[PATCH net v3 6/6] net/sched: sfq: clamp quantum to avoid signed overflow soft lockup

[Jamal Hadi Salim <jhs@mojatatu.com>]

sfq_init() sets q->quantum = psched_mtu(qdisc_dev(sch)) (unsigned). A
device with a huge MTU (e.g. dummy with max_mtu == 0 accepting MTU
2147483634) makes psched_mtu() return 0x80000000, so slot->allot = INT_MIN
and INT_MIN + INT_MIN toggles between INT_MIN and 0 forever, spinning
sfq_dequeue() under the qdisc lock.

Clamp the quantum to [256, 1 << 20] so the refill loop terminates. The
lower bound also covers q->quantum == 0 (psched_mtu() returning 0),
which spins sfq_dequeue() identically. sfq_change() already rejects a
negative quantum, so only the init path was exposed.

Conditions to recreate the bug: a device whose MTU (plus
hard_header_len) wraps psched_mtu() into the sign bit (e.g. a dummy
device with max_mtu == 0 accepting MTU 2147483634). Requires
CAP_NET_ADMIN in a user namespace.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: vega@nebusec.ai
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/sched/sch_sfq.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
index 77675f9a4c46..187d3ed578f2 100644
--- a/net/sched/sch_sfq.c
+++ b/net/sched/sch_sfq.c
@@ -799,7 +799,8 @@ static int sfq_init(struct Qdisc *sch, struct nlattr *opt,
 	q->tail = NULL;
 	q->divisor = SFQ_DEFAULT_HASH_DIVISOR;
 	q->maxflows = SFQ_DEFAULT_FLOWS;
-	q->quantum = psched_mtu(qdisc_dev(sch));
+	q->quantum = clamp_t(u32, psched_mtu(qdisc_dev(sch)),
+			     256, 1 << 20);
 	q->perturb_period = 0;
 	get_random_bytes(&q->perturbation, sizeof(q->perturbation));
 
-- 
2.43.0