Re: [Patch net] net: make pskb_trim_rcsum_slow() robust

[Eric Dumazet <eric.dumazet@gmail.com>]

On 10/29/2018 05:35 PM, Cong Wang wrote:
> Most callers of pskb_trim_rcsum() simply drops the skb when
> it fails, however, ip_check_defrag() still continues to pass
> the skb up to stack. In that case, we should restore its previous
> csum if __pskb_trim() fails.
> 
> Found this during code review.
> 
> Fixes: 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends")
> Cc: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
> ---
>  net/core/skbuff.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 946de0e24c87..5decd6e6d2b6 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -1843,6 +1843,9 @@ EXPORT_SYMBOL(___pskb_trim);
>   */
>  int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len)
>  {
> +	__wsum old_csum = skb->csum;
> +	int ret;
> +
>  	if (skb->ip_summed == CHECKSUM_COMPLETE) {
>  		int delta = skb->len - len;
>  
> @@ -1850,7 +1853,10 @@ int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len)
>  					   skb_checksum(skb, len, delta, 0),
>  					   len);
>  	}
> -	return __pskb_trim(skb, len);
> +	ret = __pskb_trim(skb, len);
> +	if (unlikely(ret))
> +		skb->csum = old_csum;

Would not it be simpler to set ip_summed to CHECKSUM_NONE (no need to save old_csum) ?

> +	return ret;
>  }
>  EXPORT_SYMBOL(pskb_trim_rcsum_slow);
>  
>