CVE-2026-72016: cpu/hotplug: Fix NULL kobject warning in cpuhp_smt_enable()

From: Greg Kroah-Hartman <[email protected]>

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

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

cpu/hotplug: Fix NULL kobject warning in cpuhp_smt_enable()

On arm64, when booting with `maxcpus` greater than the number of present
CPUs (e.g., QEMU -smp cpus=4,maxcpus=8), some CPUs are marked as 'present'
but have not yet been registered via register_cpu(). Consequently,
the per-cpu device objects for these CPUs are not yet initialized.

In cpuhp_smt_enable(), the code iterates over all present CPUs. Calling
_cpu_up() for these unregistered CPUs eventually leads to
sysfs_create_group() being called with a NULL kobject (or a kobject
without a directory), triggering the following warning in
fs/sysfs/group.c:

  WARNING: fs/sysfs/group.c:137 at internal_create_group+0x41c/0x4bc, CPU#2: sh/181
  [...]
  Call trace:
    internal_create_group+0x41c/0x4bc (P)
    sysfs_create_group+0x18/0x24
    topology_add_dev+0x1c/0x28
    cpuhp_invoke_callback+0x104/0x20c
    __cpuhp_invoke_callback_range+0x94/0x11c
    _cpu_up+0x200/0x37c

When booting with ACPI, arm64 smp_prepare_cpus() currently sets all
enumerated CPUs as "present" regardless of their status in the MADT. This
causes issues with SMT hotplug control. For instance, with QEMU's
"-smp 4,maxcpus=8" configuration, the MADT GICC entries are populated as
follows:

1. The first four CPUs: `Enabled` set but `Online Capable` not set.

2. The remaining four CPUs: `Online Capable` set but `Enabled` not set
   to support potential hot-plugging.

Fix this by:

1. When booting with ACPI, checking the ACPI_MADT_ENABLED flag in the GICC
   entry before calling set_cpu_present() during SMP initialization.

2. Properly managing the present mask in acpi_map_cpu() and
   acpi_unmap_cpu() to support actual CPU hotplug events, This aligns with
   other architectures like x86 and LoongArch.

3. Update the arm64 CPU hotplug documentation to no longer state that all
   online-capable vCPUs are marked as present by the kernel at boot time.

This ensures that only physically available or explicitly enabled CPUs
are in the present mask, keeping the SMT control logic consistent with
the actual hardware state.

The Linux kernel CVE team has assigned CVE-2026-72016 to this issue.


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

	Issue introduced in 6.15 with commit eed4583bcf9a60f8d6dd3a3c7c94dea28134b1eb and fixed in 6.18.40 with commit ccdf1770a4ba27e31599d24ad970d77a371c7912
	Issue introduced in 6.15 with commit eed4583bcf9a60f8d6dd3a3c7c94dea28134b1eb and fixed in 7.1.5 with commit 901a489d89ee9c854624c8444090e38e70aed234
	Issue introduced in 6.15 with commit eed4583bcf9a60f8d6dd3a3c7c94dea28134b1eb and fixed in 7.2-rc3 with commit f9a82544c7174851f5c7524622f5966dcafd3a47

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-2026-72016
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:
	Documentation/arch/arm64/cpu-hotplug.rst
	arch/arm64/kernel/acpi.c
	arch/arm64/kernel/smp.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/ccdf1770a4ba27e31599d24ad970d77a371c7912
	https://git.kernel.org/stable/c/901a489d89ee9c854624c8444090e38e70aed234
	https://git.kernel.org/stable/c/f9a82544c7174851f5c7524622f5966dcafd3a47