Re: [PATCH net v5] net: ipv4: add zero check for snd_cwnd_clamp

[Eric Dumazet]

On 10/20/26 00:25, 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  | 2 +-
>   net/ipv4/metrics.c | 4 ++++
>   2 files changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 4416cdf9bf30..9f605d9ecb35 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -1522,7 +1522,7 @@ 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));

No. Please do not hide bugs.

I repeat : Do not change tcp_snd_cwnd_set()

>   }
>   
>   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;
>   	}

But is it really fixing anything ? I guess this is why you added the 
max() in tcp_snd_cwnd_set() because you did not understand the issue.

An unlocked CWND of zero is harmless.

I think a better fix would be:

diff --git a/net/ipv4/metrics.c b/net/ipv4/metrics.c
index 
ad40762a8b38379e4ccdc505e3885adb8dfff763..b7f8c47f72f9707adb4ef4dde5558af23b46a502 
100644
--- a/net/ipv4/metrics.c
+++ b/net/ipv4/metrics.c
@@ -60,6 +60,12 @@ static int ip_metrics_convert(struct nlattr *fc_mx,
        if (ecn_ca)
                metrics[RTAX_FEATURES - 1] |= DST_FEATURE_ECN_CA;

+       if ((metrics[RTAX_LOCK - 1] & (1 << RTAX_CWND)) &&
+           !metrics[RTAX_CWND - 1]) {
+               NL_SET_ERR_MSG(extack, "Locked cwnd metric must be non 
zero");
+               return -EINVAL;
+       }
+
        return 0;
 }




Note that my email address is edumazet@xxxxxxxxxx

Make sure to refresh your git tree, because git should have handled the 
change automatically.

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