[CVE-2026-89768][MODERATE REGULAR] fs: fix user path of nested backing files

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

CVE: CVE-2026-89768
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fs: fix user path of nested backing files
Commit: 88c927a63dc717b6d46b20fe13ea713916e49089
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=88c927a63dc717b6d46b20fe13ea713916e49089
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89768
Analysis date: Fri, 11 Sep 2026 20:43:19 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89768 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89768

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 REGULAR
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-2026-89768	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89768 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N';CWE-706;CWE-778;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:N';BEST CVSS score: '3.3';DESCR 'A path resolution logic error in backing_file_open can associate a nested backing file with the real path of an intermediate layer instead of the outermost user visible path. Files mapped through nested overlayfs can consequently appear under a disconnected or incorrect name in proc maps and in perf or ftrace mmap records. The same behavior may affect FUSE passthrough and EROFS ishare when their user file is already a backing file. For the CVSS the PR:L value is used because a local user process must access or map a file through an already configured nested stacking filesystem. The issue is not directly network reachable. No memory corruption or denial of service primitive is present. The realistic impact is incorrect path attribution, while the paranoid assessment allows low integrity impact because audit or monitoring tools may consume misleading path records.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)  SKIP CVE-2026-89768 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	READ DISK IMPROVEONLY  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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

ActionableScore=0
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 0
* Conservative score: 0
* Raw paranoid score: 0
* Paranoid score: 0
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +0 Path attribution error. The wrong user-visible path is stored for a nested backing file, but this establishes no corruption, lifetime, ownership, or authorization primitive.
* +0 Integrity. Incorrect `/proc/<pid>/maps`, perf, and ftrace path records are demonstrated, but the rules exclude an integrity point for bookkeeping or diagnostic metadata alone.
* +0 Availability. No crash, stall, leak, or resource exhaustion is established.
* +0 Confidentiality. No protected information disclosure is established.
* Controlled reclaim, UAF, OOB access, arbitrary write, and active-state corruption are not demonstrated.
* No LSM authorization decision is shown to change from deny to allow.
* No path-based policy bypass or cross-ownership filesystem modification is established.
* Remote reachability and privilege escalation are not established.
* The supplied paranoid CVSS integrity rating does not independently create an ActionableScore security primitive.

3. Reachability analysis

A local process can observe the incorrect path after opening or mapping a file through a nested overlayfs or another nested backing-file configuration. Establishing the complete filesystem topology may require prior privileged setup, but the supplied evidence does not resolve whether all supported configurations require host administration or can be created in an attacker-controlled user and mount namespace. Therefore, both host-admin and complete namespace-scoped reachability remain unknown.

No network trigger, special hardware, race, timing window, sysctl, or boot-time prerequisite is identified. FUSE passthrough and EROFS ishare are additional affected callers, but their complete privilege requirements are not shown.

4. Primitive and severity interpretation

The exact defect is selection of the intermediate backing file path instead of the outermost user-visible path. It causes incorrect diagnostic and tracing attribution for nested backing files.

This is not a stale-object use, UAF, repeated release, semantic reinterpretation of active security state, or authorization bypass. The demonstrated consequence is an incorrect displayed or recorded pathname. A monitoring or audit consumer could theoretically make a poor decision based on that pathname, but no such security-sensitive consumer or policy bypass is established.

PrimitiveClass is `NONE`. ImportantCandidate is `NO` because there is no concrete kernel-corruption or security-boundary primitive. AutoDowngradeAllowed is `YES` because the demonstrated behavior is limited to path-reporting correctness. Operational severity is Low-like unless further call-site analysis establishes that the stored path participates in enforcement rather than reporting.

5. One-sentence report phrase

Nested backing files can retain an intermediate private path instead of the outermost user-visible path, producing incorrect names in proc maps and tracing records without an established memory-corruption, denial-of-service, or authorization-bypass primitive.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm that `file_user_path()` data is used only for reporting and audit attribution across overlayfs, FUSE passthrough, EROFS ishare, and backing-file LSM integration, and not by a path-based enforcement decision.

7. Call-site confidence

Call-site confidence: medium

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

Patch: fs: fix user path of nested backing files
Commit: 88c927a63dc717b6d46b20fe13ea713916e49089
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=88c927a63dc717b6d46b20fe13ea713916e49089

Commit description:

backing_file_open() derives the path to be stored in the new backing
file from user_file->f_path.  This is incorrect when user_file itself
is a backing file, which is the case for nested stacking filesystems,
e.g. overlayfs mounts where the lowerdir of one overlayfs is the merged
directory of another.  Since commit def3ae8 ("fs: store real path
instead of fake path in backing file f_path") the f_path of a backing
file holds the real path of the intermediate layer, not the path that
the user opened.

Commit 924577e ("ovl: Fix nested backing file paths") fixed this
for such configurations by passing file_user_path() from
ovl_open_realfile().  However, commit 6af36ae ("lsm: add
backing_file LSM hooks") changed the first argument of
backing_file_open() from the user path back to the user file and
derived the path from user_file->f_path again, silently re-introducing
the problem.

As a result, files mapped through a nested overlayfs show the wrong
path in /proc/<pid>/maps and in perf/ftrace mmap records.  For example,
with two nested overlayfs mounts:

  mkdir -p /ovl/{lower,upper,work,merged} /ovl/nested
  echo hello > /ovl/lower/foo
  mount -t overlay overlay \
	-o lowerdir=/ovl/lower,upperdir=/ovl/upper,workdir=/ovl/work \
	/ovl/merged
  # at least two lowerdirs are needed when upperdir is nonexistent
  mount -t overlay overlay \
	-o lowerdir=/ovl/merged:/ovl/lower /ovl/nested

mapping /ovl/nested/foo shows a disconnected path instead of the user
path:

  # readlink /proc/self/fd/3
  /ovl/nested/foo
  # grep foo /proc/self/maps
  7f6e2c100000-7f6e2c101000 r--s 00000000 00:24 15813027 /foo

The bogus path is derived from the f_path of the intermediate backing
file, whose mount is a private clone that d_path() cannot resolve.

Fix this by using file_user_path(), which returns the outermost
user-visible path for backing files and falls back to
&user_file->f_path for regular files.  This restores the behavior of
commit 924577e ("ovl: Fix nested backing file paths") for
overlayfs and also fixes the same problem for the other
backing_file_open() callers, fuse passthrough and erofs ishare, when
their user file is itself a backing file.

backing_tmpfile_open() has the same pattern but is not affected: it is
only called by ovl_create_tmpfile() for the upper layer, and another
overlayfs is rejected as upperdir by the DCACHE_OP_REAL check in
ovl_mount_dir_check(), so its user_file can never be a backing file.

Fixes: 6af36ae ("lsm: add backing_file LSM hooks")
Cc: stable@vger.kernel.org
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
Link: https://patch.msgid.link/20260804034204.3487077-1-libaokun@linux.alibaba.com
Tested-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/backing-file.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=88c927a63dc717b6d46b20fe13ea713916e49089

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

The original Linux kernel CVE announcement for CVE-2026-89768 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89768

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:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
  The Best / paranoid CVSS score: 3.3

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: 0

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 REGULAR

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).