CVE-2026-72016 in Linux
Summary
by MITRE • 08/15/2026
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.
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Analysis
by VulDB Data Team • 09/02/2026
The vulnerability described relates to a critical flaw in the linux kernel's cpu hotplug subsystem specifically affecting arm64 architecture when handling systems with more configured cpus than actually present. This issue manifests during system boot when the maxcpus parameter exceeds the number of physically present CPUs, creating a scenario where some CPUs are marked as 'present' but not yet registered through the standard registration process. The root cause stems from improper handling of cpu present masks during early boot initialization, particularly in systems utilizing acpi firmware interfaces with complex madt structures that define different cpu capabilities.
The technical implementation flaw occurs within the cpuhp_smt_enable() function where the kernel iterates over all cpus marked as 'present' without proper validation of whether these cpus have actually been registered. When _cpu_up() is invoked on unregistered cpus, it eventually triggers sysfs_create_group() with invalid kobject references, leading to NULL pointer dereference warnings in fs/sysfs/group.c. This represents a classic case of improper resource management where the kernel attempts to create sysfs entries for cpu devices that do not have proper underlying kernel object initialization. The warning trace indicates execution flow through internal_create_group and sysfs_create_group functions, demonstrating how the flaw propagates from the hotplug subsystem into the filesystem layer.
The operational impact of this vulnerability is significant as it can cause system instability during boot processes, particularly in virtualized environments like qemu where complex smp configurations are common. The issue affects systems with specific acpi madt configurations where cpu entries have different enabled vs online capable flags, creating inconsistencies between kernel state and actual hardware capabilities. This inconsistency particularly impacts SMT (simultaneous multithreading) hotplug control mechanisms, potentially leading to incorrect CPU state management and preventing proper hotplug operations. The vulnerability essentially creates a mismatch between the kernel's internal representation of cpu availability and the actual physical cpu configuration, which can cascade into broader system reliability issues.
The mitigation strategy involves implementing proper validation checks during acpi smp initialization by examining the ACPI_MADT_ENABLED flag in gicc entries before marking cpus as present. This approach aligns with established practices used in other architectures such as x86 and loongarch, ensuring consistency across different cpu architectures. The fix requires updating both acpi_map_cpu() and acpi_unmap_cpu() functions to properly manage the present mask during actual hotplug events rather than relying on a static boot-time assumption. Additionally, documentation updates are necessary to reflect the corrected behavior that only physically available or explicitly enabled cpus should be included in the present mask. This solution addresses the fundamental mismatch between kernel state and hardware reality while maintaining compatibility with existing acpi firmware interfaces.
This vulnerability demonstrates characteristics consistent with CWE-476 NULL Pointer Dereference (cwe-476) and CWE-833 Deadlock or Lock Inversion (cwe-833) patterns, though the primary manifestation is through improper resource handling in sysfs subsystem. The issue also relates to ATT&CK technique T1059.001 Command and Scripting Interpreter - PowerShell (attack-t1059.001) through potential system stability impacts that could affect legitimate administrative operations, though the direct attack vector is more related to improper initialization sequences than active exploitation. The fix implements proper state validation and resource management practices that align with kernel security best practices for managing cpu hotplug operations in complex multi-core systems, particularly those utilizing advanced firmware interfaces like acpi.