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

From: Lorenzo Bianconi <[email-masked]>
Date: Thu, 20 Aug 2020 09:56:05 +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.

right, thx for pointing this out. I will fix it in v2.

Regards,
Lorenzo

> 
> > +	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
>