[PATCH net v3] net/sched: cbs: perform rate conversions in signed 64-bit arithmetic

cbs_set_port_rate() and cbs_change() multiply link rates and slope values
by BYTES_PER_KBIT, an unsigned long constant.  On 32-bit architectures,
the multiplications therefore take place in 32-bit unsigned arithmetic
before the results are assigned to s64 fields.

For port rates above approximately 34.36 Gbit/s this wraps port_rate.
The same conversion turns a negative sendslope into a large positive
value, reversing the CBS credit adjustment.  This affects software CBS;
port_rate is also refreshed on NETDEV_UP and NETDEV_CHANGE notifications.

Cast the first operand of each multiplication to s64 so all intermediate
operations use signed 64-bit arithmetic and preserve the value's sign on
every architecture

Also reject a negative idleslope. A negative idleslope can arm the
watchdog in the past and busy-loop.

Fixes: 585d763af09c ("net/sched: Introduce Credit Based Shaper (CBS) qdisc")
Fixes: 397006ba5d918 ("net/sched: cbs: Fix integer overflow in cbs_set_port_rate()")

Signed-off-by: Jack Wang <redacted>
---
v3:
- Reject only negative idleslope values.
- Keep idleslope 0 accepted for compatibility with existing tc-testing
  defaults, fixing test 1820 regression reported by Victor.
- Keep the timediff_to_credits() overflow out of this series as a separate
  follow-up.

v2:
- Reject non-positive idleslope values to prevent scheduling the watchdog
  in the past.

v1: https://lore.kernel.org/netdev/20260806155253.50252-1-163wangjack@gmail.com/

 net/sched/sch_cbs.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/sched/sch_cbs.c b/net/sched/sch_cbs.c
index 1c93469c56e3..8db98d7c98a8 100644
--- a/net/sched/sch_cbs.c
+++ b/net/sched/sch_cbs.c

@@ -335,7 +335,7 @@ static void cbs_set_port_rate(struct net_device *dev, struct cbs_sched_data *q)
 		speed = ecmd.base.speed;
 
 skip:
-	port_rate = speed * 1000 * BYTES_PER_KBIT;
+	port_rate = (s64)speed * 1000 * BYTES_PER_KBIT;
 
 	atomic64_set(&q->port_rate, port_rate);
 	netdev_dbg(dev, "cbs: set %s's port_rate to: %lld, linkspeed: %d\n",

@@ -392,6 +392,10 @@ static int cbs_change(struct Qdisc *sch, struct nlattr *opt,
 	}
 
 	qopt = nla_data(tb[TCA_CBS_PARMS]);
+	if (qopt->idleslope < 0) {
+		NL_SET_ERR_MSG(extack, "Idleslope must not be negative");
+		return -EINVAL;
+	}
 
 	if (!qopt->offload) {
 		cbs_set_port_rate(dev, q);

@@ -405,8 +409,8 @@ static int cbs_change(struct Qdisc *sch, struct nlattr *opt,
 	/* Everything went OK, save the parameters used. */
 	WRITE_ONCE(q->hicredit, qopt->hicredit);
 	WRITE_ONCE(q->locredit, qopt->locredit);
-	WRITE_ONCE(q->idleslope, qopt->idleslope * BYTES_PER_KBIT);
-	WRITE_ONCE(q->sendslope, qopt->sendslope * BYTES_PER_KBIT);
+	WRITE_ONCE(q->idleslope, (s64)qopt->idleslope * BYTES_PER_KBIT);
+	WRITE_ONCE(q->sendslope, (s64)qopt->sendslope * BYTES_PER_KBIT);
 	WRITE_ONCE(q->offload, qopt->offload);
 
 	return 0;

base-commit: 7b53449540502cb21b32bca62a6258e22cd97bbe
-- 
2.53.0

On 8/13/26 8:02 AM, Jack Wang wrote:

cbs_set_port_rate() and cbs_change() multiply link rates and slope values
by BYTES_PER_KBIT, an unsigned long constant.  On 32-bit architectures,
the multiplications therefore take place in 32-bit unsigned arithmetic
before the results are assigned to s64 fields.

For port rates above approximately 34.36 Gbit/s this wraps port_rate.
The same conversion turns a negative sendslope into a large positive
value, reversing the CBS credit adjustment.  This affects software CBS;
port_rate is also refreshed on NETDEV_UP and NETDEV_CHANGE notifications.

Cast the first operand of each multiplication to s64 so all intermediate
operations use signed 64-bit arithmetic and preserve the value's sign on
every architecture

Also reject a negative idleslope. A negative idleslope can arm the
watchdog in the past and busy-loop.

Fixes: 585d763af09c ("net/sched: Introduce Credit Based Shaper (CBS) qdisc")
Fixes: 397006ba5d918 ("net/sched: cbs: Fix integer overflow in cbs_set_port_rate()")

Signed-off-by: Jack Wang <redacted>

No empty lines in the tag area.

Sashiko noted this isn't enough to avoid the busy loop timer:

https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260813060256.28748-1-163wangjack%40gmail.com

I understand that a 0 value must be accepted, but I think we need either
a better solution or drop entirely this check that does not provide much
value per se. I think the first option would be better.

Also please note saskiko has more comments.

/P