[PATCH net-next v2 3/3] tcp_cubic: fix divide by zero and endless loop on bad module params

[Jiayuan Chen <jiayuan.chen@linux.dev>]

beta_scale and cube_factor are computed once at module init. They need
1024 - beta to be positive and bic_scale * 10 to neither be 0 nor
overflow: beta == 1024 or bic_scale == 0 crash right there, and other
out of range values give garbage. Negative beta can also make
beta_scale 0. Reject them.

A small beta also gives a small beta_scale, and (cwnd * beta_scale) >> 3
truncates to 0 for a tiny cwnd, so the TCP friendliness loop never
ends. Make sure beta_scale is at least 8 at init, so this is always
>= 1 with cwnd >= 1 and the fast path is untouched.
This also caps alpha_cubic at 1 for beta < 512, which only affects
such unusual settings.

Fixes: df3271f3361b ("[TCP] BIC: CUBIC window growth (2.0)")
Suggested-by: Eric Dumazet <edumazet@kernel.org>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
Target net-next since it is not a big problem.
---
 net/ipv4/tcp_cubic.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 119bf8cbb007c..33b383e1307fb 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -504,12 +504,21 @@ 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 > 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);
 
 	cube_rtt_scale = (bic_scale * 10);	/* 1024*c/rtt */
 
-- 
2.43.0