[CVE-2026-74599][MODERATE REGULAR] mm/ptdump: always stabilise against page table freeing using init_mm

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

CVE: CVE-2026-74599
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mm/ptdump: always stabilise against page table freeing using init_mm
Commit: 7f740664aec1f832953c2e6d9b8920cd6c8bcc0c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7f740664aec1f832953c2e6d9b8920cd6c8bcc0c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74599
Analysis date: Sat, 22 Aug 2026 12:06:31 -0400
ActionableScore: 4
ActionableScore lower bound: 1
Actionable bucket: Borderline manual review recommended / not Strong Important on current evidence
Manual review required: YES

Summary:
ptdump can race with kernel page table freeing when dumping kernel mappings through a non init_mm mm, creating a local page-table lifetime UAF candidate that is most likely to cause a kernel crash but deserves manual review due to the sensitivity of page-table state.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74599 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74599

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-2026-74599	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;*CWE-416;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'ptdump can race with kernel page table freeing when it walks kernel mappings through an mm other than init_mm. Kernel page table ranges are shared across mm objects, so locking only the walked mm may not stabilize the kernel page tables that are being dumped. The likely impact is a local denial of service through a kernel crash or inconsistent page table traversal. For the CVSS the PR:L reflects a conservative review case where ptdump or related debug access is delegated to a reduced capability root, container root, or service account rather than full host admin. The issue is not network reachable and requires local access to a ptdump capable debug interface. Impact is at least local denial of service via kernel crash and in worst case may allow limited confidentiality or integrity impact due to a page table use after free candidate and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / not Strong Important on current evidence (with actual score 3)	YES	READ LOCK DEADLOCK DANGER NULLPTR INIT RACE IMPROVEONLY MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended / not Strong Important on current evidence**::

## 2. Signal breakdown

Conservative signals:

* Weak or indirect corruption candidate: +1. ptdump can walk kernel page tables while they are being freed, so stale page-table memory may be read or interpreted during traversal.
* Real lifetime corruption: +1. The patch explicitly fixes a race with page table freeing by adding init_mm locking.
* Reliable kernel crash / strong DoS: +1. The realistic failure mode is an invalid page-table walk, crash, WARN, or inconsistent traversal.
* Hard or unreliable race / special timing required: -1. Triggering requires a timing window between ptdump traversal and kernel page table freeing.
* Requires admin/root/CAP-like debug access in typical deployments: -2. ptdump is normally exposed through privileged debug interfaces, commonly debugfs.

Paranoid additional interpretation:

* Weak LPE concern: +1. Because the stale object is page-table memory, reused freed memory could theoretically be interpreted as paging structures, but the patch does not show a write primitive, callback control, or attacker-controlled reclaim.
* Reduced-privilege access model considered: no -2 penalty in paranoid score. If ptdump access is delegated to reduced-capability root, container root, a service account, or a debugging service, it is not equivalent to full host-root in practical risk terms.
* No remote/network signal. The affected path is local debug/control-plane functionality, not packet processing.
* No strong corruption primitive. There is no demonstrated arbitrary write, OOB write, type confusion with controlled replacement, or page-cache/COW ownership bypass.

Call-site confidence: high. The relevant callers and locking context are present in the supplied patch.

## 3. Reachability analysis

The bug is triggerable by a local actor who can invoke ptdump on x86 or arm64 over an mm other than init_mm while kernel page tables are concurrently freed. In typical production deployments this requires privileged debug access, so the conservative score is low. Namespaces or containers matter only if a product delegates ptdump or equivalent debug access to a reduced-privilege context. The path is not remotely reachable and is not triggered by ordinary network traffic. Realistic exploitation is mainly a local crash or unstable page-table traversal rather than a demonstrated privilege escalation.

## 4. Severity interpretation

This behaves more like a borderline manual-review Moderate than an Important-class issue on the evidence provided. The theoretical concern is a race-dependent page-table UAF candidate, which is more sensitive than a simple NULL dereference because page-table memory is security-critical. The realistic evidence still points mostly to local DoS, with no demonstrated attacker-controlled reclaim, write-after-free, arbitrary write, or reliable privilege-escalation primitive.

## 5. One-sentence report phrase

ptdump can race with kernel page table freeing when dumping kernel mappings through a non init_mm mm, creating a local page-table lifetime UAF candidate that is most likely to cause a kernel crash but deserves manual review due to the sensitivity of page-table state.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK because this is a race against page table freeing and a UAF candidate in core mm page-table traversal, even though the currently supported practical impact is mainly local DoS.

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

Patch: mm/ptdump: always stabilise against page table freeing using init_mm
Commit: 7f740664aec1f832953c2e6d9b8920cd6c8bcc0c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7f740664aec1f832953c2e6d9b8920cd6c8bcc0c

Commit description:

Previous commits have established the invariant that kernel page table
freeing is performed while an mmap read lock on init_mm is held, which
fixes races between ptdump and kernel page table freeing over init_mm.

However, x86 and arm64 can perform a ptdump over an mm other than init_mm
via ptdump_walk_pgd() and since kernel memory ranges are shared across
non-kernel mm's, this means that the race still exists for these cases.

Fix this by acquiring a nested mmap write lock for init_mm in
ptdump_walk_pgd().

This is safe as we take this after mmap write locking the mm, and nothing
acquires the init_mm lock first before locking an arbitrary mm, so no
deadlock is possible.

Also update walk_page_range_debug() to assert that init_mm is write
locked, add a comment explaining why and remove some redundant code, and
eliminate the unnecessary and confusing invocation of
walk_kernel_page_table_range().

We can safely remove the non-NULL check for walk.mm, as the mmap lock
asserts would NULL pointer deref if it was (and of course no callers do
this).

The first point at which ptdump can race kernel page table freeing is
commit b6bdb75 ("mm/vmalloc: add interfaces to free unmapped page
table"), so we target this in the Fixes tag.

Link: https://lore.kernel.org/20260723-series-vmap-race-fix-v6-4-8cc77dcc0018@kernel.org
Fixes: b6bdb75 ("mm/vmalloc: add interfaces to free unmapped page table")
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Kiryl Shutsemau <kas@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "Borah, Chaitanya Kumar" <chaitanya.kumar.borah@intel.com>
Cc: "Borislav Petkov (AMD)" <bp@alien8.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Carlier <devnexen@gmail.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Toshi Kani <toshi.kani@hpe.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Will Deacon <will@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/pagewalk.c
  mm/ptdump.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=7f740664aec1f832953c2e6d9b8920cd6c8bcc0c

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

The original Linux kernel CVE announcement for CVE-2026-74599 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74599

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:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 1
  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).