The patch titled Subject: lib/group_cpus: fix NULL pointer dereference from group_cpus_evenly() has been added to the -mm mm-hotfixes-unstable branch. Its filename is lib-group_cpus-fix-null-pointer-dereference-from-group_cpus_evenly.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days

------------------------------------------------------ From: Yu Kuai yukuai3@huawei.com Subject: lib/group_cpus: fix NULL pointer dereference from group_cpus_evenly() Date: Thu, 19 Jun 2025 21:26:55 +0800

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(), then __group_cpus_evenly() will deference the ZERO_SIZE_PTR.

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

Link: https://lkml.kernel.org/r/20250619132655.3318883-1-yukuai1@huaweicloud.com Fixes: 6a6dcae8f486 ("blk-mq: Build default queue map via group_cpus_evenly()") Signed-off-by: Yu Kuai yukuai3@huawei.com Reviewed-by: Ming Lei ming.lei@redhat.com Reviewed-by: Jens Axboe axboe@kernel.dk Cc: ErKun Yang yangerkun@huawei.com Cc: John Garry john.g.garry@oracle.com Cc: Thomas Gleinxer tglx@linutronix.de Cc: "zhangyi (F)" yi.zhang@huawei.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

lib/group_cpus.c | 3 +++ 1 file changed, 3 insertions(+)

--- a/lib/group_cpus.c~lib-group_cpus-fix-null-pointer-dereference-from-group_cpus_evenly +++ a/lib/group_cpus.c @@ -352,6 +352,9 @@ struct cpumask *group_cpus_evenly(unsign int ret = -ENOMEM; struct cpumask *masks = NULL;

+ if (numgrps == 0) + return NULL; + if (!zalloc_cpumask_var(&nmsk, GFP_KERNEL)) return NULL;

_

Patches currently in -mm which might be from yukuai3@huawei.com are

lib-group_cpus-fix-null-pointer-dereference-from-group_cpus_evenly.patch