[CVE-2026-93227][LOW] mm/mm_init: deferred_grow_zone(): fix out-of-range first_deferred_pfn

[CVE-2026-93227][LOW] mm/mm_init: deferred_grow_zone(): fix out-of-range first_deferred_pfn

[AL-KERNEL]

CVE: CVE-2026-93227
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: mm/mm_init: deferred_grow_zone(): fix out-of-range first_deferred_pfn
Commit: 45489d4f95802903ab35799b400b346fa4e13151
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=45489d4f95802903ab35799b400b346fa4e13151
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93227
Analysis date: Thu, 24 Sep 2026 13:17:21 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
An administrator-controlled deferred memory initialization layout can place first_deferred_pfn beyond the node boundary and trigger a deterministic boot-time kernel BUG, with no demonstrated memory corruption or cross-boundary security impact.

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

The original Linux kernel CVE announcement for CVE-2026-93227 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93227

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-2026-93227	LOW	https://www.kernelcve.org/list/?q=CVE-2026-93227 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';CWE-682;CWE-129;*CWE-20;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.4';DESCR 'A range tracking logic error in deferred_grow_zone can leave pgdat first_deferred_pfn pointing past the end of node memory after the entire zone memory map has already been initialized. A later deferred_init_memmap pass treats that value as a valid PFN and reaches a kernel BUG, causing a boot time panic or preventing the system from completing deferred memory initialization. The trigger requires CONFIG_DEFERRED_STRUCT_PAGE_INIT, a suitable non section aligned RAM layout, and a large CMA reservation that prevents the discovered ranges from satisfying the requested allocation. For the CVSS the PR:H value is used because reliable triggering normally requires administrative control over the kernel configuration, boot parameters, CMA placement, or virtual machine memory layout. The issue is not network reachable. Impact is denial of service only because the shown path produces an invalid tracking value and a controlled BUG check without evidence of UAF, OOB memory access, information disclosure, or a write primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)  SKIP CVE-2026-93227 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	READ OOB DANGER INIT HARDWARE TEST MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

## 1. ActionableScore

* Raw conservative score: -2
* Conservative score: 0
* Raw paranoid score: -2
* Paranoid score: 0
* Applied conservative lower bound: -2 -> 0
* Applied paranoid lower bound: -2 -> 0
* Final recommended bucket: **Ordinary Moderate**::

## 2. Signal breakdown

* **+1 Reliable kernel crash / strong DoS:** The invalid `first_deferred_pfn` reaches a documented `BUG_ON()` in `deferred_init_memmap()` and panics the kernel during initialization.
* **-2 Proven administrator-equivalent control:** Constructing the demonstrated trigger requires control over the target kernel configuration, boot command line, CMA reservation, and physical or virtual RAM layout.
* **-1 Rare and difficult-to-reach configuration:** The failure requires deferred `struct page` initialization, a large CMA reservation, a non-section-aligned RAM endpoint, and the specific condition where all zone metadata is initialized while the requested allocation remains unsatisfied.

The paranoid score remains identical because no stronger primitive is technically supported.

Not awarded:

* No memory-corruption signal. The out-of-range PFN reaches a bounds assertion, with no demonstrated OOB access.
* No UAF, stale object, repeated release, refcount mutation, or ownership violation.
* No confidentiality or integrity signal.
* No remote or ordinary local-user reachability.
* No additional Availability point because it would duplicate the same kernel panic.
* Memory-layout invariant restoration was not scored independently because it does not provide exploitation relevance beyond the already-counted assertion failure.
* No controlled payload, target selection, arbitrary write, disclosure, or privilege-escalation mechanism.

## 3. Reachability analysis

The demonstrated trigger is an early-boot configuration scenario. On physical hardware it requires administrator control over the kernel and boot parameters. In a virtualized setup, the VMM operator can construct the RAM size, alignment, kernel image, and CMA command line, but this is administrator-equivalent control over the affected guest rather than a guest-to-host attack.

The complete trigger is not namespace-scoped and cannot be created through user or network namespaces after boot. It is not reachable through network traffic or an ordinary runtime syscall interface. No special physical hardware is required, but the memory layout and boot configuration are unusual and must be established before the vulnerable kernel initializes memory.

