---
id: CVE-2026-98610
title: 'net/sched: sch_hfsc: bound the classify inner-filter walk with a drift budget'
summary: |-
  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 f…
severity: none
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= a2f79227138c71e08627af5f8961197364edbc98 <
    d4fd251e3311727c32af89344d3332f788c153a2
  - >-
    Linux >= a2f79227138c71e08627af5f8961197364edbc98 <
    30d7fea767e830bd81ac2f73d2e33c877cb2e679
  - >-
    Linux >= a2f79227138c71e08627af5f8961197364edbc98 <
    f476fb656ca2326d7b2eba98d03ced1cbcf5cd99
  - >-
    Linux >= a2f79227138c71e08627af5f8961197364edbc98 <
    e8f9979286f70dfbfe0552e2a63000950841bea6
  - >-
    Linux >= a2f79227138c71e08627af5f8961197364edbc98 <
    2bd20245dd658789d8cccb37697967b4caba6ad8
  - >-
    Linux >= a2f79227138c71e08627af5f8961197364edbc98 <
    0fa25ba50a382d79c0cf060214c2095756836f5f
  - >-
    Linux >= a2f79227138c71e08627af5f8961197364edbc98 <
    1301a1292c6742b4c96a18715d49f89c3caca0d7
  - >-
    Linux >= a2f79227138c71e08627af5f8961197364edbc98 <
    8a60ade2277e1f0e0d0578d565354e52292fa46d
  - Linux 2.6.35
published: '2026-10-18'
updated: '2026-10-18'
sourceUpdated: '2026-10-18T15:19:25.128Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-98610'
references:
  - url: 'https://git.kernel.org/stable/c/d4fd251e3311727c32af89344d3332f788c153a2'
  - url: 'https://git.kernel.org/stable/c/30d7fea767e830bd81ac2f73d2e33c877cb2e679'
  - url: 'https://git.kernel.org/stable/c/f476fb656ca2326d7b2eba98d03ced1cbcf5cd99'
  - url: 'https://git.kernel.org/stable/c/e8f9979286f70dfbfe0552e2a63000950841bea6'
  - url: 'https://git.kernel.org/stable/c/2bd20245dd658789d8cccb37697967b4caba6ad8'
  - url: 'https://git.kernel.org/stable/c/0fa25ba50a382d79c0cf060214c2095756836f5f'
  - url: 'https://git.kernel.org/stable/c/1301a1292c6742b4c96a18715d49f89c3caca0d7'
  - url: 'https://git.kernel.org/stable/c/8a60ade2277e1f0e0d0578d565354e52292fa46d'
tags:
  - cve.org
ingestedAt: '2026-10-18T15:45:12.242Z'
---

## Overview

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).

## Affected

- `Linux >= a2f79227138c71e08627af5f8961197364edbc98 < d4fd251e3311727c32af89344d3332f788c153a2`
- `Linux >= a2f79227138c71e08627af5f8961197364edbc98 < 30d7fea767e830bd81ac2f73d2e33c877cb2e679`
- `Linux >= a2f79227138c71e08627af5f8961197364edbc98 < f476fb656ca2326d7b2eba98d03ced1cbcf5cd99`
- `Linux >= a2f79227138c71e08627af5f8961197364edbc98 < e8f9979286f70dfbfe0552e2a63000950841bea6`
- `Linux >= a2f79227138c71e08627af5f8961197364edbc98 < 2bd20245dd658789d8cccb37697967b4caba6ad8`
- `Linux >= a2f79227138c71e08627af5f8961197364edbc98 < 0fa25ba50a382d79c0cf060214c2095756836f5f`
- `Linux >= a2f79227138c71e08627af5f8961197364edbc98 < 1301a1292c6742b4c96a18715d49f89c3caca0d7`
- `Linux >= a2f79227138c71e08627af5f8961197364edbc98 < 8a60ade2277e1f0e0d0578d565354e52292fa46d`
- `Linux 2.6.35`

## Remediation

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