[CVE-2026-46281][MODERATE 7.0] vmalloc: fix buffer overflow in vrealloc_node_align()

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

CVE: CVE-2026-46281
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: vmalloc: fix buffer overflow in vrealloc_node_align()
Commit: e9b057a44deff4c59c13f44672a5cc74dcd57522
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e9b057a44deff4c59c13f44672a5cc74dcd57522
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46281
Analysis date: Mon, 08 Jun 2026 15:57:48 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A shrink operation in vrealloc_node_align_noprof can allocate a smaller vmalloc buffer but copy old_size bytes into it, causing a kernel out-of-bounds write that may lead to crash and should be manually reviewed for possible local privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-46281 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46281

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-46281	MODERATE	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:H/I:H/A:H';*CWE-787;CWE-122;CWE-131;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A kernel out-of-bounds write can occur in vrealloc_node_align_noprof when a shrink request still forces a new allocation because the old pointer does not satisfy node or alignment constraints. The function allocates a new buffer of size bytes but copied old_size bytes into it, so size smaller than old_size can corrupt memory past the new vmalloc allocation. For the CVSS the PR:L is used for the paranoid score because the helper is not a direct syscall, but a local user may be able to reach it through an in-kernel caller that accepts user influenced sizes or allocation constraints. The issue is not directly network reachable based on the patch context. Impact is at least local denial of service via kernel memory corruption and in worst case may allow confidentiality or integrity impact and privilege escalation, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-46281  SKIP The Fixes patch not applied yet, so unlikely that actual: 4c5d3365882dbbc0784688784904f440d7a4c0f1 YES  NO NO unknown 	MAYBE	WRITE OOB SIMPLEFIX LINUS KPANIC MEMORY  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Memory corruption, strong corruption primitive: +2. The patch fixes a real kernel out-of-bounds write caused by copying old_size bytes into a new allocation of size bytes.
* Local unprivileged trigger: +1. No direct syscall is shown, but a local caller path may expose user-influenced realloc size or alignment constraints.
* Reliable kernel crash / strong DoS: +1. An out-of-bounds write in vmalloc memory can realistically crash the kernel.
* Broad/default/common subsystem: +1. vmalloc is core kernel memory-management infrastructure.
* Hard or special triggering conditions: -1. Triggering requires shrink plus forced realloc due to NUMA node or alignment mismatch.
* Corruption primitive constrained or uncertain: -1. The helper is not a direct user API and the overwrite payload and target depend on in-kernel callers.

Paranoid signals:

* Memory corruption, strong corruption primitive: +2.
* Privilege escalation plausible: +2. A kernel OOB write can potentially corrupt adjacent kernel objects, although no concrete exploit primitive is demonstrated.
* Local unprivileged trigger: +1.
* Reliable kernel crash / strong DoS: +1.
* Broad/default/common subsystem: +1.
* Hard or special triggering conditions: -1.

## 3. Reachability analysis

The vulnerable function is a core vmalloc realloc helper, not a direct syscall or network parser. Realistic triggering depends on whether an in-kernel caller allows a local user to influence the old allocation size, new shrink size, NUMA node, or alignment parameters enough to enter the need_realloc path. There is no evidence of remote reachability. Namespaces or containers could matter only if a namespaced interface reaches a caller using vrealloc_node_align_noprof with user-controlled sizes. The affected subsystem is common, but the exact exposed call path needs manual confirmation.

## 4. Severity interpretation

This is not an ordinary DoS-only bug. It is a concrete kernel out-of-bounds write, but practical exploitability is not established from the patch alone because the caller surface and overwrite controllability are unclear. Conservatively it is borderline manual-review severity. Paranoid handling puts it in Actionable Moderate because memory corruption in core mm code can plausibly become LPE if a controllable caller exists.

## 5. One-sentence report phrase

A shrink operation in vrealloc_node_align_noprof can allocate a smaller vmalloc buffer but copy old_size bytes into it, causing a kernel out-of-bounds write that may lead to crash and should be manually reviewed for possible local privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. This is a real kernel memory corruption fix in core memory-management code, and the key unresolved question is whether any reachable caller gives an unprivileged local user enough control over size and realloc conditions.

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

Patch: vmalloc: fix buffer overflow in vrealloc_node_align()
Commit: e9b057a44deff4c59c13f44672a5cc74dcd57522
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e9b057a44deff4c59c13f44672a5cc74dcd57522

Commit description:

Commit 4c5d336 ("mm/vmalloc: allow to set node and align in
vrealloc") added the ability to force a new allocation if the current
pointer is on the wrong NUMA node, or if an alignment constraint is not
met, even if the user is shrinking the allocation.

On this path (need_realloc), the code allocates a new object of 'size'
bytes and then memcpy()s 'old_size' bytes into it.  If the request is to
shrink the object (size < old_size), this results in an out-of-bounds
write on the new buffer.

Fix this by bounding the copy length by the new allocation size.

Link: https://lore.kernel.org/20260420114805.3572606-2-elver@google.com
Fixes: 4c5d336 ("mm/vmalloc: allow to set node and align in vrealloc")
Signed-off-by: Marco Elver <elver@google.com>
Reported-by: Harry Yoo (Oracle) <harry@kernel.org>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
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/vmalloc.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=e9b057a44deff4c59c13f44672a5cc74dcd57522

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

The original Linux kernel CVE announcement for CVE-2026-46281 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46281

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: 4
  Paranoid ActionableScore: 6

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).