[CVE-2026-72173][LOW] fs/proc/task_mmu: do not warn on seeing non-migration pmd entry

[CVE-2026-72173][LOW] fs/proc/task_mmu: do not warn on seeing non-migration pmd entry

[AL-KERNEL]

CVE: CVE-2026-72173
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: fs/proc/task_mmu: do not warn on seeing non-migration pmd entry
Commit: 319caaca072adc564ac9c5357ed013909f23d5ab
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=319caaca072adc564ac9c5357ed013909f23d5ab
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72173
Analysis date: Sun, 16 Aug 2026 03:35:40 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A stale VM_WARN_ON_ONCE in pagemap_pmd_range_thp could be triggered for valid PMD device-private entries, causing a warning and possible warning-driven DoS on panic_on_warn systems, but no memory corruption or privilege escalation primitive is indicated.

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

The original Linux kernel CVE announcement for CVE-2026-72173 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72173

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.

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

CVE-2026-72173	LOW	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:N/I:N/A:H';CWE-617;CWE-703;CWE-755;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_pmd_range_thp can emit a stale VM_WARN_ON_ONCE when it sees a non-present PMD softleaf entry that is not a migration entry. This became valid after PMD device private entries were added, so the warning can be reached by reading pagemap for a process that has such HMM or device private mappings. For the CVSS the PR:L is used because a local process can reach the proc pagemap path for an address space it is allowed to inspect, although the special PMD device private state requires HMM or device memory usage. The issue is not network reachable. Impact is warning driven denial of service only, mainly on systems where panic_on_warn turns the warning into a kernel panic. No UAF, OOB access, or arbitrary write primitive is indicated by the code change.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-72173 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: a30b48bf1b244f11bf9b6d20cdccfe0c2264130c YES  NO NO unknown 	MAYBE	DISK WARNONLY HARDWARE 	-	-	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 reach the proc pagemap path for an address space it is permitted to inspect.
* Rare AND difficult-to-reach subsystem/configuration: -1. The relevant state requires PMD-level HMM or device-private entries, which is not a common default workload.
* WARN_ON continuation rule: +0. The patch removes VM_WARN_ON_ONCE only. The diff does not show UAF, OOB access, double free, stale pointer reuse, arbitrary write, or a concrete crash path.

Conservative total: 1.

Paranoid signals:

* Local unprivileged trigger: +1. Local access to pagemap-like proc paths may be enough once the special mapping state exists.
* Warning-driven DoS concern: +1. On systems configured with panic_on_warn, the stale WARN may become a kernel panic.
* No memory corruption signals. No LPE signal. No confidentiality or integrity signal.

Paranoid total: 2.

## 3. Reachability analysis

The trigger is local through the proc pagemap path, not network reachable. The issue depends on a process having HMM or device-private PMD mappings, so practical exposure is limited to systems using device-private memory or similar heterogeneous memory configurations. Namespaces do not obviously turn this into a broader container escape or privilege boundary bypass. The path is not a common remote attack surface and the patch only removes a stale assertion.

Call-site confidence: high, because the changed code path and failure condition are directly visible in the patch.

## 4. Severity interpretation

This behaves like a Low-like or ordinary Moderate issue, not an Important-class vulnerability. The realistic impact is a false kernel warning, with DoS only in panic_on_warn deployments. Theoretical memory corruption is not supported by the patch: there is no UAF, no OOB access, no stale object reuse, no write primitive, and no privilege escalation path.

## 5. One-sentence report phrase

A stale VM_WARN_ON_ONCE in pagemap_pmd_range_thp could be triggered for valid PMD device-private entries, causing a warning and possible warning-driven DoS on panic_on_warn systems, but no memory corruption or privilege escalation primitive is indicated.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED.

Reason: this is a warning-only assertion cleanup in a special HMM/device-private pagemap path, with no supported memory corruption, confidentiality, integrity, or privilege escalation primitive.

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

Patch: fs/proc/task_mmu: do not warn on seeing non-migration pmd entry
Commit: 319caaca072adc564ac9c5357ed013909f23d5ab
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=319caaca072adc564ac9c5357ed013909f23d5ab

Commit description:

Patch series "mm/hmm: A fix and a selftest", v3.

Patch 1 fixes a stale warning present from the time when only migration
softleaf entries were supported at the PMD level.

Patch 2 adds some code into hmm-tests.c which exercises the pagemap path
for PMD device-private entries.

This patch (of 2):

pagemap_pmd_range_thp() warns if a non-present PMD is not a migration
entry.  This became false once device-private entries at the PMD level
were added.

Therefore, remove the stale migration-only assertion.

Link: https://lore.kernel.org/20260604055308.1947679-1-dev.jain@arm.com
Link: https://lore.kernel.org/20260604055308.1947679-2-dev.jain@arm.com
Fixes: a30b48b ("mm/migrate_device: implement THP migration of zone device pages")
Signed-off-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Balbir Singh <balbirs@nvidia.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Tested-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Oscar Salvador (SUSE) <osalvador@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:
  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=319caaca072adc564ac9c5357ed013909f23d5ab

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

The original Linux kernel CVE announcement for CVE-2026-72173 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72173

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).