Subject: Re: [PATCH net v2] net: neigh: Reallocate headroom if necessary in neigh_hh_bridge()

From: Ido Schimmel <[email-masked]>
Date: Wed, 13 May 2026 18:48:55 +0300

On Mon, May 11, 2026 at 05:57:39PM +0200, Lorenzo Bianconi wrote:
> diff --git a/include/net/neighbour.h b/include/net/neighbour.h
> index 2dfee6d4258a..c2b6196705ef 100644
> --- a/include/net/neighbour.h
> +++ b/include/net/neighbour.h
> @@ -487,16 +487,24 @@ static inline int neigh_event_send(struct neighbour *neigh, struct sk_buff *skb)
>  }
>  
>  #if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
> -static inline int neigh_hh_bridge(struct hh_cache *hh, struct sk_buff *skb)
> +static inline struct sk_buff *
> +neigh_hh_bridge(struct hh_cache *hh, struct sk_buff *skb)
>  {
> -	unsigned int seq, hh_alen;
> +	unsigned int seq, hh_alen = HH_DATA_ALIGN(ETH_HLEN);
> +
> +	if (unlikely(skb_headroom(skb) < hh_alen ||
> +		     skb_header_cloned(skb) || skb_shared(skb))) {
> +		skb = skb_expand_head(skb, hh_alen);

I don't think this is correct... The comment above skb_expand_head()
says that it will generate a warning if there is sufficient headroom in
the packet.

I assumed that you would just call skb_cow_head() like the AI review
suggested. There's skb_share_check() in br_handle_frame(), so no need to
worry about the skb being shared.

> +		if (!skb)
> +			return NULL;
> +	}
>  
>  	do {
>  		seq = read_seqbegin(&hh->hh_lock);
> -		hh_alen = HH_DATA_ALIGN(ETH_HLEN);
>  		memcpy(skb->data - hh_alen, hh->hh_data, ETH_ALEN + hh_alen - ETH_HLEN);
>  	} while (read_seqretry(&hh->hh_lock, seq));
> -	return 0;
> +
> +	return skb;
>  }
>  #endif
>  
> diff --git a/net/bridge/br_netfilter_hooks.c b/net/bridge/br_netfilter_hooks.c
> index 0ab1c94db4b9..8d21c88c2116 100644
> --- a/net/bridge/br_netfilter_hooks.c
> +++ b/net/bridge/br_netfilter_hooks.c
> @@ -297,7 +297,12 @@ int br_nf_pre_routing_finish_bridge(struct net *net, struct sock *sk, struct sk_
>  				goto free_skb;
>  			}
>  
> -			neigh_hh_bridge(&neigh->hh, skb);
> +			skb = neigh_hh_bridge(&neigh->hh, skb);
> +			if (!skb) {
> +				neigh_release(neigh);
> +				return -ENOMEM;
> +			}

This part looks correct.

> +
>  			skb->dev = br_indev;
>  
>  			ret = br_handle_frame_finish(net, sk, skb);
> 
> ---
> base-commit: a450063ef86b9967234ca1f896c0d77400c74f11
> change-id: 20260508-nf-neigh_hh_bridge-fix-9ab775ee23c6
> 
> Best regards,
> -- 
> Lorenzo Bianconi <[email-masked]>
>