This is not a race. Once the required layout is constructed, the failure is deterministic.

## 4. Primitive and severity interpretation

The exact defect is an out-of-range bookkeeping value. After the loop processes the entire zone, `spfn` can advance to a section beyond `zone_end_pfn()`. The old code stores that value in `pgdat->first_deferred_pfn` based only on `nr_pages`, and a later initialization pass rejects it with `BUG_ON()`.

This is `CLASS_C`: invalid-state behavior with a demonstrated kernel panic, but no independently useful corruption primitive. It involves neither active structured stale storage nor fixed stale storage because no post-lifetime reference exists. There is no first or second lifetime operation and no repeated release/refcount condition.

The patch-supported consequence is boot-time denial of service. Memory corruption, privilege escalation, and confidentiality or integrity effects are unsupported. `ImportantCandidate=NO` because no unresolved exploitation primitive exists. `AutoDowngradeAllowed=YES` because the issue is a configuration-dependent administrator-triggered assertion failure with no security-boundary crossing.

Likely operational severity is Low-like despite the CVSS availability result, because an attacker who can construct the trigger already controls the affected kernel boot environment.

## 5. One-sentence report phrase

An administrator-controlled deferred memory initialization layout can place first_deferred_pfn beyond the node boundary and trigger a deterministic boot-time kernel BUG, with no demonstrated memory corruption or cross-boundary security impact.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a bounded boot-time bookkeeping error and assertion failure without UAF, OOB memory access, authorization bypass, or another unresolved corruption primitive.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: mm/mm_init: deferred_grow_zone(): fix out-of-range first_deferred_pfn
Commit: 45489d4f95802903ab35799b400b346fa4e13151
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=45489d4f95802903ab35799b400b346fa4e13151

Commit description:

With CONFIG_DEFERRED_STRUCT_PAGE_INIT enabled, deferred_grow_zone()
initializes struct pages early in boot to satisfy an allocation.

With a large CMA reservation in place, the ranges deferred_init_memmap()
finds may not add up to the allocation it was asked for, and the function
ends up initializing the memory map of the entire zone and still falls
short.

That is fine in itself: the function accounts for it and leaves the
caller to decide whether it now has enough memory.

However, the update of pgdat->first_deferred_pfn that tracks where
uninitialized memory map starts could overflow.

If the node's RAM end is not aligned on PAGES_PER_SECTION boundaries and
some deferred struct pages were initialized, pgdat->first_deferred_pfn
would point past the end of the node's memory.

deferred_init_memmap() later picks up from pgdat->first_deferred_pfn and
hits a BUG_ON(), because it expects a pfn within its node.

For example, when running a kernel with CONFIG_DEFERRED_STRUCT_PAGE_INIT=y
and CONFIG_CMA=y using the following qemu command line

  qemu-system-x86_64 -enable-kvm -m 8032M -kernel bzImage \
      -append "nokaslr cma=4768M@0x100000000"

the kernel panics:

  kernel BUG at mm/mm_init.c:2131!
  CPU: 3 UID: 0 PID: 36 Comm: pgdatinit0 Not tainted 7.2.0-rc6 #1
  RIP: 0010:deferred_init_memmap+0x1b8/0x1c0
  RAX: 0000000000236000 R13: 0000000000238000
  Call Trace:
   kthread+0xdf/0x120
   ret_from_fork+0x187/0x250

Make sure that the update of pgdta->first_deferred_pfn does not overflow
when the entire zone's (and therefore node's) memory map is initialized.

Fixes: 3acb913 ("mm/mm_init: use deferred_init_memmap_chunk() in deferred_grow_zone()")
Cc: stable@vger.kernel.org
Assisted-by: Kiro:claude-opus-5
Signed-off-by: Alexander Graf <graf@amazon.com>
Link: https://patch.msgid.link/20260807031243.87904-1-graf@amazon.com
[rppt: massaged the changelog]
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/mm_init.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=45489d4f95802903ab35799b400b346fa4e13151

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

The original Linux kernel CVE announcement for CVE-2026-93227 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93227

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.4

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: unknown
  Paranoid ActionableScore: 0

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).