[CVE-2026-46035][MODERATE 7.0] mm/page_alloc: return NULL early from alloc_frozen_pages_nolock() in NMI on UP

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

CVE: CVE-2026-46035
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mm/page_alloc: return NULL early from alloc_frozen_pages_nolock() in NMI on UP
Commit: 05b4ed8bef30bba4f559c8d835e2dd20c48cf8a4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=05b4ed8bef30bba4f559c8d835e2dd20c48cf8a4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46035
Analysis date: Wed, 27 May 2026 15:54:31 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-46035 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46035

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: NO

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

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

CVE-2026-46035	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-667;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'alloc_frozen_pages_nolock_noprof() can be called from NMI context on UP kernels where spin_trylock() always succeeds, even if the interrupted context already owns the zone lock. This can reenter rmqueue() and corrupt page allocator freelists, causing at least a local kernel crash and potentially broader memory corruption. For the CVSS the PR:L is used in the paranoid score because a local workload may influence allocation and NMI timing on affected systems, although reliable triggering is configuration dependent and high complexity. The issue is not network reachable and requires a UP kernel without CONFIG_SMP. Impact is treated as DoS for the base score, with C:H and I:H retained for paranoid manual review because the corrupted object is core allocator metadata.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-46035  SKIP The Fixes patch not applied yet, so unlikely that actual: d7242af8643409aae32243450341ef25b28d8a8c YES  NO NO unknown 	MAYBE	DANGER HARDWARE LINUS TEST KPANIC MEMORY 	-	-	checked

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

ActionableScore=6
ActionableScoreLower=4

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

2. Signal breakdown
* Local trigger: +1. A local workload can influence allocation paths and may participate in reaching the vulnerable allocator state, though reliable triggering is not straightforward.
* Generic memory corruption: +2. The commit explicitly says reentry into `rmqueue()` can corrupt page allocator freelists.
* Weak LPE concern: +1. Core allocator freelist corruption can sometimes become security relevant, but the patch does not show attacker-controlled payload, target selection, or a reliable reclaim primitive.
* Reliable kernel crash / strong DoS: +1. With debug spinlocks this becomes a BUG, and without debug checks allocator corruption can plausibly crash the kernel.
* Broad/default/common subsystem: +1. The affected code is in the core page allocator.
* Hard or unreliable timing: -1. The issue requires NMI reentry while the interrupted context already holds the zone lock.
* Rare configuration: -1. The issue is specific to UP kernels without `CONFIG_SMP`, which are uncommon in mainstream server deployments.

3. Reachability analysis
The bug requires a UP kernel and an NMI hitting an allocation path while the interrupted context already owns allocator state. A local user may be able to create allocator pressure, but controlling NMI timing is difficult and may depend on debug, testing, perf, watchdog, or platform-specific conditions. Containers do not materially improve reachability unless they can influence host allocation and NMI behavior. The issue is not network reachable.

4. Severity interpretation
This is not a simple warning-only bug because the patch context explicitly identifies page allocator freelist corruption. Realistic exploitation is likely DoS due to the rare UP configuration and difficult NMI timing. Paranoid handling is justified because corruption of core allocator metadata can theoretically become more serious than a crash, even though no practical privilege escalation primitive is demonstrated.

5. One-sentence report phrase
On UP kernels, `alloc_frozen_pages_nolock()` can reenter the page allocator from NMI because `spin_trylock()` always succeeds, corrupting allocator freelists and causing local DoS with limited but nonzero memory-corruption concern.

6. Manual review recommendation
MANUAL CHECK RECOMMENDED. This is core allocator metadata corruption, but exposure is limited by UP-only configuration and hard NMI timing, with no demonstrated controlled overwrite or LPE chain.

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

Patch: mm/page_alloc: return NULL early from alloc_frozen_pages_nolock() in NMI on UP
Commit: 05b4ed8bef30bba4f559c8d835e2dd20c48cf8a4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=05b4ed8bef30bba4f559c8d835e2dd20c48cf8a4

Commit description:

On UP kernels (!CONFIG_SMP), spin_trylock() is a no-op that
unconditionally succeeds even when the lock is already held. As a
result, alloc_frozen_pages_nolock() called from NMI context can
re-enter rmqueue() and acquire the zone lock that the interrupted
context is already holding, corrupting the freelists.

With CONFIG_DEBUG_SPINLOCK on UP, the following BUG is triggered with
the slub_kunit test module:

  BUG: spinlock trylock failure on UP on CPU#0, kunit_try_catch/243
  [...]
  Call Trace:
   <NMI>
   dump_stack_lvl+0x3f/0x60
   do_raw_spin_trylock+0x41/0x50
   _raw_spin_trylock+0x24/0x50
   rmqueue.isra.0+0x2a9/0xa70
   get_page_from_freelist+0xeb/0x450
   alloc_frozen_pages_nolock_noprof+0x111/0x1e0
   allocate_slab+0x42a/0x500
   ___slab_alloc+0xa7/0x4c0
   kmalloc_nolock_noprof+0x164/0x310
   [...]
   </NMI>

Fix this by returning NULL early when invoked from NMI on a UP kernel.

Link: https://lore.kernel.org/linux-mm/ad_cqe51pvr1WaDg@hyeyoo
Cc: stable@vger.kernel.org
Fixes: d7242af ("mm: Introduce alloc_frozen_pages_nolock()")
Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
Link: https://patch.msgid.link/20260427-nolock-api-fix-v2-1-a6b83a92d9a4@kernel.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/page_alloc.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=05b4ed8bef30bba4f559c8d835e2dd20c48cf8a4

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

The original Linux kernel CVE announcement for CVE-2026-46035 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46035

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: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).