Description
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).
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).
Published:
2026-10-14
Score:
n/a
EPSS:
n/a
KEV:
No
Impact:
n/a
Action:
n/a
Analysis
No analysis available yet.
Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).
| Vendor | Product | Default status | Versions | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Linux | Linux | unaffected |
|
|||||||||||||||
| Linux | Linux | affected |
|
No data.
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No data available yet.
Remediation
No remediation available yet.
Tracking
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Advisories
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References
History
Wed, 14 Oct 2026 13:45:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Description | 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). | |
| Title | net/sched: codel: bound the dropping loop per dequeue call | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-10-14T13:27:53.770Z
Reserved: 2026-09-25T10:25:14.348Z
Link: CVE-2026-98427
Vulnrichment
No data.
NVD
Status : Received
Published: 2026-10-14T14:17:54.343
Modified: 2026-10-14T14:17:54.343
Link: CVE-2026-98427
Redhat
No data.
OpenCVE Enrichment
No data.
Weaknesses
No weakness.