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

[Jerry Jiang <kj2864@nyu.edu>]

I apologize for deleting the warning.

v5 is proposed:
https://lore.kernel.org/all/20261019222531.1073-1-kj2864@nyu.edu/

On Mon, Oct 19, 2026 at 4:33 AM Antoine Tenart <atenart@kernel.org> wrote:
>
> 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@vger.kernel.org
> > Reported-by: co+71b6509cb791e531@bugs.sh
> > Assisted-by: LLM
> > Signed-off-by: Kaho Jiang <kj2864@nyu.edu>
> > ---
> >  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
> >