[CVE-2026-64294][LOW] mm: do file ownership checks with the proper mount idmap [ Upstream

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

CVE: CVE-2026-64294
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: mm: do file ownership checks with the proper mount idmap [ Upstream
Commit: 744b23aa430d52f5c8e4dbff7d71496d6643bed2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=744b23aa430d52f5c8e4dbff7d71496d6643bed2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64294
Analysis date: Sat, 25 Jul 2026 08:41:34 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like, not an Important candidate
Manual review required: NO

Summary:
Incorrect mount-idmap handling in MM ownership checks could allow local users on idmapped mounts to bypass intended side-channel protections for `mincore()` or `MADV_PAGEOUT`, with limited confidentiality impact and no demonstrated memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-64294 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64294

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-2026-64294	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:L/I:L/A:L';CWE-863;CWE-284;*CWE-200;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '4.5';DESCR 'mincore and MADV_PAGEOUT used inode ownership checks with nop_mnt_idmap instead of the file mount idmap when deciding whether a caller is allowed to observe or act on shared file backed mappings. On idmapped mounts this can make the kernel evaluate ownership in the wrong namespace, which can either deny the mapped owner or allow a caller who is only the raw inode owner to pass the side channel protection check. For the CVSS the PR:L is used because a local user or process must access an idmapped mount and invoke mincore or MADV_PAGEOUT. The issue is not network reachable. Impact is mainly limited confidentiality exposure through page residency side channel information and low availability impact through page cache pageout behavior. No memory corruption or direct privilege escalation primitive is indicated by the code change.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like, not an Important candidate (with actual score 2)	YES	IMPROVEONLY LINUS 	YES	NO	checked

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

ActionableScore=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Low-like, not an Important candidate**::

## 2. Signal breakdown

* Local unprivileged trigger: +1
  A local process can call `mincore()` or `madvise(MADV_PAGEOUT)` on a relevant file-backed mapping.

* Confidentiality impact plausible: +1
  The affected checks are explicitly related to side-channel protection around shared non-exclusive mappings. The main concern is limited page-residency information exposure.

* Cross-boundary isolation/policy bypass: +1 in paranoid interpretation only
  The bug can evaluate file ownership in the wrong mount-idmap context, which is a mount namespace / idmapped-mount semantics bypass. This is policy-relevant but does not expose a strong sensitive-object theft primitive.

* Rare / configuration-dependent condition: -1
  Practical impact requires idmapped mounts and a specific ownership mismatch where `file_permission(..., MAY_WRITE)` does not already allow the operation, for example read-only files such as `0444`.

* No memory corruption: +0
  The patch changes authorization semantics only. There is no UAF, OOB access, refcount issue, stale pointer, or arbitrary write.

* No privilege escalation primitive: +0
  The patch does not indicate a direct path to credential overwrite, fd theft, code execution, or LPE.

## 3. Reachability analysis

The bug is local-only and not network reachable. A local user or process must access a file through an idmapped mount and invoke `mincore()` or `MADV_PAGEOUT` on a file-backed mapping. Creating idmapped mounts may require privileges or namespace setup, but exploitation can be relevant if such mounts already exist or are delegated to containers. The affected paths are common MM syscalls, but the vulnerable condition is configuration-specific and depends on idmapped mount ownership translation.

Call-site confidence: high. The relevant call sites in `mm/mincore.c`, `mm/madvise.c`, and `mm/filemap.c` are visible in the patch.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like access-control semantics bug, not an Important-class kernel vulnerability. The theoretical impact is limited confidentiality exposure through page-residency side-channel behavior and possibly minor policy inconsistency for `MADV_PAGEOUT`. Realistic exploitation evidence for privilege escalation, sensitive file theft, or memory corruption is not present.

## 5. One-sentence report phrase

Incorrect mount-idmap handling in MM ownership checks could allow local users on idmapped mounts to bypass intended side-channel protections for `mincore()` or `MADV_PAGEOUT`, with limited confidentiality impact and no demonstrated memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a local, configuration-dependent authorization semantics issue with limited side-channel impact and no concrete memory-corruption, LPE, fd-theft, or network-triggerable primitive.

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

Patch: mm: do file ownership checks with the proper mount idmap [ Upstream
Commit: 744b23aa430d52f5c8e4dbff7d71496d6643bed2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=744b23aa430d52f5c8e4dbff7d71496d6643bed2

Changed files:
  include/linux/fs.h
  mm/filemap.c
  mm/madvise.c
  mm/mincore.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=744b23aa430d52f5c8e4dbff7d71496d6643bed2

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

The original Linux kernel CVE announcement for CVE-2026-64294 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64294

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:L/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 4.5

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: unknown
  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).