[CVE-2026-89593][LOW] hugetlb: only adjust reservation during unmapping if mapcount is 0

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

CVE: CVE-2026-89593
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: hugetlb: only adjust reservation during unmapping if mapcount is 0
Commit: a3c65af20cceb7f997847727636b4017326f845d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a3c65af20cceb7f997847727636b4017326f845d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89593
Analysis date: Fri, 11 Sep 2026 21:39:04 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89593 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89593

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-2026-89593	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89593 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-191;CWE-682;**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 'hugetlb reservation accounting can underflow when the same anonymous huge folio is mapped by both a parent and a child and one mapping is unmapped before the other. The issue is caused by restoring the reservation before the last unmap, while the folio is still mapped elsewhere, which can leave the reserved count in an incorrect state until the remaining mapping is removed. For the CVSS the PR:L is used because a local user or local process must be able to create and unmap HugeTLB mappings, usually depending on hugetlbfs access, configured huge pages, and local resource limits. The issue is not network reachable and is triggered through local memory management operations rather than packet or remote input. Impact is primarily denial of service or incorrect HugeTLB resource accounting, with no clear evidence of a UAF, OOB write, information leak, or privilege escalation primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	READ OOB SIMPLEFIX IMPROVEONLY LINUS MEMORY  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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

ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1. No floor or cap applied.
* Raw paranoid score: 2
* Paranoid score: 2. No floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Conservative signals:

* +1. Kernel resource accounting invalid state. The patch and commit establish a hugetlb reservation count underflow when the same anonymous huge folio remains mapped by a child while the parent unmaps first.
* 0. No reliable kernel crash is established. The described consequence is reservation underflow and later restoration when the child unmaps.
* 0. No concrete persistent reference leak is established. The count may be temporarily incorrect, but the commit says it is restored after the child unmaps.
* 0. No UAF, stale pointer use, OOB access, arbitrary write, controlled reclaim, or refcount double release is established.
* 0. No confidentiality or integrity primitive is established. Incorrect hugetlb reservation accounting alone does not show unauthorized modification of another principal data or disclosure.
* 0. No network reachability is established.
* 0. No race penalty. This is a parent child unmap ordering problem, not a clearly narrow attacker uncontrollable timing race.
* 0. No host admin penalty. Host admin requirement is not positively established from the supplied evidence.

Paranoid additional signal:

* +1. Possible local availability impact in environments where a non admin local process can create and unmap HugeTLB mappings through configured hugetlbfs access, huge page limits, or group based access. This remains resource accounting DoS only and does not become memory corruption.

Non-awarded material signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* UAF not demonstrated.
* Concrete repeated release or refcount mutation not demonstrated.
* Security boundary bypass not demonstrated.
* Remote reachability not established.
* C and I impact not awarded because the patch supports accounting underflow and possible availability impact, not a disclosure or modification primitive.
* Extra availability not awarded because no independent persistent host wide failure is established.

3. Reachability analysis

The likely trigger is local memory management activity involving anonymous hugetlb mappings shared between a parent and child, followed by unmapping in the parent before the child unmaps. A normal local process may be able to reach this if HugeTLB pages and hugetlbfs access are configured for that user, but the supplied evidence does not prove that every required step is reachable by an ordinary unprivileged user.

Host admin equivalent privilege is therefore UNKNOWN, not YES and not NO. Namespace scoped privilege is NO because the supplied patch does not involve a namespace capability gate or a namespace local object. Prior setup may exist in practice because HugeTLB pages and permissions are usually configured by the system, but the patch does not prove that host administrator action is required for every trigger attempt. No special hardware is required. No direct network trigger exists.

4. Primitive and severity interpretation

The exact primitive is a hugetlb reservation accounting underflow caused by adjusting reservation state before the last mapping of an anonymous huge folio is gone. This is not an active structured UAF and not fixed stale storage. It is an accounting and resource state inconsistency.

There is no concrete repeated release or refcount operation. There is no evidence that the first operation frees an object and that the same dangling reference later receives a second lifetime mutation. The demonstrated consequence is incorrect reserved count handling, with possible local availability impact for HugeTLB users. The speculative consequence is broader HugeTLB resource disruption if the accounting error affects allocation or reservation decisions, but the supplied evidence does not support memory corruption, privilege escalation, or information disclosure.

PrimitiveClass is CLASS_C because the behavior is invalid resource state and possible DoS, without an independently useful corruption or authorization primitive. ImportantCandidate is NO because no Class A or strong unresolved Class B primitive is present. AutoDowngradeAllowed is YES because this appears suitable for ordinary deferred handling unless product specific evidence shows broad unprivileged HugeTLB exposure or a stronger downstream consequence.

5. One-sentence report phrase

A hugetlb reservation accounting bug can underflow the reserved count when a parent unmaps an anonymous huge folio that is still mapped by a child, causing possible local HugeTLB resource accounting disruption but no established memory corruption or privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the patch supports a local resource accounting underflow with possible availability impact only, and does not establish UAF, OOB access, authorization bypass, network reachability, or a concrete escalation primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: hugetlb: only adjust reservation during unmapping if mapcount is 0
Commit: a3c65af20cceb7f997847727636b4017326f845d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a3c65af20cceb7f997847727636b4017326f845d

Commit description:

Since df7a6d1 , __unmap_hugepage_range can adjust reservations.  In
the case of folio mapped in both a parent and a child, if the parent
unmaps the range first, the reservation adjustment will result in an
underflow of the reserved count.  Once the child unmaps the range, the
count is restored.  Change __unmap_hugepage_range() to check the mapcount
before adjusting the reservation.

Link: https://lore.kernel.org/all/alEJkwn5VlTTH_ZX@bender.morinfr.org/
Link: https://lore.kernel.org/amkC_1Ya6OiUoiLZ@bender.morinfr.org
Fixes: df7a6d1 ("mm/hugetlb: restore the reservation if needed")
Signed-off-by: Guillaume Morin <guillaume@morinfr.org>
Reviewed-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Rik van Riel <riel@surriel.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
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/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=a3c65af20cceb7f997847727636b4017326f845d

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

The original Linux kernel CVE announcement for CVE-2026-89593 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89593

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: 1
  Paranoid ActionableScore: 2

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).