Subject: Re: [PATCH net-next 4/6] xdp: add multi-buff support to xdp_return_{buff/frame}

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 20 Aug 2020 09:52:22 +0200

On Wed, 19 Aug 2020 15:13:49 +0200
Lorenzo Bianconi <[email-masked]> wrote:

> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 884f140fc3be..006b24b5d276 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -370,19 +370,55 @@ static void __xdp_return(void *data, struct xdp_mem_info *mem, bool napi_direct)
>  
>  void xdp_return_frame(struct xdp_frame *xdpf)
>  {
> +	struct skb_shared_info *sinfo;
> +	int i;
> +
>  	__xdp_return(xdpf->data, &xdpf->mem, false);

There is a use-after-free race here.  The xdpf->data contains the
shared_info (xdp_get_shared_info_from_frame(xdpf)). Thus you cannot
free/return the page and use this data area below.

> +	if (!xdpf->mb)
> +		return;
> +
> +	sinfo = xdp_get_shared_info_from_frame(xdpf);
> +	for (i = 0; i < sinfo->nr_frags; i++) {
> +		struct page *page = skb_frag_page(&sinfo->frags[i]);
> +
> +		__xdp_return(page_address(page), &xdpf->mem, false);
> +	}
>  }
>  EXPORT_SYMBOL_GPL(xdp_return_frame);
>  
>  void xdp_return_frame_rx_napi(struct xdp_frame *xdpf)
>  {
> +	struct skb_shared_info *sinfo;
> +	int i;
> +
>  	__xdp_return(xdpf->data, &xdpf->mem, true);

Same issue.

> +	if (!xdpf->mb)
> +		return;
> +
> +	sinfo = xdp_get_shared_info_from_frame(xdpf);
> +	for (i = 0; i < sinfo->nr_frags; i++) {
> +		struct page *page = skb_frag_page(&sinfo->frags[i]);
> +
> +		__xdp_return(page_address(page), &xdpf->mem, true);
> +	}
>  }
>  EXPORT_SYMBOL_GPL(xdp_return_frame_rx_napi);
>  
>  void xdp_return_buff(struct xdp_buff *xdp)
>  {
> +	struct skb_shared_info *sinfo;
> +	int i;
> +
>  	__xdp_return(xdp->data, &xdp->rxq->mem, true);

Same issue.

> +	if (!xdp->mb)
> +		return;
> +
> +	sinfo = xdp_get_shared_info_from_buff(xdp);
> +	for (i = 0; i < sinfo->nr_frags; i++) {
> +		struct page *page = skb_frag_page(&sinfo->frags[i]);
> +
> +		__xdp_return(page_address(page), &xdp->rxq->mem, true);
> +	}
>  }

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer