linux-cve-announce - CVE-2025-38255: lib/group_cpus: fix NULL pointer dereference from group_cpus

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Message-ID: <2025070935-CVE-2025-38255-57aa@gregkh>
Date: Wed,  9 Jul 2025 12:42:47 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-38255: lib/group_cpus: fix NULL pointer dereference from group_cpus_evenly()

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

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@...wei.com: also fix the non-SMP version]

The Linux kernel CVE team has assigned CVE-2025-38255 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.3 with commit 6a6dcae8f486c3f3298d0767d34505121c7b0b81 and fixed in 6.6.96 with commit 64a99eff8dcf1f951a544e6058341b2b19a8fdbd
	Issue introduced in 6.3 with commit 6a6dcae8f486c3f3298d0767d34505121c7b0b81 and fixed in 6.12.36 with commit 29d39e0d5f16c060e32542b2cf351c09fd22b250
	Issue introduced in 6.3 with commit 6a6dcae8f486c3f3298d0767d34505121c7b0b81 and fixed in 6.15.5 with commit 911ef2e8a7de5b2bae8ff11fb0bd01f699e6db65
	Issue introduced in 6.3 with commit 6a6dcae8f486c3f3298d0767d34505121c7b0b81 and fixed in 6.16-rc4 with commit df831e97739405ecbaddb85516bc7d4d1c933d6b

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-38255
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	lib/group_cpus.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/64a99eff8dcf1f951a544e6058341b2b19a8fdbd
	https://git.kernel.org/stable/c/29d39e0d5f16c060e32542b2cf351c09fd22b250
	https://git.kernel.org/stable/c/911ef2e8a7de5b2bae8ff11fb0bd01f699e6db65
	https://git.kernel.org/stable/c/df831e97739405ecbaddb85516bc7d4d1c933d6b