# CVE-2025-38255: lib/group_cpus: fix NULL pointer dereference from group_cpus_evenly()

Status: published. Published: 2025-07-09. Updated: 2026-11-05.

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]

## Remediation

No remediation reported in collected sources

## Affected products and versions

- Linux / Linux: ≥ 6a6dcae8f486c3f3298d0767d34505121c7b0b81; < 64a99eff8dcf1f951a544e6058341b2b19a8fdbd (affected, git); ≥ 6a6dcae8f486c3f3298d0767d34505121c7b0b81; < 29d39e0d5f16c060e32542b2cf351c09fd22b250 (affected, git); ≥ 6a6dcae8f486c3f3298d0767d34505121c7b0b81; < 911ef2e8a7de5b2bae8ff11fb0bd01f699e6db65 (affected, git); ≥ 6a6dcae8f486c3f3298d0767d34505121c7b0b81; < df831e97739405ecbaddb85516bc7d4d1c933d6b (affected, git). Source: CNA: Linux
- Linux / Linux: 6.3 (affected); ≥ 0; < 6.3 (unaffected, semver); ≥ 6.6.96; ≤ 6.6.* (unaffected, semver); ≥ 6.12.36; ≤ 6.12.* (unaffected, semver); ≥ 6.15.5; ≤ 6.15.* (unaffected, semver); ≥ 6.16; ≤ * (unaffected, original_commit_for_fix). Source: CNA: Linux

## Severity and prioritization

- CISA KEV: No listing in collected sources

- EPSS: 0.17% as of 2026-11-05; percentile 5.85%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: Not reported

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://git.kernel.org/stable/c/64a99eff8dcf1f951a544e6058341b2b19a8fdbd (CNA: Linux)

- https://git.kernel.org/stable/c/29d39e0d5f16c060e32542b2cf351c09fd22b250 (CNA: Linux)

- https://git.kernel.org/stable/c/911ef2e8a7de5b2bae8ff11fb0bd01f699e6db65 (CNA: Linux)

- https://git.kernel.org/stable/c/df831e97739405ecbaddb85516bc7d4d1c933d6b (CNA: Linux)

## Provenance

- CNA: Linux: https://www.cve.org/CVERecord?id=CVE-2025-38255; retrieved 2026-11-05; source updated 2026-11-05

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2025-38255; retrieved 2026-11-05; source updated Not reported

Revision: e424377d7a8d5c8da4efed2c56624dadb3c9cfe1d2bf0abad79d432b0a36dc9e

Canonical: https://y2.dev/cve/CVE-2025-38255

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