---
id: CVE-2025-38255
title: 'lib/group_cpus: fix NULL pointer dereference from group_cpus_evenly()'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  lib/group_cpus: fix NULL pointer dereference from group_cpus_evenly()

  While testing null_blk with configfs, echo 0 > poll_queues will trigger
  following panic:

  BUG: ke…
severity: none
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 6a6dcae8f486c3f3298d0767d34505121c7b0b81 <
    64a99eff8dcf1f951a544e6058341b2b19a8fdbd
  - >-
    Linux >= 6a6dcae8f486c3f3298d0767d34505121c7b0b81 <
    29d39e0d5f16c060e32542b2cf351c09fd22b250
  - >-
    Linux >= 6a6dcae8f486c3f3298d0767d34505121c7b0b81 <
    911ef2e8a7de5b2bae8ff11fb0bd01f699e6db65
  - >-
    Linux >= 6a6dcae8f486c3f3298d0767d34505121c7b0b81 <
    df831e97739405ecbaddb85516bc7d4d1c933d6b
  - Linux 6.3
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-11-05T20:52:25.572397Z'
published: '2025-07-09'
updated: '2026-11-05'
sourceUpdated: '2026-11-05T21:02:29.328Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2025-38255'
references:
  - url: 'https://git.kernel.org/stable/c/64a99eff8dcf1f951a544e6058341b2b19a8fdbd'
  - url: 'https://git.kernel.org/stable/c/29d39e0d5f16c060e32542b2cf351c09fd22b250'
  - url: 'https://git.kernel.org/stable/c/911ef2e8a7de5b2bae8ff11fb0bd01f699e6db65'
  - url: 'https://git.kernel.org/stable/c/df831e97739405ecbaddb85516bc7d4d1c933d6b'
tags:
  - cve.org
epss: 0.00166
epssPercentile: 0.05828
ingestedAt: '2026-11-05T22:38:31.945Z'
---

## Overview

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

lib/group_cpus: fix NULL pointer dereference from group_cpus_evenly()

While testing null_blk with configfs, echo 0 > poll_queues will trigger
following panic:

BUG: kernel NULL pointer dereference, address: 0000000000000010
Oops: Oops: 0000 [#1] SMP NOPTI
CPU: 27 UID: 0 PID: 920 Comm: bash Not tainted 6.15.0-02023-gadbdb95c8696-dirty #1238 PREEMPT(undef)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.1-2.fc37 04/01/2014
RIP: 0010:__bitmap_or+0x48/0x70
Call Trace:
 <TASK>
 __group_cpus_evenly+0x822/0x8c0
 group_cpus_evenly+0x2d9/0x490
 blk_mq_map_queues+0x1e/0x110
 null_map_queues+0xc9/0x170 [null_blk]
 blk_mq_update_queue_map+0xdb/0x160
 blk_mq_update_nr_hw_queues+0x22b/0x560
 nullb_update_nr_hw_queues+0x71/0xf0 [null_blk]
 nullb_device_poll_queues_store+0xa4/0x130 [null_blk]
 configfs_write_iter+0x109/0x1d0
 vfs_write+0x26e/0x6f0
 ksys_write+0x79/0x180
 __x64_sys_write+0x1d/0x30
 x64_sys_call+0x45c4/0x45f0
 do_syscall_64+0xa5/0x240
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Root cause is that numgrps is set to 0, and ZERO_SIZE_PTR is returned from
kcalloc(), and later ZERO_SIZE_PTR will be deferenced.

Fix the problem by checking numgrps first in group_cpus_evenly(), and
return NULL directly if numgrps is zero.

[yukuai3@huawei.com: also fix the non-SMP version]

## Affected

- `Linux >= 6a6dcae8f486c3f3298d0767d34505121c7b0b81 < 64a99eff8dcf1f951a544e6058341b2b19a8fdbd`
- `Linux >= 6a6dcae8f486c3f3298d0767d34505121c7b0b81 < 29d39e0d5f16c060e32542b2cf351c09fd22b250`
- `Linux >= 6a6dcae8f486c3f3298d0767d34505121c7b0b81 < 911ef2e8a7de5b2bae8ff11fb0bd01f699e6db65`
- `Linux >= 6a6dcae8f486c3f3298d0767d34505121c7b0b81 < df831e97739405ecbaddb85516bc7d4d1c933d6b`
- `Linux 6.3`

## Remediation

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