[CVE-2026-72175][MODERATE REGULAR] fs/proc/task_mmu: fix make_uffd_wp_huge_pte() prot-update race

[CVE-2026-72175][MODERATE REGULAR] fs/proc/task_mmu: fix make_uffd_wp_huge_pte() prot-update race

[AL-KERNEL]

CVE: CVE-2026-72175
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fs/proc/task_mmu: fix make_uffd_wp_huge_pte() prot-update race
Commit: 50a25249a6355db74c2c1b6be541b4caab9f3655
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=50a25249a6355db74c2c1b6be541b4caab9f3655
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72175
Analysis date: Sat, 15 Aug 2026 03:41:38 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline manual review recommended, not Important
Manual review required: YES

Summary:
A local PAGEMAP_SCAN and UFFD write-protect race on HugeTLB PTEs can lose hardware Dirty or Accessed updates, causing incorrect page-state tracking but no demonstrated memory-corruption or privilege-escalation primitive.

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

The original Linux kernel CVE announcement for CVE-2026-72175 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72175

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.

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

CVE-2026-72175	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:N/I:L/A:L';CWE-362;CWE-667;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '3.6';DESCR 'make_uffd_wp_huge_pte can lose hardware Dirty or Accessed updates on a present HugeTLB PTE because the code committed a stale PTE snapshot without first using huge_ptep_modify_prot_start. A local process using PAGEMAP_SCAN with UFFD write protection on HugeTLB mappings can trigger the race window while hardware updates the live PTE state. For the CVSS the PR:L is used because reliable triggering requires local code execution and access to the relevant pagemap userfaultfd and HugeTLB workflow, not just remote reachability. The issue is not network reachable. Impact is mainly incorrect page state tracking and possible low integrity impact for dirty or accessed accounting. There is no clear UAF, OOB write, arbitrary write, or privilege escalation primitive in this patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended, not Important (with actual score 3)  SKIP CVE-2026-72175 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 52526ca7fdb905a768a93f8faa418e9b988fc34b YES  NO NO unknown 	MAYBE	WRITE OOB DEADLOCK DISK SIMPLEFIX RACE HARDWARE MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: **2**
* Paranoid score: **3**
* Final recommended bucket: **Borderline manual review recommended, not Important**::

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged trigger: +1**
  A local process can plausibly reach this through PAGEMAP_SCAN / UFFD write-protect workflows on its own mappings, assuming the relevant HugeTLB and userfaultfd setup is available.

* **Integrity impact plausible: +1**
  The bug can lose hardware Dirty / Accessed PTE updates, causing incorrect page-state tracking. This is integrity of memory-management metadata, not arbitrary data corruption.

* **Broad/default/common subsystem: +1**
  The affected code is in core mm / proc task_mmu page-table handling, although the trigger is feature-specific.

* **Hard or unreliable race / special timing required: -1**
  The problematic window requires hardware Dirty / Accessed updates to occur between the stale PTE snapshot and the commit.

Paranoid difference:

* Paranoid score does **not** subtract the timing penalty, because repeated PAGEMAP_SCAN / UFFD-WP operations may make the race easier to hit in a controlled stress scenario.

Not awarded:

* **No generic memory corruption**. The patch does not show UAF, OOB write, double free, arbitrary write, refcount takeover, or object reuse.
* **No LPE signal**. There is no demonstrated privilege-escalation primitive.
* **No reliable kernel crash / strong DoS**. The described impact is lost Dirty / Accessed tracking, not panic or Oops.
* **No network reachability**. This is a local mm/proc/userfaultfd path.

## 3. Reachability analysis

The bug is locally reachable by a process that can use PAGEMAP_SCAN with UFFD write protection over HugeTLB mappings. In typical systems this may depend on userfaultfd policy, HugeTLB availability, and local configuration. Containers or namespaces may reduce or remove access depending on how userfaultfd and hugetlbfs are exposed. The path is not network-triggerable and does not involve firmware or external device input. Realistic exploitation conditions are specialized but not purely theoretical for workloads using userfaultfd-based tracking.

Call-site confidence: **high**, because the changed function and the relevant PTE update sequence are present in the patch.

## 4. Severity interpretation

This behaves like an **ordinary Moderate / Low-like correctness race** with manual-review value because it affects page-table state tracking. The theoretical concern is that lost Dirty / Accessed state can affect higher-level tracking decisions, but the patch does not support memory corruption, sensitive data disclosure, or privilege escalation. The realistic impact is incorrect UFFD/PAGEMAP_SCAN tracking rather than a strong security primitive.

