PT-2026-114406 · Linux · Linux
CVE-2026-98427
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Published
2026-10-14
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Updated
2026-10-14
None
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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:
-
Make the control-law step at least 1 tick so the dropping loop always moves forward.
-
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).
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Related Identifiers
Affected Products
Linux