[CVE-2026-46280][MODERATE 7.0] lib: test_hmm: evict device pages on file close to avoid use-after-free [ Upstream

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

CVE: CVE-2026-46280
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: lib: test_hmm: evict device pages on file close to avoid use-after-free [ Upstream
Commit: bf477abd448c76bb8ea51c9b4f63a3a17c4b6239
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf477abd448c76bb8ea51c9b4f63a3a17c4b6239
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46280
Analysis date: Mon, 08 Jun 2026 11:56:03 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
`test_hmm can leave device private pages with a stale zone_device_data pointer after dmirror is freed, allowing a later page fault to dereference freed kernel memory and crash the system, with higher risk only if the HMM selftest driver is exposed to untrusted local users.`

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

The original Linux kernel CVE announcement for CVE-2026-46280 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46280

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-46280	MODERATE	CHECK	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-672;*CWE-664;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'lib/test_hmm can leave device private pages with a stale zone_device_data pointer after dmirror_fops_release frees the dmirror object without first migrating those pages back to system memory. A later fault on the same pages, for example during coredump VMA walking after SIGABRT in the HMM selftest, can call dmirror_devmem_fault and dereference the freed dmirror pointer. For the CVSS the PR:L is used in the paranoid score because a local user or test process with access to the HMM test device may be enough once the test module is present, even though loading the module and configuring the test environment is usually administrative. The issue is not network reachable. Impact is at least local denial of service via kernel crash. Because this is a real use-after-free on a kernel pointer, worst case confidentiality and integrity impact are plausible and the case should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 4)	YES	DANGER UAF SKIP HARDWARE DEBUGFS TEST  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

Practical reading: this is an **Actionable Moderate** because it is a real UAF/lifetime bug, but it is in `lib/test_hmm.c`, a kernel selftest driver path, so production exposure is likely limited unless `test_hmm` is built, loaded, and accessible.

## 2. Signal breakdown

Conservative score:

* `+1 Local trigger`: the trigger is local through the HMM test device or selftest workflow.
* `+1 Memory corruption, weak primitive`: stale `zone_device_data` points to freed `dmirror`, but the demonstrated primitive is stale pointer dereference leading to panic, not controlled reclaim or overwrite.
* `+1 Real lifetime corruption`: device private pages retain a pointer to freed `dmirror`.
* `+1 Reliable kernel crash / strong DoS`: later fault path such as coredump VMA walking can dereference the stale pointer and panic.
* `-1 Rare AND difficult-to-reach subsystem/configuration`: this is `test_hmm`, a kernel selftest driver, not a broad default production path.

Conservative total: `1 + 1 + 1 + 1 - 1 = 3`.

Paranoid score:

* `+1 Local trigger`: a local user or test process may trigger the fault path once the test driver is present.
* `+2 Generic memory corruption`: the bug is a real use-after-free class issue involving a freed kernel object pointer.
* `+1 Real lifetime corruption`: stale pointer survives file close and object free.
* `+1 Reliable kernel crash / strong DoS`: kernel panic is explicitly described.
* No rare-config subtraction in paranoid mode because the score assumes an affected test or CI kernel where `test_hmm` is intentionally enabled and reachable.

Paranoid total: `1 + 2 + 1 + 1 = 5`.

Not counted:

* No remote reachability.
* No Dirty-Pipe-like primitive.
* No demonstrated arbitrary write.
* No demonstrated attacker-controlled reclaim.
* No strong LPE bonus.
* No confidentiality or integrity bonus in the conservative score.

## 3. Reachability analysis

The bug is locally reachable through the HMM selftest driver path. A process opens the `dmirror` device, causes device private pages to exist, closes the file so `dmirror_fops_release()` frees the `dmirror` object, and a later page fault on those stale device private pages can reach `dmirror_devmem_fault()`.

In typical production kernels, `test_hmm` is not a normal exposed subsystem. It is mainly used for kernel selftests, CI, development, and HMM validation. If the module is not built, not loaded, or the device node is not accessible to untrusted users, practical exposure is low.

Namespaces and containers do not by themselves make this broadly reachable. The key question is whether an untrusted local process can access the `test_hmm` device after it has been enabled by an administrator or by a test environment.

## 4. Severity interpretation

This behaves more like **Actionable Moderate** than ordinary Low because the patch fixes a real lifetime bug and a stale pointer to freed kernel memory. The demonstrated impact is a local kernel crash during a later fault, especially during coredump processing.

Theoretical memory corruption potential exists because the stale `zone_device_data` pointer references freed kernel memory. However, the patch and commit message do not show attacker-controlled reclaim, type confusion, write-after-free, callback hijacking, refcount takeover, or an arbitrary write primitive. Therefore, Important classification is not strongly supported for ordinary production triage, but the UAF class justifies manual review if the affected driver is exposed.

## 5. One-sentence report phrase

`test_hmm can leave device private pages with a stale zone_device_data pointer after dmirror is freed, allowing a later page fault to dereference freed kernel memory and crash the system, with higher risk only if the HMM selftest driver is exposed to untrusted local users.`

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is a real UAF/lifetime bug involving a stale kernel pointer, even though the affected code is a selftest driver and the demonstrated impact is DoS. Manual review should confirm whether `CONFIG_TEST_HMM` is enabled, whether the module is loaded, and whether untrusted users can access the device node.

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

Patch: lib: test_hmm: evict device pages on file close to avoid use-after-free [ Upstream
Commit: bf477abd448c76bb8ea51c9b4f63a3a17c4b6239
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf477abd448c76bb8ea51c9b4f63a3a17c4b6239

Changed files:
  mm/ksft_hmm.sh
  lib/test_hmm.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=bf477abd448c76bb8ea51c9b4f63a3a17c4b6239

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

The original Linux kernel CVE announcement for CVE-2026-46280 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46280

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:L/PR:L/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: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: 3
  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: NO
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).