned int to int. @Michael S . Tsirkin
> - Some cleaner codes. @Michael S . Tsirkin
> 
>  drivers/net/virtio_net.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index aaa6fe9b214a..8102861785a2 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -716,7 +716,7 @@ static unsigned int virtnet_get_headroom(struct virtnet_info *vi)
>   * have enough headroom.
>   */
>  static struct page *xdp_linearize_page(struct receive_queue *rq,
> -				       u16 *num_buf,
> +				       int *num_buf,
>  				       struct page *p,
>  				       int offset,
>  				       int page_off,
> @@ -816,7 +816,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
>  		if (unlikely(xdp_headroom < virtnet_get_headroom(vi))) {
>  			int offset = buf - page_address(page) + header_offset;
>  			unsigned int tlen = len + vi->hdr_len;
> -			u16 num_buf = 1;
> +			int num_buf = 1;
>  
>  			xdp_headroom = virtnet_get_headroom(vi);
>  			header_offset = VIRTNET_RX_PAD + xdp_headroom;
> @@ -989,7 +989,7 @@ static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
>  				      void *buf,
>  				      unsigned int len,
>  				      unsigned int frame_sz,
> -				      u16 *num_buf,
> +				      int *num_buf,
>  				      unsigned int *xdp_frags_truesize,
>  				      struct virtnet_rq_stats *stats)
>  {
> @@ -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

Ouch. Why is this here? Merged so pls remove by a follow up patch, the
rest of the code handles 0 fine. I'm not sure this introduces a bug by
we don't want spaghetti code.

> @@ -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;
>  
> -	while ((--*num_buf) >= 1) {
> +	while (--*num_buf > 0) {
>  		buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx);
>  		if (unlikely(!buf)) {
>  			pr_debug("%s: rx error: %d buffers out of %d missing\n",
> @@ -1076,7 +1079,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>  					 struct virtnet_rq_stats *stats)
>  {
>  	struct virtio_net_hdr_mrg_rxbuf *hdr = buf;
> -	u16 num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers);
> +	int num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers);
>  	struct page *page = virt_to_head_page(buf);
>  	int offset = buf - page_address(page);
>  	struct sk_buff *head_skb, *curr_skb;
> -- 
> 2.19.1.6.gb485710b