In the Linux kernel, the following vulnerability has been resolved: net/sched: codel: bound the dropping loop per dequeue call The CoDel control law schedules the next drop one interval/sqrt(count) after the previous drop, using the co…

Overview

In the Linux kernel, the following vulnerability has been resolved:

net/sched: codel: bound the dropping loop per dequeue call

The CoDel control law schedules the next drop one interval/sqrt(count) after the previous drop, using the configured interval (codel_params.interval). For very small intervals the scheduled step rounds down to zero, so the dropping loop in codel_dequeue() never advances and drains the entire backlog under the qdisc lock in one call - an unprivileged user can trigger a soft lockup this way.

Fix in the shared codel code used by both codel and fq_codel:

1. Make the control-law step at least 1 tick so the dropping loop always moves forward.

2. Cap the dropping loop at CODEL_MAX_DROPS_PER_DEQUEUE (256) drops per codel_dequeue() call, resyncing drop_next to now when the cap is hit: the catch-up owed to the loop grows with the idle gap and the backlog, which no interval threshold can bound. This is a deliberate behaviour change after long idle gaps.

The cap applies to fq_codel (4b549a2ef4be) and the mac80211 TXQ path (fixed interval, cap only).

The target sojourn delay (codel_params.target) is not validated: it does not feed the control law, so a sub-tick value is aggressive rather than deadlock-prone.

Conditions to recreate the bug:

• tc qdisc add dev lo root handle 1: tbf rate 1kbit burst 2kb limit 1000000
• tc qdisc add dev lo parent 1:1 handle 10: codel interval 2us target 1ms noecn limit 1000000 (same for fq_codel)
• unpatched kernel: tc accepts it; a UDP flood under the 1kbit tbf soft-lockups (watchdog: BUG: soft lockup) while one codel_dequeue() call drops the backlog under the qdisc lock
• patched kernel: same setup, at most 256 drops per dequeue call, no soft lockup

Testing: claim reproducer and interval 2us/3us variants run clean; tdc qdisc category passes (see the selftests patch).

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Vendor advisories

• Red Hat VEX · Low · affected: Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat OpenShift Container Platform 4 · no fix planned: Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat OpenShift Container Platform 4 · updated 2026-10-18 · vex