linux-kernel - Re: [PATCH net] net: bridge: fix corrupted ethernet header on multicast-to-unicast

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Message-ID: <9a1825ff-3a55-4c2a-86bc-0709d4ff8153@blackwall.org>
Date: Tue, 7 May 2024 15:15:43 +0300
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Felix Fietkau <nbd@....name>, netdev@...r.kernel.org,
 Roopa Prabhu <roopa@...dia.com>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>
Cc: bridge@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: bridge: fix corrupted ethernet header on
 multicast-to-unicast

On 05/05/2024 21:42, Felix Fietkau wrote:
> The change from skb_copy to pskb_copy unfortunately changed the data
> copying to omit the ethernet header, since it was pulled before reaching
> this point. Fix this by calling __skb_push/pull around pskb_copy.
> 
> Fixes: 59c878cbcdd8 ("net: bridge: fix multicast-to-unicast with fraglist GSO")
> Signed-off-by: Felix Fietkau <nbd@....name>
> ---
>   net/bridge/br_forward.c | 9 +++++++--
>   1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/net/bridge/br_forward.c b/net/bridge/br_forward.c
> index d7c35f55bd69..d97064d460dc 100644
> --- a/net/bridge/br_forward.c
> +++ b/net/bridge/br_forward.c
> @@ -258,6 +258,7 @@ static void maybe_deliver_addr(struct net_bridge_port *p, struct sk_buff *skb,
>   {
>   	struct net_device *dev = BR_INPUT_SKB_CB(skb)->brdev;
>   	const unsigned char *src = eth_hdr(skb)->h_source;
> +	struct sk_buff *nskb;
>   
>   	if (!should_deliver(p, skb))
>   		return;
> @@ -266,12 +267,16 @@ static void maybe_deliver_addr(struct net_bridge_port *p, struct sk_buff *skb,
>   	if (skb->dev == p->dev && ether_addr_equal(src, addr))
>   		return;
>   
> -	skb = pskb_copy(skb, GFP_ATOMIC);
> -	if (!skb) {
> +	__skb_push(skb, ETH_HLEN);
> +	nskb = pskb_copy(skb, GFP_ATOMIC);
> +	__skb_pull(skb, ETH_HLEN);
> +	if (!nskb) {
>   		DEV_STATS_INC(dev, tx_dropped);
>   		return;
>   	}
>   
> +	skb = nskb;
> +	__skb_pull(skb, ETH_HLEN);
>   	if (!is_broadcast_ether_addr(addr))
>   		memcpy(eth_hdr(skb)->h_dest, addr, ETH_ALEN);
>   

This dance is getting ugly, but better to have it correct.
It'd be nice if you could add a selftest that exercises it.

Acked-by: Nikolay Aleksandrov <razor@...ckwall.org>