[CVE-2026-46318][LOW] Revert "mm/hugetlbfs: update hugetlbfs to use mmap_prepare"

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-46318
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: Revert "mm/hugetlbfs: update hugetlbfs to use mmap_prepare"
Commit: 3af5fc3f0ac98c624c109c8c0796fa46e814344c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3af5fc3f0ac98c624c109c8c0796fa46e814344c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46318
Analysis date: Tue, 09 Jun 2026 12:53:54 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A local hugetlbfs mmap error path could leak a per-VMA hugetlb lock after mmap_prepare, causing possible gradual resource exhaustion under specific huge page configuration and late failure conditions.

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

The original Linux kernel CVE announcement for CVE-2026-46318 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46318

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: LOW
Manual review required: NO

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

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

CVE-2026-46318	LOW	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'hugetlbfs mmap handling could leak a per VMA hugetlb lock when a later failure occurs after mmap_prepare has already prepared the mapping state. A local process may be able to trigger the leak by repeatedly exercising hugetlbfs mmap error paths, but reliable triggering depends on specific hugetlbfs availability and a late allocation failure window. For the CVSS the PR:L is used because the attacker needs local code execution and access to a hugetlbfs mapping path, but full administrative privileges are not necessarily required if hugetlbfs files or huge page mappings are available to the user. The issue is not network reachable. Impact is denial of service through gradual kernel memory leakage or resource exhaustion, with no supported evidence of information disclosure or privilege escalation. NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)  SKIP CVE-2026-46318  SKIP The Fixes patch not applied yet, so unlikely that actual: ea52cb24cd3fb121283754ab82b2cb3044609359 YES  NO NO unknown 	MAYBE	WRITE OOB DISK INIT RACE LEAK HARDWARE LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=1
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate / Low-like**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process may trigger hugetlbfs mmap paths if it has access to hugetlbfs files or configured huge page mappings.

* Availability impact realistic: +1 in paranoid scoring only
  Repeated leakage of per-VMA hugetlb lock objects could theoretically contribute to kernel memory pressure or resource exhaustion.

Negative signals:

* Hard or unreliable trigger condition: -1
  The leak requires a failure after mmap_prepare has already prepared the hugetlb mapping state. This is a narrow late-failure path, not a simple deterministic crash trigger.

* Rare or configuration-dependent subsystem: -1
  hugetlbfs and usable huge page mappings are not universally available to unprivileged users and often require explicit system configuration.

Not triggered:

* No generic memory corruption
  The patch describes a leaked lock allocation, not UAF, double free, OOB write, arbitrary write, or object confusion.

* No privilege escalation plausibility
  There is no supported primitive for controlled overwrite, reclaim, credential manipulation, or cross-object corruption.

* No network reachability
  The affected path is local mmap handling.

## 3. Reachability analysis

The bug is reachable through local hugetlbfs mmap activity, assuming the system has hugetlbfs or huge page mappings configured and accessible to the user. It does not require packet input or remote protocol exposure.

Privileges are environment-dependent. A normal user may be able to access hugetlbfs mappings if the administrator exposed appropriate files or permissions, so PR:L is reasonable for CVSS-style analysis. In many deployments, however, huge page usage is restricted or not configured for untrusted users, reducing practical exposure.

Containers and namespaces do not automatically make this broadly reachable. If huge page resources are delegated into a container, a container process could potentially exercise the path, but the impact remains resource leakage rather than memory corruption.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like resource leak, not an Important-class kernel vulnerability.

The theoretical worst case is local DoS via repeated kernel memory leakage. The realistic evidence points to a narrow cleanup bug where a per-VMA hugetlb lock allocation can be leaked after a late mmap failure. There is no indication of UAF, stale pointer traversal, attacker-controlled overwrite, information disclosure, or privilege escalation.

The issue should not be elevated merely because it is in memory-management code. The primitive is resource leakage only.

## 5. One-sentence report phrase

A local hugetlbfs mmap error path could leak a per-VMA hugetlb lock after mmap_prepare, causing possible gradual resource exhaustion under specific huge page configuration and late failure conditions.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a resource leak without memory corruption, network exposure, privilege escalation evidence, or sensitive-object access. It can be handled as deferred or low-priority unless the affected environment exposes hugetlbfs heavily to untrusted local users.

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

Patch: Revert "mm/hugetlbfs: update hugetlbfs to use mmap_prepare"
Commit: 3af5fc3f0ac98c624c109c8c0796fa46e814344c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3af5fc3f0ac98c624c109c8c0796fa46e814344c

Commit description:

This reverts commit ea52cb2 ("mm/hugetlbfs: update hugetlbfs to use
mmap_prepare") with conflict resolution to account for changes in commit
ea52cb2 ("mm/hugetlbfs: update hugetlbfs to use mmap_prepare").

The patch incorrectly handled hugetlb VMA lock allocation at the
mmap_prepare stage, where a failed allocation occurring after mmap_prepare
is called might result in the lock leaking.

There is no risk of a merge causing a similar issues, as
VMA_DONTEXPAND_BIT is set for hugetlb mappings.

As a first step in addressing this issue, simply revert the change so we
can rework how we do this having corrected the underlying issues.

We maintain the VMA flags changes as best we can, accounting for the fact
that we were working with a VMA descriptor previously and propagating
like-for-like changes for this.

Note that we invoke vma_set_flags() and do not call vma_start_write() as
vm_flags_set() does.  This is OK as it's being done in an .mmap hook where
the VMA is not yet linked into the tree so nobody else can be accessing
it.

Link: https://lore.kernel.org/20260512160643.266960-1-ljs@kernel.org
Fixes: ea52cb2 ("mm/hugetlbfs: update hugetlbfs to use mmap_prepare")
Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
Reported-by: Mingyu Wang <25181214217@stu.xidian.edu.cn>
Closes: https://lore.kernel.org/linux-mm/20260425070700.562229-1-25181214217@stu.xidian.edu.cn/
Acked-by: Muchun Song <muchun.song@linux.dev>
Acked-by: Oscar Salvador <osalvador@suse.de>
Cc: David Hildenbrand <david@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/20260425070700.562229-1-25181214217
  fs/hugetlbfs/inode.c
  include/linux/hugetlb.h
  include/linux/hugetlb_inline.h
  mm/hugetlb.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=3af5fc3f0ac98c624c109c8c0796fa46e814344c

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

The original Linux kernel CVE announcement for CVE-2026-46318 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46318

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 0
  Paranoid ActionableScore: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).