Re: [PATCH net-next 3/3] tcp_cubic: fix divide by zero and endless loop on bad module params

[Eric Dumazet]

On Wed, Sep 30, 2026 at 12:10 PM Jiayuan Chen <jiayuan.chen@xxxxxxxxx> wrote:
>
> beta_scale and cube_factor are computed once at module init, and they
> need 1024 - beta and bic_scale * 10 to be positive: beta == 1024 or
> bic_scale == 0 crash right there, beta > 1024 or a negative bic_scale
> gives garbage or wraps to 0. Negative beta can also make beta_scale 0.
> Reject them.
>
> A small beta also gives a small beta_scale, and (cwnd * scale) >> 3
> truncates to 0 for a tiny cwnd (e.g. 2), so the TCP friendliness loop
> never ends. Clamp delta to 1.
>
> Fixes: df3271f3361b ("[TCP] BIC: CUBIC window growth (2.0)")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>
> ---
> Target net-next since it is not a big problem.
> ---
>  net/ipv4/tcp_cubic.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
> index 119bf8cbb007c..2f04dca5be095 100644
> --- a/net/ipv4/tcp_cubic.c
> +++ b/net/ipv4/tcp_cubic.c
> @@ -298,7 +298,7 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
>         if (tcp_friendliness) {
>                 u32 scale = beta_scale;
>
> -               delta = (cwnd * scale) >> 3;

I would prefer not adding a test in the fast path to work around
silly module parameters.

beta_scale is computed once at module init, we can make sure it is >= 8
there. tcp_snd_cwnd() is >= 1, so delta would be >= 1.

With the integer divisions, beta_scale >= 8 iff beta >= 512,
so the default beta (717 -> beta_scale = 15) is not affected.


> +               delta = max((cwnd * scale) >> 3, 1U);
>                 while (ca->ack_cnt > delta) {           /* update tcp cwnd */
>                         ca->ack_cnt -= delta;
>                         ca->tcp_cwnd++;
> @@ -504,6 +504,11 @@ static int __init cubictcp_register(void)
>
>         BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
>
> +       if (beta < 0 || beta >= BICTCP_BETA_SCALE || bic_scale <= 0) {

bic_scale * 10 can overflow if bic_scale > INT_MAX / 10

> +               pr_err("tcp_cubic: invalid beta %d or bic_scale %d\n", beta, bic_scale);
> +               return -EINVAL;
> +       }
> +

Something like this (untested) :

if (beta < 0 || beta >= BICTCP_BETA_SCALE ||
   bic_scale <= 0 || bic_scale > INT_MAX / 10) {
    pr_err("tcp_cubic: invalid beta %d or bic_scale %d\n",
      beta, bic_scale);
    return -EINVAL;
}
/* Precompute a bunch of the scaling factors that are used per-packet
* based on SRTT of 100ms
*/
beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
/ (BICTCP_BETA_SCALE - beta);
/* bictcp_update() needs (cwnd * beta_scale) >> 3 to be >= 1 */
beta_scale = max(beta_scale, 8U);

Thanks.