[CVE-2026-72174][LOW] fs/proc/task_mmu: fix hugetlb self-deadlock in pagemap_scan_pte_hole()

[CVE-2026-72174][LOW] fs/proc/task_mmu: fix hugetlb self-deadlock in pagemap_scan_pte_hole()

[AL-KERNEL]

CVE: CVE-2026-72174
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: fs/proc/task_mmu: fix hugetlb self-deadlock in pagemap_scan_pte_hole()
Commit: a6ac03652d9edc30c2912037ac83beb42cc67f8e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a6ac03652d9edc30c2912037ac83beb42cc67f8e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72174
Analysis date: Sat, 15 Aug 2026 16:32:38 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
PAGEMAP_SCAN with PM_SCAN_WP_MATCHING on a hugetlb VMA can self-deadlock on unpopulated hugepage ranges, causing an unkillable local hung task and denial of service without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-72174 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72174

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-2026-72174	LOW	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:N/A:H';CWE-667;*CWE-833;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'PAGEMAP_SCAN with PM_SCAN_WP_MATCHING on a hugetlb VMA can deadlock the calling task when the scan reaches an unpopulated huge page range. The page table walker holds the hugetlb VMA lock for read and the hole handling path can re enter hugetlb change protection which tries to take the same lock for write. This results in an unkillable hung task and a local denial of service rather than memory corruption. For the CVSS the PR:L is used for the paranoid score because a local user or process able to use the pagemap scan interface on a suitable hugetlb mapping may trigger the condition without full administrator privileges in some deployments. The issue is not network reachable. Impact is primarily availability loss. Confidentiality and integrity remain N because the bug class is a self-deadlock and no UAF, OOB access, info leak, or write primitive is shown by the code path.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-72174 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 52526ca7fdb905a768a93f8faa418e9b988fc34b YES  NO NO unknown 	MAYBE	WRITE OOB LOCK DEADLOCK DISK INIT NOMEMCHK LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local process may be able to invoke PAGEMAP_SCAN on a suitable mapping it controls, depending on procfs access policy and hugetlb availability.

* Rare AND difficult-to-reach subsystem/configuration: -1
  The trigger requires a hugetlb VMA, PM_SCAN_WP_MATCHING, an unpopulated hugepage range, and the PAGEMAP_SCAN ioctl path. This is not a broad default kernel data path.

* Availability impact realistic: +1
  The bug can create an unkillable hung calling task due to self-deadlock.

Paranoid additional signal:

* Reliable kernel crash / strong DoS: +1
  Not a crash, but the unkillable hung task is a strong local DoS symptom. In the paranoid interpretation, repeated triggering could consume resources and cause broader availability impact.

Not applied:

* No memory corruption signal. The patch shows a lock ordering self-deadlock, not UAF, OOB access, double free, type confusion, or stale pointer reuse.
* No privilege escalation signal. There is no supported write primitive, object replacement, callback control, or credential boundary bypass.
* No confidentiality or integrity signal. The affected path hangs while trying to acquire a write lock already blocked by its own read lock.

## 3. Reachability analysis

The bug is locally reachable through PAGEMAP_SCAN with PM_SCAN_WP_MATCHING on a hugetlb VMA when scanning an unpopulated part of the range. It is not network reachable. A fully unprivileged trigger depends on whether the local user can create or access suitable hugetlb mappings and use the pagemap scan interface for the target process. Containers or namespaces may reduce practical barriers if hugetlb resources and procfs access are delegated, but this still remains a local interface issue.

Call-site confidence: high. The commit message includes the exact call chain from do_pagemap_scan() through walk_hugetlb_range(), pagemap_scan_pte_hole(), uffd_wp_range(), and hugetlb_change_protection().

## 4. Severity interpretation

This behaves like a local DoS-only bug, not an Important-class vulnerability. The theoretical impact is an unkillable hung task caused by a self-deadlock. The realistic exploitation evidence does not support memory corruption, privilege escalation, sensitive data exposure, or cross-boundary policy bypass. The paranoid score is only slightly higher because repeated hung tasks could have broader availability consequences.

## 5. One-sentence report phrase

PAGEMAP_SCAN with PM_SCAN_WP_MATCHING on a hugetlb VMA can self-deadlock on unpopulated hugepage ranges, causing an unkillable local hung task and denial of service without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a clearly described self-deadlock in a specialized local memory-management path, with no supported UAF, OOB access, write primitive, information leak, or privilege escalation path.

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

Patch: fs/proc/task_mmu: fix hugetlb self-deadlock in pagemap_scan_pte_hole()
Commit: a6ac03652d9edc30c2912037ac83beb42cc67f8e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a6ac03652d9edc30c2912037ac83beb42cc67f8e

Commit description:

A PAGEMAP_SCAN ioctl requesting PM_SCAN_WP_MATCHING on a hugetlb VMA hangs
the calling thread, unkillably, as soon as the scan reaches an unpopulated
part of the range:

  do_pagemap_scan()
    walk_page_range()
      walk_hugetlb_range()
        hugetlb_vma_lock_read()           # take the vma lock for read ...
        pagemap_scan_pte_hole()           # ... ->pte_hole() for a hole
          uffd_wp_range()
            change_protection()
              hugetlb_change_protection()
                hugetlb_vma_lock_write()  # ... and block taking it for write

walk_hugetlb_range() holds the hugetlb vma lock for read across the whole
walk.  A present entry goes to ->hugetlb_entry(); an unpopulated one goes
to ->pte_hole(), i.e.  pagemap_scan_pte_hole().  To write-protect the hole
that handler calls uffd_wp_range(), which on a hugetlb VMA reaches
hugetlb_change_protection() and takes the same vma lock for write.  The
thread then blocks in down_write() waiting for the read lock it is itself
holding.

The populated path avoids this: pagemap_scan_hugetlb_entry()
write-protects the entry inline under the page-table lock and never enters
hugetlb_change_protection().

Do the same for holes.  Fault in the page table and install the uffd-wp
marker directly with make_uffd_wp_huge_pte() under the page-table lock,
rather than routing through uffd_wp_range().  That is the same sequence
hugetlb_change_protection() runs for an unpopulated entry, minus the vma
write lock -- which is safe to skip because PMD sharing is disabled on
uffd-wp VMAs (hugetlb_unshare_all_pmds() runs at registration), leaving
nothing for that lock to serialise against.

Link: https://lore.kernel.org/20260529172331.356655-4-kas@kernel.org
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>
Assisted-by: Claude:claude-opus-4-8
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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=a6ac03652d9edc30c2912037ac83beb42cc67f8e

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

The original Linux kernel CVE announcement for CVE-2026-72174 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72174

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1
  Paranoid ActionableScore: 2

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).