CVE-2026-80718 in Linux
Summary
by MITRE • 08/28/2026
In the Linux kernel, the following vulnerability has been resolved:
mm/percpu-km: fix bitmap overflow and accounting in pcpu_create_chunk()
In pcpu_create_chunk(), nr_pages is the total contiguous backing allocation, i.e., nr_units * pcpu_unit_pages, but pcpu_chunk_populated() uses it to set chunk->populated, whose size is pcpu_unit_pages, bitmap. Since bit N in chunk->populated means page offset N inside every unit is backed. When nr_units > 1, the function writes beyond chunk->populated. Fix it by using chunk->nr_pages.
It also fixes the global pcpu_nr_empty_pop_pages accounting, since pcpu_balance_free() only iterates up to chunk->nr_pages.
Commit a63d4ac4ab609 ("percpu: make percpu-km set chunk->populated bitmap properly") introduced the bitmap overflow issue. Later, commit b539b87fed37f ("percpu: implmeent pcpu_nr_empty_pop_pages and chunk->nr_populated") added pcpu_nr_empty_pop_pages and caused the accounting issue.
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Analysis
by VulDB Data Team • 08/28/2026
The Linux kernel's per-CPU memory management subsystem contains a critical implementation flaw within the pcpu_create_chunk function that results in both buffer overflow conditions and incorrect resource accounting. This vulnerability stems from a fundamental mismatch between the scope of data structures used for tracking page population status and the actual size of those structures relative to the allocated chunks. Specifically, the variable nr_pages represents the total number of contiguous pages backing an entire chunk, calculated as the product of the number of units and the unit page count. However, the function pcpu_chunk_populated incorrectly utilizes this aggregate value when setting the chunk->populated bitmap field. The populated bitmap is designed to track population status on a per-unit basis, with its size limited to pcpu_unit_pages rather than the total chunk capacity. Consequently, when multiple units are present in a single chunk, the write operation exceeds the allocated bounds of the bitmap array, leading to memory corruption that can destabilize kernel operations or potentially be exploited for privilege escalation if an attacker can influence allocation parameters.
Beyond the immediate buffer overflow risk, this defect introduces significant accounting errors within the global per-CPU subsystem state management. The variable pcpu_nr_empty_pop_pages tracks the number of empty populated pages across all chunks to assist in memory balancing decisions. Because the flawed logic writes beyond the intended bitmap boundaries, it corrupts adjacent memory structures that are critical for maintaining accurate counts of free and allocated resources. This corruption directly impacts the effectiveness of pcpu_balance_free(), which relies on iterating up to chunk->nr_pages to identify and reclaim unused pages. When the underlying accounting data is corrupted due to the overflow, the kernel may fail to properly balance memory usage across CPUs, leading to inefficient resource utilization or potential denial-of-service conditions where legitimate processes are starved of necessary memory resources despite available physical RAM being theoretically accessible but logically mismanaged by the broken accounting logic.
The root cause of this vulnerability was introduced in commit a63d4ac4ab609, which attempted to correct how percpu-km sets the chunk populated bitmap properly but inadvertently created an off-by-one or range error regarding unit boundaries. This issue was further exacerbated and made more impactful by subsequent changes in commit b539b87fed37f, which implemented pcpu_nr_empty_pop_pages and associated accounting mechanisms without fully reconciling them with the existing overflow-prone logic in pcpu_create_chunk. The interaction between these two commits created a scenario where not only was memory written outside its allocated bounds, but that corrupted data was then actively used by higher-level subsystems for critical decision-making processes regarding memory allocation and balancing. This layered complexity makes the vulnerability particularly dangerous as it affects both low-level memory integrity and high-level system stability metrics simultaneously.
From a classification perspective, this issue aligns with CWE-120 Buffer Overflow without check of buffer boundaries in C/C++, specifically manifesting as an out-of-bounds write that corrupts kernel data structures. It also relates to CWE-665 Improper Initialization regarding the incorrect population status tracking and potentially CWE-789 Uncontrolled Memory Allocation if the overflow leads to unpredictable system states affecting resource availability. In terms of adversarial tactics, this vulnerability could be leveraged within the ATT&CK framework under techniques related to Defense Evasion or Privilege Escalation, as memory corruption in kernel space often provides a pathway for attackers to gain control over execution flow or modify critical security controls. The impact extends beyond simple crashes; it undermines the reliability of the per-CPU allocator, which is foundational to Linux performance and stability on multi-core systems.
Mitigation strategies primarily involve applying the upstream kernel patch that corrects the usage of chunk->nr_pages instead of nr_units multiplied by unit pages when interacting with the populated bitmap. System administrators should ensure their kernels are updated to versions where this specific accounting logic has been rectified, particularly for deployments relying on percpu-km allocator configurations with multiple units per chunk. For environments unable to immediately patch, monitoring kernel logs for signs of memory corruption or unexpected oopses related to page allocation and balancing may provide early detection indicators. Long-term architectural improvements should include stricter bounds checking in low-level memory management functions and enhanced static analysis coverage for bitmap manipulation routines to prevent similar off-by-one errors from recurring in future development cycles.