[CVE-2026-97943][MODERATE 7.0] x86/mm/pat: Acquire init_mm write lock on collapse to avoid UAF

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

CVE: CVE-2026-97943
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: x86/mm/pat: Acquire init_mm write lock on collapse to avoid UAF
Commit: 35820cf8dd521f5a651e221483015b0586794516
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=35820cf8dd521f5a651e221483015b0586794516
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97943
Analysis date: Sat, 26 Sep 2026 14:55:40 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-97943 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97943

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.

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

CVE-2026-97943	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-97943 CHECK WITH IMPACT FROM ORIG NN LOW	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-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'An unlocked x86 CPA large page collapse can free lower level kernel page tables while ptdump or another CPA walker is still accessing them, creating a race and a use after free of page table memory. The directly supported impact is at least a local denial of service through an invalid page table access or kernel crash. Reuse of the freed page table memory may also create confidentiality or integrity impact and a potential privilege escalation path because the affected objects control kernel mappings, although a reliable arbitrary write primitive is not demonstrated. For the CVSS the PR:L value is used for the paranoid score because BPF, execmem, container root, or a delegated service may reach executable mapping changes without having unrestricted host administration rights. AC:H applies because successful triggering depends on winning a narrow race between page table collapse and a concurrent walker. The issue is not network reachable and requires local execution or control of a process that can reach the relevant CPA path. Impact is at least a local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to page table use after free and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)  SKIP CVE-2026-97943 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	READ INIT BPF RACE UAF HARDWARE KPANIC MEMORY 	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 5
* Conservative floor applied: 3 -> 4 under the real UAF floor.
* Paranoid floor applied: 4 -> 5 because an active page-table hierarchy can be traversed after its lower-level tables are freed, and concurrent CPA walkers leave the depth of subsequent processing unresolved.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF. CPA collapse frees lower-level page tables while ptdump may still traverse them.
* +1 Strong DoS. Traversal of freed page-table memory can realistically cause an invalid kernel access and host crash.
* Total: 3.
* Floor 4 applies because this is an actual stale use of active structured page-table memory, not fixed stale storage, and host-admin-equivalent privilege is not positively established.

Paranoid signals:

* +2 Real UAF.
* +1 Strong DoS.
* +1 High-value MMU lifetime corruption context. The affected storage is an active kernel page-table hierarchy, and the commit also identifies concurrent CPA walkers rather than only a diagnostic stale pointer.
* Total: 4.
* Floor 5 applies because stale page-table state can enter nontrivial structured traversal by concurrent kernel walkers. This warrants exploitation-leverage review despite the absence of a demonstrated controlled reclaim.

Not awarded:

* No remote-reachability points.
* No local-unprivileged point because the complete below-host-admin trigger is not established.
* No race penalty because the commit does not establish that repeated attempts are impractical or that the timing is externally uncontrollable.
* No structured stale-object leverage point because attacker-controlled reclaim, replacement contents, or useful post-free field consumption is not demonstrated.
* No strong-LPE point, arbitrary-write point, confidentiality point, or integrity point.
* No extra Availability point because it would duplicate the same crash.
* No concrete repeated release/refcount point. The patch does not show a second put, free, or refcount mutation through the same dangling reference.
* No BPF-specific reachability bonus. BPF is a caller, but the complete load, JIT, execution, collapse, and concurrent-walker privilege model is unresolved.
* FLOOR 5 is not based on a repeated release and the repeated-refcount paranoid bonus is withheld.

3. Reachability analysis

The vulnerable collapse is reachable only through `CPA_COLLAPSE`, set by `set_memory_rox()`. Identified users include execmem and BPF executable mappings. The competing operation may be ptdump or another CPA collapse.

Ordinary unprivileged reachability is not established. BPF loading and JIT use are commonly privilege-gated, but the supplied evidence does not establish whether host-admin-equivalent privilege is mandatory for every complete trigger. Access to the relevant ptdump operation is also not resolved. Therefore `HostAdminRequired=UNKNOWN`, with no privilege penalty or unprivileged bonus.

No evidence establishes that all required BPF, executable-mapping, ptdump, and CPA operations are available through attacker-created namespaces. Therefore `NamespaceScopedPrivilege=UNKNOWN`.

