From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	vega@nebusec.ai,
	Victor Nogueira <victor@mojatatu.com>,
	Jamal Hadi Salim <jhs@mojatatu.com>,
	Paolo Abeni <pabeni@redhat.com>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.12 1298/1376] net/sched: fq: add overflow bounds to quantum and initial quantum
Date: Sat, 12 Sep 2026 09:02:04 +0200
Message-ID: <20260912065636.463177355@linuxfoundation.org>
In-Reply-To: <20260912065607.535295758@linuxfoundation.org>
References: <20260912065607.535295758@linuxfoundation.org>
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6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jamal Hadi Salim <jhs@mojatatu.com>

[ Upstream commit 709f34f7c28dc4dd6c40343d101850f11e172312 ]

fq_init() computes quantum = 2 * psched_mtu() and initial_quantum = 10 *
psched_mtu() with no overflow check. A device with a huge MTU (e.g. dummy
with max_mtu == 0 accepting MTU 2147483634) makes psched_mtu() return
0x80000000; the 2 * and 10 * multiplications wrap to 0 in 32-bit
arithmetic, so q->quantum == 0. Then in fq_dequeue() the credit-refill
loop adds 0 to f->credit (which stays <= 0) and goto begin loops
forever under the qdisc lock, creating a soft lockup.

Clamp psched_mtu() to [1, 1 << 20] before multiplying so the product
cannot wrap, then cap the result at 1 << 20, matching the bound already
enforced on TCA_FQ_QUANTUM in fq_change().

Conditions to recreate the bug: a device whose MTU (plus
hard_header_len) is large enough that 2 * psched_mtu() wraps (e.g. a
dummy device with max_mtu == 0 accepting MTU 2147483634). Requires
CAP_NET_ADMIN in a user namespace.

Fixes: afe4fd062416 ("pkt_sched: fq: Fair Queue packet scheduler")
Reported-by: vega@nebusec.ai
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260822195509.112717-2-jhs@mojatatu.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/sched/sch_fq.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
index 682daf79af373..afb76d7595917 100644
--- a/net/sched/sch_fq.c
+++ b/net/sched/sch_fq.c
@@ -1141,12 +1141,14 @@ static int fq_init(struct Qdisc *sch, struct nlattr *opt,
 		   struct netlink_ext_ack *extack)
 {
 	struct fq_sched_data *q = qdisc_priv(sch);
+	u32 mtu;
 	int i, err;
 
 	sch->limit		= 10000;
 	q->flow_plimit		= 100;
-	q->quantum		= 2 * psched_mtu(qdisc_dev(sch));
-	q->initial_quantum	= 10 * psched_mtu(qdisc_dev(sch));
+	mtu = clamp_t(u32, psched_mtu(qdisc_dev(sch)), 1, 1 << 20);
+	q->quantum		= min_t(u32, 2 * mtu, 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;
 	q->time_next_delayed_flow = ~0ULL;
-- 
2.53.0