netdev - Re: net: Fix skb csum races when peeking

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Message-ID: <1436854313.1374.5.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 14 Jul 2015 08:11:53 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Eric Dumazet <edumazet@...gle.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: net: Fix skb csum races when peeking

On Mon, 2015-07-13 at 20:01 +0800, Herbert Xu wrote:

> ---8<---
> When we calculate the checksum on the recv path, we store the
> result in the skb as an optimisation in case we need the checksum
> again down the line.
> 
> This is in fact bogus for the MSG_PEEK case as this is done without
> any locking.  So multiple threads can peek and then store the result
> to the same skb, potentially resulting in bogus skb states.
> 
> This patch fixes this by only storing the result if the skb is not
> shared.  This preserves the optimisations for the few cases where
> it can be done safely due to locking or other reasons, e.g., SIOCINQ.
> 
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
> 
> diff --git a/net/core/datagram.c b/net/core/datagram.c
> index b80fb91..4967262 100644
> --- a/net/core/datagram.c
> +++ b/net/core/datagram.c
> @@ -622,7 +657,8 @@ __sum16 __skb_checksum_complete_head(struct sk_buff *skb, int len)
>  		    !skb->csum_complete_sw)
>  			netdev_rx_csum_fault(skb->dev);
>  	}
> -	skb->csum_valid = !sum;
> +	if (!skb_shared(skb))
> +		skb->csum_valid = !sum;
>  	return sum;
>  }
>  EXPORT_SYMBOL(__skb_checksum_complete_head);
> @@ -642,11 +678,13 @@ __sum16 __skb_checksum_complete(struct sk_buff *skb)
>  			netdev_rx_csum_fault(skb->dev);
>  	}
>  
> -	/* Save full packet checksum */
> -	skb->csum = csum;
> -	skb->ip_summed = CHECKSUM_COMPLETE;
> -	skb->csum_complete_sw = 1;
> -	skb->csum_valid = !sum;
> +	if (!skb_shared(skb)) {
> +		/* Save full packet checksum */
> +		skb->csum = csum;
> +		skb->ip_summed = CHECKSUM_COMPLETE;
> +		skb->csum_complete_sw = 1;
> +		skb->csum_valid = !sum;
> +	}
>  
>  	return sum;
>  }

Acked-by: Eric Dumazet <edumazet@...gle.com>

Thanks !


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