No special hardware requirement is shown. The race can involve concurrent CPA operations, but the evidence does not establish whether an attacker can reliably and repeatedly drive both sides. A hard-race penalty is therefore not applied.

4. Primitive and severity interpretation

The exact primitive is a page-table UAF: collapse replaces lower-level page-table entries with a huge entry and frees the old page-table pages without synchronization against concurrent walkers.

This is active structured storage, not a fixed asynchronous buffer. The stale path can traverse page-table entries and hierarchy state after lifetime termination. It is not limited to cleanup or destructor processing.

There is no concrete repeated release/refcount sequence. The patch does not establish that the stale walker performs another free or lifetime mutation on the same table page.

The issue is CLASS_B because a real active-object lifetime violation exists, but controlled reclaim, attacker-shaped replacement contents, directed kernel access, callback control, and arbitrary read/write are not demonstrated. The supported practical consequence is kernel crash or host DoS. Privilege escalation and confidentiality or integrity compromise remain speculative.

`ImportantCandidate=YES` because the UAF affects active kernel page tables and concurrent CPA walkers, leaving meaningful exploitation leverage unresolved. `AutoDowngradeAllowed=NO` because automatically treating this as an ordinary crash-only race could discard a concrete MMU lifetime-corruption primitive that requires manual review.

5. One-sentence report phrase

An unsynchronized x86 CPA large-page collapse can free lower-level kernel page tables while ptdump or another CPA walker still traverses them, causing a page-table use-after-free and at least a local kernel crash, while stronger exploitation remains unresolved.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The bug is a concrete active page-table UAF in kernel MMU management, but the available evidence does not resolve attacker privilege, reclaim control, or whether concurrent CPA processing can convert the stale traversal into a useful corruption primitive.

7. Call-site confidence

Call-site confidence: medium

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

Patch: x86/mm/pat: Acquire init_mm write lock on collapse to avoid UAF
Commit: 35820cf8dd521f5a651e221483015b0586794516
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=35820cf8dd521f5a651e221483015b0586794516

Commit description:

x86 implements page attribute modification using its Change Page
Attributes (CPA) mechanism.

This tracks properties of ranges such as cache mode through x86 page
attributes, and as part of that logic manipulates kernel page tables.

Since commit:

  41d8848 ("x86/mm/pat: restore large ROX pages after fragmentation")

ranges of kernel page table entries can be collapsed into
huge page table entries as part of this logic.

As part of this collapse, it frees the page tables which the collapsed
entries previously pointed to, and it does so without any relevant locks
being held to preclude concurrent kernel page table walkers.

The only way this code can be reached is if CPA_COLLAPSE is specified, and
this is only set in set_memory_rox() via:

set_memory_rox()
-> change_page_attr_set_clr()
-> cpa_flush()
-> cpa_collapse_large_pages()

Notable users of this are execmem and BPF when manipulating executable
mappings.

However, this is problematic for ptdump as it walks ranges it does not own
and thus runs the risk of a use-after-free on page tables freed underneath
it.

In addition, concurrent CPA collapse operations are possible which can also
cause races.

Resolve the issue by acquiring the mmap write lock on init_mm across the
whole operation.

It is safe to acquire a sleeping lock as all the callers invoke
set_memory_rox() from process context and in any case,
change_page_attr_set_clr() calls vm_unmap_alias() which ultimately takes a
mutex, disallowing atomic context here.

Fixes: 41d8848 ("x86/mm/pat: restore large ROX pages after fragmentation")
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Will Deacon <will@kernel.org>
Reviewed-by: David Carlier <devnexen@gmail.com>
Tested-by: Atish Patra <atishp@meta.com>
Tested-by: Nikunj A Dadhania <nikunj@amd.com>
Cc:stable@vger.kernel.org
Link: https://patch.msgid.link/20260813-cpa-fixes-v2-1-39b4ff90f91d@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/mm/pat/set_memory.c
  include/linux/mmap_lock.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=35820cf8dd521f5a651e221483015b0586794516

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

The original Linux kernel CVE announcement for CVE-2026-97943 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97943

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: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: 5

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).