[PATCH net-next 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, 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;
+		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) {
+		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
 	 */
-- 
2.43.0