Re: [PATCH net v4] net: ipv4: add zero check for snd_cwnd_clamp

[Antoine Tenart]

On Sat, Oct 17, 2026 at 05:48:46PM -0400, Kaho Jiang wrote:
> User can pass zero to snd_cwnd_clamp via RTAX_CWND type.
> This could lead to division of zero error if not checked properly.
> 
> Oops: divide error: 0000 [#1] SMP KASAN NOPTI
> Call Trace:
>  <IRQ>
>  tcp_reno_cong_avoid (net/ipv4/tcp_cong.c:510 (discriminator 1))
>  tcp_ack (net/ipv4/tcp_input.c:3517 net/ipv4/tcp_input.c:3873 net/ipv4/tcp_input.c:4440)
>  tcp_rcv_established (net/ipv4/tcp_input.c:6685)
>  tcp_v4_do_rcv (net/ipv4/tcp_ipv4.c:1854)
>  tcp_v4_rcv (net/ipv4/tcp_ipv4.c:2248)
>  ip_protocol_deliver_rcu (net/ipv4/ip_input.c:207 (discriminator 1))
>  ip_local_deliver_finish (net/ipv4/ip_input.c:241 (discriminator 1))
>  ip_local_deliver (./include/linux/netfilter.h:325 ./include/linux/netfilter.h:319 net/ipv4/ip_input.c:262)
>  ip_rcv (./include/net/dst.h:481 (discriminator 6) net/ipv4/ip_input.c:492 (discriminator 6) ./include/linux/netfilter.h:325 (discriminator 6) ./include/li)
>  __netif_receive_skb_one_core (net/core/dev.c:6297 (discriminator 4))
>  __netif_receive_skb (net/core/dev.c:6410)
>  process_backlog (net/core/dev.c:6761)
>  __napi_poll.constprop.0 (net/core/dev.c:7830)
>  net_rx_action (net/core/dev.c:7893 net/core/dev.c:8050)
>  handle_softirqs (kernel/softirq.c:645)
>  __irq_exit_rcu (kernel/softirq.c:679 kernel/softirq.c:519 kernel/softirq.c:758)
>  irq_exit_rcu (kernel/softirq.c:775)
>  sysvec_apic_timer_interrupt (arch/x86/kernel/apic/apic.c:1062 (discriminator 35) arch/x86/kernel/apic/apic.c:1062 (discriminator 35))
> RIP: 0010:tcp_cong_avoid_ai (net/ipv4/tcp_cong.c:482)
> 
> Fix by checking if user pass in zero as parameter,
> if so reject the value zero.
> 
> Fixes: 814d488c6126 ("tcp: fix the timid additive increase on stretch ACKs")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: co+71b6509cb791e531@xxxxxxx
> Assisted-by: LLM
> Signed-off-by: Kaho Jiang <kj2864@xxxxxxx>
> ---
>  include/net/tcp.h  | 3 +--
>  net/ipv4/metrics.c | 4 ++++
>  2 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 4416cdf9bf30..ad80b8fb8a52 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -1521,8 +1521,7 @@ static inline u32 tcp_snd_cwnd(const struct tcp_sock *tp)
>  
>  static inline void tcp_snd_cwnd_set(struct tcp_sock *tp, u32 val)
>  {
> -	WARN_ON_ONCE((int)val <= 0);
> -	WRITE_ONCE(tp->snd_cwnd, val);
> +	WRITE_ONCE(tp->snd_cwnd, max(val, 1U));

Please keep the warning (it's not only about 0).

>  }
>  
>  static inline bool tcp_in_slow_start(const struct tcp_sock *tp)
> diff --git a/net/ipv4/metrics.c b/net/ipv4/metrics.c
> index ad40762a8b38..cb36686973e5 100644
> --- a/net/ipv4/metrics.c
> +++ b/net/ipv4/metrics.c
> @@ -54,6 +54,10 @@ static int ip_metrics_convert(struct nlattr *fc_mx,
>  			NL_SET_ERR_MSG(extack, "Unknown flag set in feature mask in metrics attribute");
>  			return -EINVAL;
>  		}
> +		if (type == RTAX_CWND && !val) {
> +			NL_SET_ERR_MSG(extack, "RTAX_CWND must be greater than zero");
> +			return -EINVAL;
> +		}
>  		metrics[type - 1] = val;
>  	}
>  
> -- 
> 2.43.0
>