[CVE-2026-80718][MODERATE 7.0] mm/percpu-km: fix bitmap overflow and accounting in pcpu_create_chunk()

[CVE-2026-80718][MODERATE 7.0] mm/percpu-km: fix bitmap overflow and accounting in pcpu_create_chunk()

[AL-KERNEL]

CVE: CVE-2026-80718
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mm/percpu-km: fix bitmap overflow and accounting in pcpu_create_chunk()
Commit: 5f43d2c1bea280dcdfabaf156c25e7402fb8039f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5f43d2c1bea280dcdfabaf156c25e7402fb8039f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80718
Analysis date: Fri, 28 Aug 2026 06:00:25 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
No one-sentence report phrase was found.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-80718 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80718

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE 7.0
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-80718	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-119;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'pcpu_create_chunk used the total backing allocation page count when marking the per unit populated bitmap. When more than one percpu unit is present, this can write past chunk populated and corrupt adjacent kernel memory. For the CVSS the PR:L is used because a local user or local process is needed to create allocation pressure that may force percpu chunk creation, although reliable triggering depends on kernel configuration and workload. The issue is not directly network reachable. Impact is at least local denial of service via kernel memory corruption and kernel crash. In the worst case it may allow confidentiality or integrity impact because the primitive is a kernel bitmap overflow rather than a pure resource leak. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-80718 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	SIMPLEFIX LINUS KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=7
ActionableScoreLower=5

1. ActionableScore
* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

2. Signal breakdown
* Local unprivileged trigger: +1. A local process may be able to create percpu allocation pressure that eventually forces `pcpu_create_chunk()`, although reliability is workload and configuration dependent.
* Constrained kernel buffer overwrite: +1 conservative. The bug is an explicit bitmap overflow, but the write pattern is constrained by bitmap setting and does not show attacker controlled payload or target selection.
* Generic memory corruption: +2 paranoid. The patch states that `pcpu_chunk_populated()` writes beyond `chunk->populated` when `nr_units > 1`, so the paranoid interpretation treats this as kernel OOB write memory corruption.
* Memory layout invariant restoration: +1. The fix restores consistency between total backing allocation size and per-unit bitmap size by using `chunk->nr_pages` instead of `nr_pages`.
* Integrity impact plausible: +1. Adjacent kernel metadata may be corrupted by the bitmap overflow.
* Availability impact realistic: +1. Kernel allocator metadata corruption can plausibly cause crashes or allocator malfunction.
* Broad/default/common subsystem exposure: +1. This is in the percpu allocator area, a core kernel memory-management subsystem, even if the exact `percpu-km` backend may be configuration dependent.
* Weak LPE concern: +1 paranoid only. The primitive is an OOB write in kernel allocator metadata, so privilege escalation cannot be dismissed, but no reclaim, arbitrary write, callback control, or demonstrated exploit path is shown.
* Hard or unreliable trigger: -1. Reliable triggering likely depends on specific percpu-km configuration, `nr_units > 1`, allocation pressure, and allocator state.
* No attacker-controlled payload or overwrite target: -1. The overflow appears to set bitmap bits derived from allocator accounting rather than writing attacker chosen data.

3. Reachability analysis
The affected path is local and kernel-internal through percpu chunk creation. There is no realistic network trigger from the supplied patch. A local unprivileged user may be able to influence allocation pressure indirectly, but reliable triggering is not as direct as a syscall parser bug and depends on whether the target uses the affected `percpu-km` backend and has `nr_units > 1`. Containers and namespaces may matter if container workloads can create enough kernel objects to force percpu allocations, but this is not proven from the patch alone. Call-site confidence: medium, because the changed function and bug mechanism are visible, but detailed external allocation call paths are not included.

4. Severity interpretation
This is not an ordinary low-risk accounting bug because the commit explicitly describes a bitmap overflow in kernel memory. Conservative handling is Actionable Moderate because the write is constrained and exploitation reliability is uncertain. Paranoid handling reaches Strong Important candidate because allocator metadata OOB writes have historically been underestimated and may become privilege escalation primitives after manual analysis. The realistic demonstrated impact is likely local DoS or allocator corruption, while privilege escalation remains plausible but not demonstrated.

5. One-sentence report phrase
A bitmap overflow in `pcpu_create_chunk()` can write past the per-unit `chunk->populated` bitmap when the total backing allocation size is used instead of `chunk->nr_pages`, creating local kernel memory corruption with realistic DoS impact and plausible worst-case privilege-escalation concern.

6. Manual review recommendation
MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.
Reason: explicit kernel OOB bitmap write in a core memory-management allocator path, with constrained but real memory corruption and enough worst-case LPE concern to avoid auto-close.

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: mm/percpu-km: fix bitmap overflow and accounting in pcpu_create_chunk()
Commit: 5f43d2c1bea280dcdfabaf156c25e7402fb8039f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5f43d2c1bea280dcdfabaf156c25e7402fb8039f

Commit description:

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 a63d4ac ("percpu: make percpu-km set chunk->populated bitmap
properly") introduced the bitmap overflow issue.  Later, commit
b539b87 ("percpu: implmeent pcpu_nr_empty_pop_pages and
chunk->nr_populated") added pcpu_nr_empty_pop_pages and caused the
accounting issue.

Link: https://lore.kernel.org/20260709-fix-pcpu_create_chunk-in-percpu-km-v1-1-1f64745a84cc@nvidia.com
Fixes: a63d4ac ("percpu: make percpu-km set chunk->populated bitmap properly")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260703-keep-subpage-private-zero-at-free-v2-0-2970fe777dd6%40nvidia.com?part=1
Assisted-by: Codex:GPT-5
Signed-off-by: Zi Yan <ziy@nvidia.com>
Acked-by: Dennis Zhou <dennis@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/percpu-km.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5f43d2c1bea280dcdfabaf156c25e7402fb8039f

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-80718 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80718

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 5
  Paranoid ActionableScore: 7

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).