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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , stable@vger.kernel.org, vega@nebusec.ai, Victor Nogueira Subject: [PATCH net v3 2/6] net/sched: fq_codel: clamp default quantum and mtu Date: Sat, 22 Aug 2026 15:55:05 -0400 Message-Id: <20260822195509.112717-3-jhs@mojatatu.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260822195509.112717-1-jhs@mojatatu.com> References: <20260822195509.112717-1-jhs@mojatatu.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit fq_codel_init() sets q->quantum = psched_mtu(qdisc_dev(sch)) without clamping. A device with a huge MTU (e.g. dummy with max_mtu == 0 accepting MTU 2147483634) makes psched_mtu() return 0x80000000, which overflows the signed flow->deficit to INT_MIN in fq_codel_dequeue(), causing an infinite loop and soft lockup. Emulate fq_codel_change() and constrain to [256, FQ_CODEL_QUANTUM_MAX]. The same unclamped psched_mtu() is assigned to q->cparams.mtu a bit below, and fq_codel_change() never updates it. codel_should_drop() tests "*backlog <= params->mtu"; with mtu == 0x80000000 (~2 GiB) and the default 32 MiB memory_limit, the test is always true, so CoDel is silently and completely disabled (no drops, no ECN). Declare a single clamped mtu and assign both q->quantum and q->cparams.mtu from it, which also removes the double psched_mtu() call. 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: 4b549a2ef4be ("fq_codel: Fair Queue Codel AQM") Reported-by: vega@nebusec.ai Tested-by: Victor Nogueira Signed-off-by: Jamal Hadi Salim --- net/sched/sch_fq_codel.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c index 6cce86ba383c..969b2510b0b8 100644 --- a/net/sched/sch_fq_codel.c +++ b/net/sched/sch_fq_codel.c @@ -509,6 +509,7 @@ static int fq_codel_init(struct Qdisc *sch, struct nlattr *opt, struct netlink_ext_ack *extack) { struct fq_codel_sched_data *q = qdisc_priv(sch); + u32 mtu; int i; int err; @@ -516,13 +517,14 @@ static int fq_codel_init(struct Qdisc *sch, struct nlattr *opt, q->flows_cnt = 1024; q->memory_limit = 32 << 20; /* 32 MBytes */ q->drop_batch_size = 64; - q->quantum = psched_mtu(qdisc_dev(sch)); + mtu = clamp_t(u32, psched_mtu(qdisc_dev(sch)), 256, FQ_CODEL_QUANTUM_MAX); + q->quantum = mtu; INIT_LIST_HEAD(&q->new_flows); INIT_LIST_HEAD(&q->old_flows); codel_params_init(&q->cparams); codel_stats_init(&q->cstats); q->cparams.ecn = true; - q->cparams.mtu = psched_mtu(qdisc_dev(sch)); + q->cparams.mtu = mtu; if (opt) { err = fq_codel_change(sch, opt, extack); -- 2.43.0