linux-kernel - Re: [PATCH net] net: gro: set the last skb->next to NULL when it get merged

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Date:   Wed, 27 Oct 2021 12:20:04 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     kerneljasonxing@...il.com, davem@...emloft.net, kuba@...nel.org,
        alobakin@...me, jonathan.lemon@...il.com, willemb@...gle.com,
        pabeni@...hat.com, vvs@...tuozzo.com, cong.wang@...edance.com
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jason Xing <xingwanli@...ishou.com>
Subject: Re: [PATCH net] net: gro: set the last skb->next to NULL when it get
 merged



On 10/26/21 6:18 AM, kerneljasonxing@...il.com wrote:
> From: Jason Xing <xingwanli@...ishou.com>
> 
> Setting the @next of the last skb to NULL to prevent the panic in future
> when someone does something to the last of the gro list but its @next is
> invalid.
> 
> For example, without the fix (commit: ece23711dd95), a panic could happen
> with the clsact loaded when skb is redirected and then validated in
> validate_xmit_skb_list() which could access the error addr of the @next
> of the last skb. Thus, "general protection fault" would appear after that.
> 

If this a bug, please provide a Fixes: tag

> Signed-off-by: Jason Xing <xingwanli@...ishou.com>
> ---
>  net/core/skbuff.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 2170bea..7b248f1 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -4396,6 +4396,7 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
>  		skb_shinfo(p)->frag_list = skb;
>  	else
>  		NAPI_GRO_CB(p)->last->next = skb;
> +	skb->next = NULL;

Really at this point skb->next must be already NULL.

Please provide a stack trace so that we fix the caller instead
of adding more code in GRO layer.


>  	NAPI_GRO_CB(p)->last = skb;
>  	__skb_header_release(skb);
>  	lp = p;
>