[PATCH net-next 2/3] tcp_hybla: fix divide by zero on rtt0 == 0

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

rtt0 is the reference RTT in ms, so 0 makes no sense, and
hybla_recalc_param() divides by it right from hybla_init().

Reject values below 1 when the parameter is written. Also cap it at
U32_MAX / USEC_PER_MSEC, since rtt0 * USEC_PER_MSEC can wrap to 0 on
32-bit. An rtt0 above that (about 71 minutes) makes no sense anyway.

Fixes: 835b3f0c0d7e ("[TCP]: Add TCP Hybla congestion control module.")
Fixes: 740b0f1841f6 ("tcp: switch rtt estimations to usec resolution")
Reported-by: VEGA <vega@nebusec.ai>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
Target net-next since it is not a big problem.
---
 net/ipv4/tcp_hybla.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_hybla.c b/net/ipv4/tcp_hybla.c
index abd7d91807e54..b9b482180d30b 100644
--- a/net/ipv4/tcp_hybla.c
+++ b/net/ipv4/tcp_hybla.c
@@ -26,8 +26,20 @@ struct hybla {
 };
 
 /* Hybla reference round trip time (default= 1/40 sec = 25 ms), in ms */
-static int rtt0 = 25;
-module_param(rtt0, int, 0644);
+static unsigned int rtt0 = 25;
+
+static int rtt0_set(const char *val, const struct kernel_param *kp)
+{
+	/* avoid rtt0 * USEC_PER_MSEC overflow */
+	return param_set_uint_minmax(val, kp, 1, U32_MAX / USEC_PER_MSEC);
+}
+
+static const struct kernel_param_ops rtt0_ops = {
+	.set = rtt0_set,
+	.get = param_get_uint,
+};
+
+module_param_cb(rtt0, &rtt0_ops, &rtt0, 0644);
 MODULE_PARM_DESC(rtt0, "reference rout trip time (ms)");
 
 /* This is called to refresh values for hybla parameters */
-- 
2.43.0