CVE-2026-98610

Received Received - Intake

Linux Kernel HFSC Classify Soft Lockup Vulnerability

Publication date: 2026-10-18

Last updated on: 2026-10-19

Assigner: kernel.org

Description

In the Linux kernel, the following vulnerability has been resolved: net/sched: sch_hfsc: bound the classify inner-filter walk with a drift budget hfsc_classify() applies the "filter may only point downwards" level check only when the filter result carries no bound class. A filter created with a flowid gets res.class set once at bind time, so the check never runs for it during classification. hfsc_adjust_levels() can later raise a class's level without revalidating existing bindings, leaving two binds that were each legal at bind time pointing at each other; the classify walk then bounces between two interior classes forever with the qdisc lock held and BH disabled — a soft lockup from a single packet. The stuck walk trips the watchdog: watchdog: BUG: soft lockup - CPU#3 stuck for 13s! [ping:444] RIP: 0010:u32_classify+0x542/0x17f0 ... tcf_classify+0x66/0xa0 hfsc_enqueue+0x166/0xdf0 Bound the traversal with a budget of non-descending hops, the only way a configured walk can move without descending the class tree once levels drift after bind time. The budget is cumulative over the whole walk and is deliberately not reset on a descending hop: a chain that alternates a descent with a lateral hop would return the budget every lap and never trip. Descending hops never decrement it, so legitimately deep trees are unaffected and a terminating lateral chain still classifies normally. Drop the packet with a rate-limited warning once the budget is exhausted, mirroring the merged HTB fix. This is a follow-up to commit 729c4896ab82 ("net/sched: sch_htb: limit htb_classify inner-class filter hops"), which bounded the same classify loop on the HTB side but left the HFSC walk unbounded. Conditions to recreate the bug: - CONFIG_NET_SCHED, CONFIG_NET_SCH_HFSC, CONFIG_NET_CLS_U32, CONFIG_LOCKUP_DETECTOR. - Build a cycle with two legal-at-bind-time flowid binds and a level drift: class X 1:1 (child of root) with leaf child 1:10; class Y 1:2 (sibling of X) with children 1:20 and 1:200; root u32 filter flowid 1:1; filter on X flowid 1:2 (legal when Y is a leaf); after Y's level rises to 2, filter on Y flowid 1:1 (legal then). Send one packet (ping on the device). Unfixed kernel: classify spins with the qdisc lock held; with softlockup_panic=1 it panics. - Reachable from unprivileged user via unshare -Urn (CAP_NET_ADMIN).

CVSS Scores

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Meta Information

Published

2026-10-18

Last Modified

2026-10-19

Generated

2026-10-19

AI Q&A

2026-10-18

EPSS Evaluated

N/A

NVD

CVE-2026-98610

EUVD

EUVD-2026-103655

Affected Vendors & Products

Vendor	Product	Version / Range
Linux	Linux	a2f79227138c71e08627af5f8961197364edbc98
Linux	Linux	a2f79227138c71e08627af5f8961197364edbc98
Linux	Linux	a2f79227138c71e08627af5f8961197364edbc98
Linux	Linux	a2f79227138c71e08627af5f8961197364edbc98
Linux	Linux	a2f79227138c71e08627af5f8961197364edbc98
Linux	Linux	a2f79227138c71e08627af5f8961197364edbc98
Linux	Linux	a2f79227138c71e08627af5f8961197364edbc98
Linux	Linux	a2f79227138c71e08627af5f8961197364edbc98
Linux	Linux	2.6.35

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-UNKNOWN

Attack-Flow Graph

Executive Summary

This is a Linux kernel vulnerability where a packet classification loop in the HFSC traffic control scheduler can get stuck in an infinite loop. It occurs when two traffic classes are legally bound to each other but their levels drift, causing the classifier to bounce between them indefinitely while holding a lock. This leads to a soft lockup detected by the kernel watchdog.

Detection Guidance

This vulnerability can be detected by monitoring for soft lockups or CPU hangs during packet classification on systems using the HFSC qdisc. Check kernel logs for messages like 'soft lockup - CPU# stuck for Xs!' or watchdog timeouts. Commands like 'dmesg | grep -i softlockup' or 'journalctl -k | grep softlockup' may reveal issues. If CONFIG_LOCKUP_DETECTOR is enabled, the system will log these events.

Impact Analysis

An attacker with CAP_NET_ADMIN privileges (e.g., via unshare -Urn) could trigger a kernel soft lockup by sending a single packet. This causes the affected CPU core to become unresponsive for over 13 seconds, potentially leading to system slowdowns, service disruptions, or a kernel panic if softlockup_panic is enabled.

Compliance Impact

This vulnerability does not directly affect compliance with GDPR, HIPAA, or similar standards. It is a Linux kernel networking issue causing a soft lockup due to an unbounded classify walk in the HFSC qdisc. No data exposure or privacy violation is described.

Mitigation Strategies

Apply the kernel patch that bounds the classify inner-filter walk with a drift budget. Disable CONFIG_NET_SCH_HFSC if not needed. Update to a fixed kernel version. Monitor for soft lockups post-update. If affected, reboot the system to clear the qdisc lock.

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