## 5. One-sentence report phrase

A local PAGEMAP_SCAN and UFFD write-protect race on HugeTLB PTEs can lose hardware Dirty or Accessed updates, causing incorrect page-state tracking but no demonstrated memory-corruption or privilege-escalation primitive.

## 6. Manual review recommendation

**MANUAL CHECK RECOMMENDED**

Reason: this is a race in mm page-table handling and userfaultfd tracking code, so it should not be blindly auto-closed, but the supplied patch supports only low integrity impact and does not justify Important-class handling.

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

Patch: fs/proc/task_mmu: fix make_uffd_wp_huge_pte() prot-update race
Commit: 50a25249a6355db74c2c1b6be541b4caab9f3655
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=50a25249a6355db74c2c1b6be541b4caab9f3655

Commit description:

Patch series "userfaultfd/pagemap: pre-existing fixes".

These are pre-existing bug fixes that were carried at the front of the
userfaultfd RWP working-set-tracking series up to v5 [1].  Per review
feedback that fixes should not sit in the middle of a feature series, they
are split out and sent on their own; the RWP series is reposted rebased on
top of this.

All six were flagged by the Sashiko AI review of the RWP series and carry
Reported-by: Sashiko AI review <sashiko-bot@kernel.org>.  They are
independent of RWP, apply to mm-new directly, and carry Cc: stable@.

  1: fs/proc/task_mmu: a missing huge_ptep_modify_prot_start() in
     make_uffd_wp_huge_pte() can lose hardware Dirty/Accessed updates
     when PAGEMAP_SCAN write-protects a hugetlb PTE.

  2: fs/proc/task_mmu: pagemap_scan_hugetlb_entry() compares the range
     against HPAGE_SIZE rather than the hstate page size, so it never
     write-protects gigantic hugetlb pages.

  3: fs/proc/task_mmu: PAGEMAP_SCAN with PM_SCAN_WP_MATCHING over an
     unpopulated hugetlb range self-deadlocks -- pagemap_scan_pte_hole()
     calls uffd_wp_range() while walk_hugetlb_range() holds the hugetlb
     vma lock for read, and hugetlb_change_protection() then takes it
     for write. Install the marker inline instead.

  4: mm/huge_memory: change_non_present_huge_pmd() drops pmd_swp_uffd_wp
     on a device-private PMD permission downgrade, silently losing the
     uffd-wp marker.

  5: userfaultfd: must_wait() applies pte_write() to a locklessly read
     PTE without checking pte_present(), so swap/migration entries
     decode random offset bits and a thread can stay parked on a stale
     fault.

  6: userfaultfd: __VMA_UFFD_FLAGS feeds VMA_UFFD_MINOR_BIT (41) to
     mk_vma_flags() unconditionally, an out-of-bounds write into the
     single-word vma_flags_t on 32-bit. Build the mask from config-gated
     per-mode masks so an unavailable bit is never materialised.

This patch (of 6):

make_uffd_wp_huge_pte() arms the UFFD_WP bit on a present HugeTLB PTE by
calling huge_ptep_modify_prot_commit() with a ptent snapshot that was
fetched without the corresponding huge_ptep_modify_prot_start().  The
start helper is what atomically clears the entry so the kernel-owned
snapshot stays consistent until the commit; without it, the hardware may
set Dirty or Accessed in the live PTE between the original read and the
commit, and huge_ptep_modify_prot_commit() (whose generic implementation
just calls set_huge_pte_at()) then writes the stale snapshot back over the
live hardware bits, losing the update.

The non-hugetlb sibling make_uffd_wp_pte() does this correctly via
ptep_modify_prot_start() / ptep_modify_prot_commit().  Mirror that pattern
for the present-PTE branch.  The migration case stays as-is -- migration
entries are non-present, so there's no hardware update to race against.

Link: https://lore.kernel.org/20260529172331.356655-1-kas@kernel.org
Link: https://lore.kernel.org/20260529172331.356655-2-kas@kernel.org
Link: https://lore.kernel.org/all/20260526130509.2748441-1-kirill@shutemov.name/ [1]
Fixes: 52526ca ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reported-by: Sashiko AI review <sashiko-bot@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[ kas: adapt to the pre-softleaf idiom; apply the
  huge_ptep_modify_prot_start()/commit() fix to the present-PTE
  (!huge_pte_none()) branch ]
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  fs/proc/task_mmu.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=50a25249a6355db74c2c1b6be541b4caab9f3655

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

The original Linux kernel CVE announcement for CVE-2026-72175 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72175

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:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS score: 3.6

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: 2
  Paranoid ActionableScore: 3

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).