---
id: CVE-2026-31421
title: |-
  In the Linux kernel, the following vulnerability has been resolved:

  net/sched: cls_fw: fix NULL pointer dereference on shared blocks

  The old-method path in fw_classify() calls tcf_block_q() and
  dereferences q->handle
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  net/sched: cls_fw: fix NULL pointer dereference on shared blocks

  The old-method path in fw_classify() calls tcf_block_q() and
  dereferences q->handle.  Shared blocks le…
severity: medium
cvss: 5.5
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'
cwe:
  - CWE-476
vendor: linux
product: linux_kernel
affected:
  - 'linux_kernel >= 4.15, < 5.10.253'
  - 'linux_kernel >= 5.11, < 5.15.203'
  - 'linux_kernel >= 5.16, < 6.1.168'
  - 'linux_kernel >= 6.2, < 6.6.134'
  - 'linux_kernel >= 6.7, < 6.12.81'
  - 'linux_kernel >= 6.13, < 6.18.22'
  - 'linux_kernel >= 6.19, < 6.19.12'
  - linux_kernel = 7.0
patched:
  - linux_kernel 6.19.12
published: '2026-04-13'
updated: '2026-10-15'
sourceUpdated: '2026-10-15T02:19:03.837'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-31421'
references:
  - url: 'https://git.kernel.org/stable/c/18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/3cb055df9e8625ce699a259d8178d67b37f2b160'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/3d41f9a314afa94b1c7c7c75405920123220e8cd'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/5cf41031922c154aa5ccda8bcdb0f5e6226582ec'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/96426c348def662b06bfdc65be3002905604927a'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/d6d5bd62a09650856e1e2010eb09853eba0d64e1'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/faeea8bbf6e958bf3c00cb08263109661975987c'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/febf64ca79a2d6540ab6e5e197fa0f4f7e84473e'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://cert-portal.siemens.com/productcert/html/ssa-019113.html'
    label: 0b142b55-0307-4c5a-b3c9-f314f3fb7c5e
  - url: 'https://cert-portal.siemens.com/productcert/html/ssa-082556.html'
    label: 0b142b55-0307-4c5a-b3c9-f314f3fb7c5e
  - url: 'https://cert-portal.siemens.com/productcert/html/ssa-457936.html'
    label: 0b142b55-0307-4c5a-b3c9-f314f3fb7c5e
tags:
  - nvd
  - cve.org
epss: 0.0012
epssPercentile: 0.01802
ingestedAt: '2026-09-08T15:33:26.991Z'
---

## Overview

In the Linux kernel, the following vulnerability has been resolved:

net/sched: cls_fw: fix NULL pointer dereference on shared blocks

The old-method path in fw_classify() calls tcf_block_q() and
dereferences q->handle.  Shared blocks leave block->q NULL, causing a
NULL deref when an empty cls_fw filter is attached to a shared block
and a packet with a nonzero major skb mark is classified.

Reject the configuration in fw_change() when the old method (no
TCA_OPTIONS) is used on a shared block, since fw_classify()'s
old-method path needs block->q which is NULL for shared blocks.

The fixed null-ptr-deref calling stack:
 KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f]
 RIP: 0010:fw_classify (net/sched/cls_fw.c:81)
 Call Trace:
  tcf_classify (./include/net/tc_wrapper.h:197 net/sched/cls_api.c:1764 net/sched/cls_api.c:1860)
  tc_run (net/core/dev.c:4401)
  __dev_queue_xmit (net/core/dev.c:4535 net/core/dev.c:4790)

## Affected

- `linux_kernel >= 4.15, < 5.10.253`
- `linux_kernel >= 5.11, < 5.15.203`
- `linux_kernel >= 5.16, < 6.1.168`
- `linux_kernel >= 6.2, < 6.6.134`
- `linux_kernel >= 6.7, < 6.12.81`
- `linux_kernel >= 6.13, < 6.18.22`
- `linux_kernel >= 6.19, < 6.19.12`
- `linux_kernel = 7.0`

## Remediation

Upgrade past the affected range:

- `linux_kernel 6.19.12`