[CVE-2026-74600][MODERATE REGULAR] mm/page_table_check: skip special zero mappings

[CVE-2026-74600][MODERATE REGULAR] mm/page_table_check: skip special zero mappings

[AL-KERNEL]

CVE: CVE-2026-74600
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mm/page_table_check: skip special zero mappings
Commit: b726eb3c94d23e09da0e0f46b0fa09fb2b5d99cc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b726eb3c94d23e09da0e0f46b0fa09fb2b5d99cc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74600
Analysis date: Sat, 22 Aug 2026 15:35:57 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended
Manual review required: YES

Summary:
An unprivileged local process can abuse special zero-page PTE or huge zero PMD mappings to overflow page_table_check file mapping accounting and hit a BUG_ON, causing a kernel panic on systems with page_table_check enabled.

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

The original Linux kernel CVE announcement for CVE-2026-74600 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74600

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 REGULAR
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-2026-74600	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-400;CWE-617;CWE-682;BEST CVSS score: '5.5';DESCR 'page_table_check can incorrectly account shared zero page PTEs and huge zero PMDs as normal file backed mappings. An unprivileged local process can populate many zero mappings until file_map_count overflows and an existing BUG_ON is reached, causing a kernel panic and denial of service. For the CVSS the PR:L is used because local code execution as an unprivileged user is enough to create the mappings, but the issue is not remotely reachable over the network. Impact is denial of service only. No confidentiality or integrity impact is assigned because the patch shows an accounting and assertion failure path rather than UAF, OOB write, arbitrary write, or information disclosure. The issue depends on page_table_check being enabled, so affected exposure may be limited to kernels built with that hardening feature.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended (with actual score 4)	YES	SIMPLEFIX HARDWARE LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: **3**
* Paranoid score: **4**
* Final recommended bucket: **Borderline manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged trigger: +1**
  The commit explicitly says an unprivileged process can populate enough zero mappings to trigger the issue.

* **Reliable kernel crash / strong DoS: +1**
  The described outcome is overflow of `file_map_count` followed by an existing `BUG_ON()`, which is a kernel panic style DoS.

* **Availability impact realistic: +1**
  This is a host-wide crash from local memory mapping activity, not just a harmless warning.

* **Broad/core MM path when enabled: +1**
  The affected logic is in `mm/page_table_check.c`, in user page-table accounting paths.

* **Rare or configuration-dependent feature: -1**
  Practical exposure depends on `page_table_check` being enabled. This is a hardening/debug-style kernel feature and may not be enabled in all production kernels.

Paranoid difference:

* The paranoid score drops the configuration penalty because affected products that enable `page_table_check` expose this through ordinary unprivileged memory mapping behavior.

No added signals:

* **No generic memory corruption bonus**
  The patch shows accounting of special zero mappings and a reachable `BUG_ON()`, not UAF, OOB write, double free, arbitrary write, type confusion, or attacker-controlled object reuse.

* **No privilege escalation bonus**
  There is no supported LPE primitive beyond denial of service.

* **No confidentiality or integrity bonus**
  No information disclosure or unauthorized write primitive is shown.

## 3. Reachability analysis

An unprivileged local process can trigger the issue by creating enough shared zero-page PTE mappings or huge zero PMD mappings to overflow the accounting counter. No network path is involved. Namespaces or containers do not remove the local trigger if the process can perform ordinary memory mappings inside the affected kernel. The main exposure limiter is whether the kernel is built and run with `page_table_check` enabled.

Call-site confidence: **high**, because the patch includes the directly affected set and clear paths and the commit message states the unprivileged trigger and resulting `BUG_ON()`.

## 4. Severity interpretation

This behaves like an **actionable local DoS / borderline Moderate**, not an Important-class memory corruption issue. The theoretical impact is a reliable local kernel crash. The realistic evidence does not support privilege escalation, confidentiality loss, or integrity loss. Manual review is still reasonable because the trigger is unprivileged and the crash is kernel-wide, but this should not be treated as a strong Important candidate without additional evidence of corruption before the assertion.

## 5. One-sentence report phrase

An unprivileged local process can abuse special zero-page PTE or huge zero PMD mappings to overflow page_table_check file mapping accounting and hit a BUG_ON, causing a kernel panic on systems with page_table_check enabled.

## 6. Manual review recommendation

**MANUAL CHECK RECOMMENDED**

Reason: the issue is an unprivileged local kernel panic in core MM accounting when the feature is enabled, but the patch supports DoS only and does not show a memory corruption or privilege escalation primitive.

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

Patch: mm/page_table_check: skip special zero mappings
Commit: b726eb3c94d23e09da0e0f46b0fa09fb2b5d99cc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b726eb3c94d23e09da0e0f46b0fa09fb2b5d99cc

Commit description:

page_table_check_set() and page_table_check_clear() account mappings based
on PageAnon().  Shared zero-page PTEs and huge zero PMDs are special
mappings, but page_table_check can still account them as file-backed
pages.

An unprivileged process can populate enough zero mappings to overflow
file_map_count and hit the existing BUG_ON().  The PTE path can do this
with the shared zero page, and the PMD path can do the same with huge zero
mappings.

Skip special zero mappings in the user page-table accounting paths.  Keep
the PTE-side pte_special() check, and identify huge zero PMDs from the
mapped folio instead of pmd_special().  That covers architectures where
pmd_special() is a no-op without adding huge_zero_pfn checks to the
generic counter helpers.

Link: https://lore.kernel.org/cover.1784717203.git.zhilinz@nebusec.ai
Link: https://lore.kernel.org/e94478e4fb7912fb7e8ebebed5ce85d00dc9a69d.1784717203.git.zhilinz@nebusec.ai
Fixes: df4e817 ("mm: page table check")
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Signed-off-by: Ren Wei <enjou1224z@gmail.com>
Reported-by: Vega <vega@nebusec.ai>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Assisted-by: Codex:gpt-5.4
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/page_table_check.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=b726eb3c94d23e09da0e0f46b0fa09fb2b5d99cc

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

The original Linux kernel CVE announcement for CVE-2026-74600 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74600

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: Not available
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 3
  Paranoid ActionableScore: 4

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).