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net/sched: drr: clamp quantum in change class

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drr_change_class() rejects explicit quantum==0 but falls back to
psched_mtu() with no floor. With a crafted size table qdisc_pkt_len
reaches ~2 GiB, so quantum=1 (or a zero psched_mtu on a headerless
device) makes the deficit-refill loop spin under the qdisc lock.

Add clamp_t(u32, quantum, 256, 1<<20) after the zero reject and on the
fallback path. The explicit-zero reject is preserved.

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

  tc qdisc add dev dummy0 root drr
  tc class add dev dummy0 parent 1: classid 1:1 drr quantum 1

Fixes: 13d2a1d2b0 ("pkt_sched: add DRR scheduler")
Reported-by: Vega <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>
Link: https://patch.msgid.link/QDISC-0CFC.v3.20260901204856@mojatatu.com.8
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jamal Hadi Salim 2026-09-01 17:39:28 -04:00 committed by Jakub Kicinski
parent 54370e44c0
commit 8382abec0f
1 changed files with 2 additions and 1 deletions
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3
net/sched/sch_drr.c
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@ -82,8 +82,9 @@ static int drr_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
NL_SET_ERR_MSG(extack, "Specified DRR quantum cannot be zero");
return -EINVAL;
}
quantum = clamp_t(u32, quantum, 256, 1 << 20);
} else
quantum = psched_mtu(qdisc_dev(sch));
quantum = clamp_t(u32, (u32)psched_mtu(qdisc_dev(sch)), 256, 1 << 20);
if (cl != NULL) {
if (tca[TCA_RATE]) {