[CVE-2026-64368][MODERATE REGULAR] mm/slab: do not limit zeroing to orig_size when only red zoning is enabled [ Upstream

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

CVE: CVE-2026-64368
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mm/slab: do not limit zeroing to orig_size when only red zoning is enabled [ Upstream
Commit: 6256899c3a34674bba6076884aedbba49fc695e4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6256899c3a34674bba6076884aedbba49fc695e4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64368
Analysis date: Sat, 25 Jul 2026 09:19:59 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline Actionable Moderate candidate. Manual review recommended. Not an Important candidate
Manual review required: YES

Summary:
A SLUB zeroing logic bug can break the krealloc __GFP_ZERO guarantee under red-zoning-only debug configurations, potentially exposing uninitialized kernel heap bytes through suitable local allocation and reallocation paths.

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

The original Linux kernel CVE announcement for CVE-2026-64368 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64368

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64368	MODERATE	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:H/I:L/A:N';CWE-665;CWE-908;*CWE-200;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.3';DESCR 'A zeroing logic bug in SLUB can break the krealloc __GFP_ZERO guarantee when only red zoning is enabled without requested size tracking. In this configuration kmalloc may zero only the requested size even though the actual bucket object is larger, and a later krealloc can expose previously uninitialized bytes from the same object to kernel code. For the CVSS the PR:L is used because a local user or local process is normally needed to drive a kernel allocation and reallocation path that reaches this allocator behavior. The issue is not directly network reachable and requires a specific non default SLUB debug configuration plus a suitable kmalloc and krealloc caller. Impact is primarily a potential confidentiality issue from uninitialized memory exposure, with limited integrity concern in the paranoid score if affected kernel callers make decisions based on such data.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Actionable Moderate candidate. Manual review recommended. Not an Important candidate (with actual score 4)	YES	SIMPLEFIX IMPROVEONLY LINUS MEMORY  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline Actionable Moderate candidate. Manual review recommended. Not an Important candidate**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process may be able to drive kernel kmalloc and krealloc paths indirectly through syscalls, but no concrete vulnerable caller is shown in the patch.

* Memory layout invariant restoration: +1
  The patch restores the invariant between requested allocation size, actual kmalloc object size, and krealloc __GFP_ZERO semantics.

* Confidentiality impact plausible: +1
  The bug can leave bytes beyond orig_size unzeroed and later visible to kernel code after krealloc, which may become an information exposure if a caller copies or exposes that data.

* Broad allocator subsystem: +1
  SLUB is a core allocator path, so the bug class deserves more scrutiny than a narrow driver-local issue.

Subtracted signals:

* Non-default debug configuration required: -1
  The issue requires SLAB_RED_ZONE without SLAB_STORE_USER or equivalent red-zoning-only debug behavior, which is not a normal production default.

Paranoid-only signal:

* Integrity impact plausible but weak: +1
  If a caller makes decisions based on supposedly zeroed data, limited integrity impact is possible, but the patch does not show corruption, overwrite, UAF, or attacker-controlled object replacement.

Not counted:

* No generic memory corruption bonus.
  This is not UAF, double-free, OOB write, type confusion, or arbitrary write.

* No reliable kernel crash bonus.
  The commit describes a broken zeroing contract, not a panic or strong DoS.

* No privilege escalation bonus.
  No concrete LPE primitive is shown.

## 3. Reachability analysis

The likely trigger is local. A local user or process would need to reach a kernel path that allocates a kmalloc object with zeroing semantics and later expands it with krealloc using __GFP_ZERO.

The path is not network reachable by itself. Network services might indirectly exercise affected allocation patterns, but the patch does not identify such a call path.

The issue depends on a non-default SLUB debug configuration where red zoning is enabled without requested-size tracking. This significantly reduces practical exposure in normal production kernels.

Namespaces and containers may not materially change the primitive. A container workload could trigger allocator paths, but exploitation still requires the host kernel to run the affected debug configuration and a suitable kernel caller that exposes the uninitialized bytes.

Call-site confidence: medium. The allocator behavior is clear, but no concrete caller exposing data to userspace is provided in the patch.

## 4. Severity interpretation

This behaves more like a borderline actionable Moderate than an Important issue.

Theoretical risk is information exposure from uninitialized memory after krealloc. The realistic evidence is weaker because the bug requires a specific non-default SLUB debug mode and a suitable caller path. There is no demonstrated memory corruption, no crash, no arbitrary write, and no direct privilege-escalation path.

The paranoid score keeps the issue from being auto-closed because allocator-generic zeroing contract violations can affect many callers and sometimes hide sensitive data exposure paths.

## 5. One-sentence report phrase

A SLUB zeroing logic bug can break the krealloc __GFP_ZERO guarantee under red-zoning-only debug configurations, potentially exposing uninitialized kernel heap bytes through suitable local allocation and reallocation paths.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is generic allocator behavior with possible confidentiality impact across unknown callers, but it is not strong enough for Important classification without a concrete caller-level leak, memory corruption primitive, or LPE path.

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

Patch: mm/slab: do not limit zeroing to orig_size when only red zoning is enabled [ Upstream
Commit: 6256899c3a34674bba6076884aedbba49fc695e4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6256899c3a34674bba6076884aedbba49fc695e4

Changed files:
  mm/slab.h

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=6256899c3a34674bba6076884aedbba49fc695e4

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

The original Linux kernel CVE announcement for CVE-2026-64368 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64368

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:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:L/A:N
  The Best / paranoid CVSS score: 5.3

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).