[CVE-2026-74484][MODERATE REGULAR] binfmt_misc: don't let an 'F' entry pin its own instance

[CVE-2026-74484][MODERATE REGULAR] binfmt_misc: don't let an 'F' entry pin its own instance

[AL-KERNEL]

CVE: CVE-2026-74484
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: binfmt_misc: don't let an 'F' entry pin its own instance
Commit: 1cc2decee06acc939337304e9b3f737fd5d8c4bd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1cc2decee06acc939337304e9b3f737fd5d8c4bd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74484
Analysis date: Sat, 15 Aug 2026 18:22:48 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline Moderate, but not Important candidate
Manual review required: NO

Summary:
A local user may create a self-pinning binfmt_misc F entry or stacked mount configuration that permanently leaks the binfmt_misc superblock and user namespace charges, causing persistent local resource-exhaustion DoS without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-74484 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74484

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-2026-74484	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:N/I:N/A:H';**CWE-400;*CWE-772;CWE-404;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A binfmt_misc entry registered with the F flag can keep its interpreter file open for the lifetime of the entry. If that interpreter is placed on the same binfmt_misc instance, or if the instance is used as an overlayfs lower layer, the mount and superblock references can form a self pinning cycle. After the mount namespace is gone, there may be no remaining path to unregister the entry, so the superblock, mount objects, and the related user namespace charges remain pinned. For the CVSS the PR:L is used because on systems with sandboxed binfmt_misc mounts and user namespaces, a local unprivileged process may be able to create the needed namespace context and consume its namespace charges. The issue is not network reachable and requires local code execution. Impact is denial of service through persistent resource leakage and possible exhaustion of namespace or kernel memory resources. No direct memory corruption, information disclosure, or privilege escalation primitive is indicated.';NO MANUAL CHECK; ,and ActionableScore result is Borderline Moderate, but not Important candidate (with actual score 4)	YES	DISK LEAK LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline Moderate, but not Important candidate**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  On kernels with sandboxed binfmt_misc mounts and user namespaces, a local unprivileged process may be able to create the relevant namespace and mount context.

* Reliable persistent resource DoS: +1
  The reference cycle can permanently pin the binfmt_misc superblock, mount objects, interpreter file, and user namespace charges after the mount namespace disappears.

* Availability impact realistic: +1
  The leak is persistent and repeatable, and repeated iterations can consume namespace charges or kernel resources.

Paranoid-only signal:

* Cross-boundary isolation or resource-accounting concern: +1
  The bug pins user namespaces and can consume per-user namespace accounting in a way that persists beyond the lifetime of the triggering mount namespace. This is still resource exhaustion, not memory corruption.

Not awarded:

* No remote/network trigger.
* No UAF, OOB write, double free, type confusion, or arbitrary write.
* No confidentiality or integrity impact.
* No privilege escalation primitive.
* No Dirty-Pipe-like page-cache or COW ownership violation.

## 3. Reachability analysis

The bug is locally reachable when binfmt_misc sandboxed mounts are available and user namespaces allow a local user to create the needed mount context. The relevant stable scope starts with kernels where sandboxed binfmt_misc mounts exist, so PR:L is more appropriate than PR:H for modern affected configurations. Before sandboxed mounts, triggering would have required global root against the shared instance, but that is not the main affected scenario described by the patch.

The path is not network reachable. Exploitation requires local code execution and either pointing an F entry interpreter at the binfmt_misc instance itself or using the instance as a stacked filesystem lower layer such as overlayfs. The realistic impact is persistent local resource exhaustion.

## 4. Severity interpretation

This behaves like an actionable resource-exhaustion Moderate only in environments where unprivileged user namespaces and sandboxed binfmt_misc mounts are enabled. It does not behave like an Important-class memory corruption issue. The patch prevents a reference-cycle lifetime leak, not a stale freed object access.

Theoretical worst case is exhaustion of user namespace charges or kernel resources after repeated triggering. Realistic evidence supports DoS only.

## 5. One-sentence report phrase

A local user may create a self-pinning binfmt_misc F entry or stacked mount configuration that permanently leaks the binfmt_misc superblock and user namespace charges, causing persistent local resource-exhaustion DoS without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a persistent resource leak and DoS issue, but the patch and commit message do not support memory corruption, privilege escalation, information disclosure, or remote reachability.

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

Patch: binfmt_misc: don't let an 'F' entry pin its own instance
Commit: 1cc2decee06acc939337304e9b3f737fd5d8c4bd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1cc2decee06acc939337304e9b3f737fd5d8c4bd

Commit description:

An entry registered with 'F' opens its interpreter at registration time
and holds that file until the entry is freed. Any entry nobody removes
by hand only gets closed once the binfmt_misc superblock is shut down.
If the interpreter lives on a mount that keeps that superblock alive the
two pin each other:

    binfmt_misc sb -> inode -> entry -> interp_file -> vfsmount -> binfmt_misc sb

TL;DR the file is never closed. Once the mount namespace is gone there
is nothing left to unregister through either.

There are two ways to trigger this bug:

- Point the interpreter at the instance itself. Its files are regular
  files owned by the mounter and both bm_get_inode() and
  simple_fill_super() leave i_op at empty_iops. So notify_change() falls
  back to simple_setattr() and chmod +x works. We never set SB_I_NOEXEC
  and so open_exec() accepts it.

- Use the instance as an overlayfs lower layer. The overlay superblock
  holds a clone_private_mount() of every layer until it is destroyed and
  that clone is in no namespace. So umount_tree() never reaches it.

That's a DoS. And it isn't only the superblock that leaks. It pins the
user namespace it was mounted in, so every iteration permanently eats
one of the caller's user namespace charges.

So let's just do the sane thing. SB_I_NOEXEC makes open_exec() fail on
the instance's own files and s_stack_depth makes overlayfs reject the
layer before it ever takes a clone. That also covers the ecryptfs and
fuse passthrough variants. What 'F' promises is unchanged.

The stable tag is narrower than the Fixes tags on purpose. Before
sandboxed mounts this needed global root against the single instance
everyone shares, and the change doesn't apply to those trees anyway.

Note that SB_I_NODEV is implicitly raised for userns mounts but raise it
explicitly here as well.

Link: https://patch.msgid.link/20260728-work-binfmt_misc-selfpin-v1-1-74df5daeca5b@kernel.org
Fixes: 948b701 ("binfmt_misc: add persistent opened binary handler for containers")
Fixes: 21ca59b ("binfmt_misc: enable sandboxed mounts")
Cc: stable@vger.kernel.org # v6.7+
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/binfmt_misc.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=1cc2decee06acc939337304e9b3f737fd5d8c4bd

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

The original Linux kernel CVE announcement for CVE-2026-74484 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74484

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:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 3
  Paranoid ActionableScore: 4

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).