linux-kernel - Re: [PATCH 1/1 v2] skbuff: Fix a potential race while recycling page

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Date:   Thu, 15 Jul 2021 12:00:57 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        <netdev@...r.kernel.org>
CC:     Alexander Duyck <alexanderduyck@...com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexander Lobakin <alobakin@...me>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Guillaume Nault <gnault@...hat.com>,
        Cong Wang <cong.wang@...edance.com>,
        "Jesper Dangaard Brouer" <brouer@...hat.com>,
        Matteo Croce <mcroce@...rosoft.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1 v2] skbuff: Fix a potential race while recycling
 page_pool packets

On 2021/7/9 14:29, Ilias Apalodimas wrote:
> As Alexander points out, when we are trying to recycle a cloned/expanded
> SKB we might trigger a race.  The recycling code relies on the
> pp_recycle bit to trigger,  which we carry over to cloned SKBs.
> If that cloned SKB gets expanded or if we get references to the frags,
> call skbb_release_data() and overwrite skb->head, we are creating separate
> instances accessing the same page frags.  Since the skb_release_data()
> will first try to recycle the frags,  there's a potential race between
> the original and cloned SKB, since both will have the pp_recycle bit set.
> 
> Fix this by explicitly those SKBs not recyclable.
> The atomic_sub_return effectively limits us to a single release case,
> and when we are calling skb_release_data we are also releasing the
> option to perform the recycling, or releasing the pages from the page pool.
> 
> Fixes: 6a5bcd84e886 ("page_pool: Allow drivers to hint on SKB recycling")
> Reported-by: Alexander Duyck <alexanderduyck@...com>
> Suggested-by: Alexander Duyck <alexanderduyck@...com>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@...aro.org>
> ---
> Changes since v1:
> - Set the recycle bit to 0 during skb_release_data instead of the 
>   individual fucntions triggering the issue, in order to catch all 
>   cases
>  net/core/skbuff.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 12aabcda6db2..f91f09a824be 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -663,7 +663,7 @@ static void skb_release_data(struct sk_buff *skb)
>  	if (skb->cloned &&
>  	    atomic_sub_return(skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1,
>  			      &shinfo->dataref))
> -		return;
> +		goto exit;

Is it possible this patch may break the head frag page for the original skb,
supposing it's head frag page is from the page pool and below change clears
the pp_recycle for original skb, causing a page leaking for the page pool?

>  
>  	skb_zcopy_clear(skb, true);
>  
> @@ -674,6 +674,8 @@ static void skb_release_data(struct sk_buff *skb)
>  		kfree_skb_list(shinfo->frag_list);
>  
>  	skb_free_head(skb);
> +exit:
> +	skb->pp_recycle = 0;
>  }
>  
>  /*
>