[PATCH net v2 1/8] net/sched: fq: clamp quantum and initial_quantum in change path

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

The fq change path accepts TCA_FQ_QUANTUM in [1, INT_MAX] and
TCA_FQ_INITIAL_QUANTUM up to INT_MAX, while fq_init() already clamps to
[1, 1<<20]. A user can override the init clamp via tc qdisc change,
restoring the small-quantum deficit spin that the init clamp prevents.

Narrow iq_range.max to 1<<20 so TCA_FQ_INITIAL_QUANTUM is rejected at
parse time. Clamp TCA_FQ_QUANTUM to [256, 1<<20] in fq_change() and
fq_init() quantum to [256, 1<<20] for tiny-MTU devices.

Conditions to recreate the bug:
  CONFIG_NET_SCH_FQ=y. Requires CAP_NET_ADMIN (namespace-local via
  unshare -Urn suffices).

  tc qdisc add dev dummy0 root fq
  tc qdisc change dev dummy0 root fq quantum 1 stab data 32768 size_log 15 cell_log 0

Fixes: 709f34f7c28d ("net/sched: fq: add overflow bounds to quantum and initial quantum")
Reported-by: vega@nebusec.ai
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: stable@vger.kernel.org
---
 net/sched/sch_fq.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
index 6144b5686f13..35f940b2205d 100644
--- a/net/sched/sch_fq.c
+++ b/net/sched/sch_fq.c
@@ -980,7 +980,7 @@ static int fq_resize(struct Qdisc *sch, u32 log)
 }
 
 static const struct netlink_range_validation iq_range = {
-	.max = INT_MAX,
+	.max = 1 << 20,
 };
 
 static const struct nla_policy fq_policy[TCA_FQ_MAX + 1] = {
@@ -1106,14 +1106,10 @@ static int fq_change(struct Qdisc *sch, struct nlattr *opt,
 			   nla_get_u32(tb[TCA_FQ_FLOW_PLIMIT]));
 
 	if (tb[TCA_FQ_QUANTUM]) {
-		u32 quantum = nla_get_u32(tb[TCA_FQ_QUANTUM]);
+		u32 quantum = clamp_t(u32, nla_get_u32(tb[TCA_FQ_QUANTUM]),
+				     256, 1 << 20);
 
-		if (quantum > 0 && quantum <= (1 << 20)) {
-			WRITE_ONCE(q->quantum, quantum);
-		} else {
-			NL_SET_ERR_MSG_MOD(extack, "invalid quantum");
-			err = -EINVAL;
-		}
+		WRITE_ONCE(q->quantum, quantum);
 	}
 
 	if (tb[TCA_FQ_INITIAL_QUANTUM])
@@ -1232,7 +1228,7 @@ static int fq_init(struct Qdisc *sch, struct nlattr *opt,
 	sch->limit		= 10000;
 	q->flow_plimit		= 100;
 	mtu = clamp_t(u32, psched_mtu(qdisc_dev(sch)), 1, 1 << 20);
-	q->quantum		= min_t(u32, 2 * mtu, 1 << 20);
+	q->quantum		= clamp_t(u32, 2 * mtu, 256, 1 << 20);
 	q->initial_quantum	= min_t(u32, 10 * mtu, 1 << 20);
 	q->flow_refill_delay	= msecs_to_jiffies(40);
 	q->flow_max_rate	= ~0UL;
-- 
2.43.0