CVE-2026-98011 Linux CVE debrief

Who should care

Defenders responsible for Linux kernel configurations, especially those with CAP_NET_ADMIN privileges, should assess exposure and prioritize verification of HHF qdisc configurations.

Why it matters

Defenders should assess exposure and prioritize verification of HHF qdisc configurations, especially where CAP_NET_ADMIN privileges are present, to prevent potential denial-of-service conditions.

• Denial-of-service conditions due to soft lockup.
• Potential for excessive CPU utilization.
• Need for verification of HHF qdisc configurations.
• Potential impact on network performance.

Technical summary

The Linux kernel's Hierarchical Fair Queueing (HHF) qdisc implementation is vulnerable to a denial-of-service attack when the `quantum` parameter is set to 1. This causes the deficit-refill loop to spin excessively under the qdisc lock, leading to a soft lockup. The vulnerability was resolved by adding a clamp to the `quantum` parameter in the `hhf_change` and `hhf_init` paths, similar to the fq_codel_change function. The clamp ensures that the `quantum` parameter is set to a value between 256 and 1<<20, preventing the deficit-refill loop from spinning excessively under the qdisc lock.

Defensive priority

Defenders should assess exposure and prioritize verification of HHF qdisc configurations, especially where CAP_NET_ADMIN privileges are present.

Recommended defensive actions

• Review and verify HHF qdisc configurations to ensure the quantum parameter is not set to 1.
• Assess exposure and prioritize verification of CAP_NET_ADMIN privileges.
• Monitor for potential soft lockup conditions in the Linux kernel.
• Review compensating controls for exposed systems while remediation is scheduled and verified.
• Check relevant monitoring, detection, and logs for exposed assets that need extra review.
• Track exceptions, retest remediated assets, and close the item only after evidence is documented.
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed.

Evidence notes

The vulnerability was resolved by adding a clamp to the `quantum` parameter in the `hhf_change` and `hhf_init` paths, similar to the fq_codel_change function. The clamp ensures that the `quantum` parameter is set to a value between 256 and 1<<20, preventing the deficit-refill loop from spinning excessively under the qdisc lock. Defenders should verify that the HHF qdisc configurations are properly set and that the `quantum` parameter is not set to 1. Additionally, defenders should review the Linux kernel patch notes and verify that

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.