Re: [PATCH net] netfilter: nft_synproxy: stop bypassing the priv->info snapshot

[Pablo Neira Ayuso]

Hi,

On Thu, Jun 11, 2026 at 12:21:20PM +0800, Runyu Xiao wrote:
> nft_synproxy_eval_v4() and nft_synproxy_eval_v6() already take a
> whole-object READ_ONCE() snapshot of the shared priv->info state before
> building the SYNACK reply, but nft_synproxy_tcp_options() still masks
> opts->options with priv->info.options from the live shared object.
> 
> When a named synproxy object is updated concurrently with SYN traffic,
> the eval path can then mix mss and timestamp handling from the local
> snapshot with an options mask taken from a newer configuration, so one
> SYNACK no longer reflects a coherent synproxy configuration.
> 
> Use info->options so nft_synproxy_tcp_options() stays on the same local
> snapshot that the eval path already copied from priv->info.

I'll incorporate this to nf-next, this is good for correctness, but no
crash is associated.

> Fixes: ee394f96ad75 ("netfilter: nft_synproxy: add synproxy stateful object support")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Runyu Xiao <runyu.xiao@xxxxxxxxxx>
> ---
>  net/netfilter/nft_synproxy.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/net/netfilter/nft_synproxy.c b/net/netfilter/nft_synproxy.c
> index 7641f249614c..9ed288c9d168 100644
> --- a/net/netfilter/nft_synproxy.c
> +++ b/net/netfilter/nft_synproxy.c
> @@ -24,14 +24,13 @@ static const struct nla_policy nft_synproxy_policy[NFTA_SYNPROXY_MAX + 1] = {
>  static void nft_synproxy_tcp_options(struct synproxy_options *opts,
>  				     const struct tcphdr *tcp,
>  				     struct synproxy_net *snet,
> -				     struct nf_synproxy_info *info,
> -				     const struct nft_synproxy *priv)
> +				     struct nf_synproxy_info *info)
>  {
>  	this_cpu_inc(snet->stats->syn_received);
>  	if (tcp->ece && tcp->cwr)
>  		opts->options |= NF_SYNPROXY_OPT_ECN;
>  
> -	opts->options &= priv->info.options;
> +	opts->options &= info->options;
>  	opts->mss_encode = opts->mss_option;
>  	opts->mss_option = info->mss;
>  	if (opts->options & NF_SYNPROXY_OPT_TIMESTAMP)
> @@ -56,7 +55,7 @@ static void nft_synproxy_eval_v4(const struct nft_synproxy *priv,
>  
>  	if (tcp->syn) {
>  		/* Initial SYN from client */
> -		nft_synproxy_tcp_options(opts, tcp, snet, &info, priv);
> +		nft_synproxy_tcp_options(opts, tcp, snet, &info);
>  		synproxy_send_client_synack(net, skb, tcp, opts);
>  		consume_skb(skb);
>  		regs->verdict.code = NF_STOLEN;
> @@ -87,7 +86,7 @@ static void nft_synproxy_eval_v6(const struct nft_synproxy *priv,
>  
>  	if (tcp->syn) {
>  		/* Initial SYN from client */
> -		nft_synproxy_tcp_options(opts, tcp, snet, &info, priv);
> +		nft_synproxy_tcp_options(opts, tcp, snet, &info);
>  		synproxy_send_client_synack_ipv6(net, skb, tcp, opts);
>  		consume_skb(skb);
>  		regs->verdict.code = NF_STOLEN;
> -- 
> 2.34.1