[CVE-2026-72379][MODERATE 7.0] fs: refuse O_TMPFILE creation with an unmapped fsuid or fsgid [ Upstream

[CVE-2026-72379][MODERATE 7.0] fs: refuse O_TMPFILE creation with an unmapped fsuid or fsgid [ Upstream

[AL-KERNEL]

CVE: CVE-2026-72379
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fs: refuse O_TMPFILE creation with an unmapped fsuid or fsgid [ Upstream
Commit: bac8fb0d60254846f3b56957435dcd870ae12948
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bac8fb0d60254846f3b56957435dcd870ae12948
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72379
Analysis date: Sat, 15 Aug 2026 18:07:29 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum, manual review required
Manual review required: YES

Summary:
`vfs_tmpfile() failed to reject O_TMPFILE creation when the caller fsuid or fsgid was unmapped on an idmapped mount, allowing a local process to create a linkable file with INVALID_UID or INVALID_GID ownership and potentially bypass filesystem ownership policy.`

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

The original Linux kernel CVE announcement for CVE-2026-72379 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72379

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
======================================================================

CVE-2026-72379	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N';CWE-282;CWE-284;CWE-863;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '4.4';DESCR 'vfs_tmpfile() did not verify that the caller fsuid and fsgid are mapped into the target filesystem on an idmapped mount before creating an O_TMPFILE inode. If the idmap does not cover the caller IDs, inode initialization can store INVALID_UID or INVALID_GID and the linkable tmpfile can later be attached to the namespace with linkat(2). This is a local control plane and filesystem policy issue rather than a memory corruption bug. For the CVSS the PR:L is used because a local user or process with access to a writable affected idmapped mount can trigger O_TMPFILE without full host administrator privileges in that mount context. The issue is not network reachable. Impact is mainly integrity policy bypass through creation of an object with an owner the filesystem should not represent, with possible low confidentiality concerns in paranoid triage if local policy or tooling makes decisions based on ownership.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum, manual review required (with actual score 5)	YES	DISK IMPROVEONLY LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: **4**
* Paranoid score: **5**
* Final recommended bucket: **Actionable Moderate at minimum, manual review required**::

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged trigger: +1**
  A local process with write access to an affected idmapped mount can use `O_TMPFILE` and later `linkat(2)` the object into the namespace.

* **Cross-boundary isolation/policy bypass: +1**
  The bug bypasses the intended VFS/idmapped-mount ownership mapping boundary by allowing creation of an inode whose owner cannot be represented by the filesystem mapping.

* **Integrity impact plausible: +1**
  A linkable file can be created with `INVALID_UID` / `INVALID_GID`, which may violate expected ownership and local filesystem policy.

* **Important filesystem/storage path: +1**
  The affected helper is `vfs_tmpfile()`, and the issue applies across filesystems supporting idmapped mounts and `->tmpfile()`, including tmpfs, ext4, btrfs, xfs, f2fs, and overlayfs paths.

Paranoid additional signal:

* **Broad/common subsystem exposure: +1**
  Although the trigger requires an idmapped mount with incomplete fsuid/fsgid coverage, the vulnerable check is in a central VFS creation path and affects multiple common filesystems.

Not applied:

* **No memory corruption signals**
  The patch does not indicate UAF, OOB access, arbitrary write, refcount misuse, or object lifetime corruption.

* **No reliable DoS signal**
  The primary issue is incorrect object ownership, not crash or resource exhaustion.

* **No direct sensitive file theft primitive**
  The patch supports an ownership/policy bypass concern, but not direct access to `/etc/shadow`, privileged FDs, credentials, or secrets.

## 3. Reachability analysis

The issue is locally reachable through `O_TMPFILE` on an affected idmapped mount where the caller fsuid or fsgid is not mapped into the filesystem. Creating such mounts usually requires privileges, but the actual trigger can be a less privileged local process if the mount already exists inside a container, namespace, or managed environment.

Namespaces and containers matter here because idmapped mounts are commonly relevant to containerized or delegated filesystem setups. This should not be scored as a fully remote issue. It is not network reachable. It is also not a full host-root-only issue if a reduced-privilege process can operate inside a preconfigured writable idmapped mount.

## 4. Severity interpretation

This behaves like an **actionable Moderate** access-control and filesystem policy issue, not like a memory-corruption vulnerability. The realistic impact is creation of a linkable filesystem object with an invalid/unmapped owner, which can undermine expected ownership semantics on idmapped mounts.

Theoretical privilege escalation is not strongly supported by the patch alone. However, because this is a VFS-level ownership invariant violation affecting multiple filesystems and overlayfs-related paths, it should not be auto-closed as a low-risk correctness fix without manual review.

## 5. One-sentence report phrase

`vfs_tmpfile() failed to reject O_TMPFILE creation when the caller fsuid or fsgid was unmapped on an idmapped mount, allowing a local process to create a linkable file with INVALID_UID or INVALID_GID ownership and potentially bypass filesystem ownership policy.`

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is not memory corruption, but it affects a VFS ownership and idmapped-mount security boundary across multiple filesystems. The practical impact depends on container/idmapped-mount deployment details and should be reviewed manually rather than auto-closed.

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

Patch: fs: refuse O_TMPFILE creation with an unmapped fsuid or fsgid [ Upstream
Commit: bac8fb0d60254846f3b56957435dcd870ae12948
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bac8fb0d60254846f3b56957435dcd870ae12948

Changed files:
  fs/namei.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=bac8fb0d60254846f3b56957435dcd870ae12948

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

The original Linux kernel CVE announcement for CVE-2026-72379 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72379

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:L/PR:L/UI:N/S:U/C:L/I:L/A:N
  The Best / paranoid CVSS score: 4.4

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: 4
  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).