Re: [PATCH net-next 3/3] tcp_cubic: fix divide by zero and endless loop on bad module params
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From: Eric Dumazet <hidden>
Date: 2026-09-30 11:21:52
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On Wed, Sep 30, 2026 at 12:10 PM Jiayuan Chen [off-list ref] wrote:
quoted hunk ↗ jump to hunk
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@linux.dev> --- 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.
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+ 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
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+ 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.