[CVE-2026-74487][LOW] binfmt_misc: restore write access when removing an entry

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

CVE: CVE-2026-74487
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: binfmt_misc: restore write access when removing an entry
Commit: fdc1d702bf3001586221fa07e598e876a0a854c5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fdc1d702bf3001586221fa07e598e876a0a854c5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74487
Analysis date: Sat, 15 Aug 2026 12:47:17 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A `binfmt_misc` cleanup bug leaves the interpreter inode write-denied after removing a `MISC_FMT_OPEN_FILE` entry, causing persistent `ETXTBSY` write failures until inode eviction, with local administrative or delegated management access required.

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

The original Linux kernel CVE announcement for CVE-2026-74487 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74487

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-2026-74487	LOW	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-772;CWE-404;CWE-459;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'binfmt_misc entries created with MISC_FMT_OPEN_FILE keep the interpreter opened through open_exec, which denies write access to the interpreter inode while the entry exists. On entry removal the file was closed without restoring write access, leaving i_writecount unbalanced and causing later write opens of the interpreter to fail with ETXTBSY until inode eviction. For the CVSS the PR:L is used for the paranoid interpretation because some deployments may delegate binfmt_misc management to reduced root, container root, or service accounts rather than full host administration. The issue is local and is not network reachable. Impact is denial of service against file modification or update workflows, with no evidence of memory corruption, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)	YES	WRITE OOB LOCK DISK SIMPLEFIX LEAK ERRORPATH LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Availability impact realistic: +1. Removing a `binfmt_misc` entry can leave the interpreter inode with permanently denied write access, causing later write opens to fail with `ETXTBSY` until inode eviction.
* Reliable operational DoS: +1. The failure mode is deterministic once the affected entry lifecycle occurs, but it is limited to write denial on the interpreter file rather than a kernel crash.
* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2. Creating and removing `binfmt_misc` entries is normally an administrative operation.
* Legacy / non-default execution mode: -1. The issue depends on `MISC_FMT_OPEN_FILE`, which is a specific persistent-open handler mode rather than the default path.

Conservative total floors at 0.

Paranoid signals:

* Local reduced-privilege trigger: +1. Some container or service-account deployments may delegate `binfmt_misc` management to reduced root rather than full host root.
* Availability impact realistic: +1. The write-denial state can persist after entry removal until inode eviction.
* Reliable operational DoS: +1. The affected interpreter remains unwritable with `ETXTBSY`.
* Non-default `MISC_FMT_OPEN_FILE` mode: -1.

Paranoid total: 2.

Not awarded:

* No generic memory corruption. The patch restores a missing `exe_file_allow_write_access()` call before `filp_close()`.
* No UAF, double-free, OOB write, refcount takeover, or type confusion.
* No confidentiality or integrity impact is supported.
* No privilege escalation primitive is visible.
* No remote or network trigger.

Call-site confidence: high. The affected cleanup path and the missing balancing call are directly visible in the provided patch.

## 3. Reachability analysis

The bug is triggered by registering a `binfmt_misc` entry with `MISC_FMT_OPEN_FILE` and then removing it. In normal deployments this requires administrative control over `binfmt_misc`, usually root or equivalent capability. In some container-oriented setups, reduced root or a service account may be allowed to manage entries, which supports the paranoid PR:L-style interpretation, but not a remote or unauthenticated one.

The path is local and configuration-dependent. It is not packet-triggerable and does not depend on attacker-controlled kernel memory layout. Realistic exploitation is an operational denial of service against updates or writes to the interpreter inode, not kernel compromise.

## 4. Severity interpretation

This behaves like a low-end Moderate or Low-like operational DoS. Theoretical impact is persistent write denial on a specific interpreter file until inode eviction. Realistic exploitation evidence does not support memory corruption, data disclosure, privilege escalation, or arbitrary modification of protected data.

The issue should not be treated as Important-class. It is mainly a resource-accounting imbalance in the executable write-denial mechanism.

## 5. One-sentence report phrase

A `binfmt_misc` cleanup bug leaves the interpreter inode write-denied after removing a `MISC_FMT_OPEN_FILE` entry, causing persistent `ETXTBSY` write failures until inode eviction, with local administrative or delegated management access required.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch shows a straightforward missing cleanup balance for write-access denial, with no memory corruption or privilege-escalation primitive. This can be handled as an ordinary Moderate or Low-like local DoS unless the product specifically delegates `binfmt_misc` management to untrusted container users.

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

Patch: binfmt_misc: restore write access when removing an entry
Commit: fdc1d702bf3001586221fa07e598e876a0a854c5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fdc1d702bf3001586221fa07e598e876a0a854c5

Commit description:

Registering an entry with the MISC_FMT_OPEN_FILE flag opens the
interpreter via open_exec() which denies write access to it for as
long as the entry exists. Removing the entry closes the interpreter
file via filp_close() but never restores write access, leaving the
inode's i_writecount permanently negative. Opening the interpreter
for writing keeps failing with ETXTBSY long after the entry is gone
until the inode is evicted from the inode cache.

Commit 90f601b ("binfmt_misc: restore write access before
closing files opened by open_exec()") fixed the same imbalance in the
error path of bm_register_write() but the actual removal path has
been leaking the write denial since the introduction of the flag.

Restore write access in put_binfmt_handler() before closing the
interpreter file.

Link: https://patch.msgid.link/20260710-work-binfmt_misc-locking-v3-1-a162f7cb58d6@kernel.org
Fixes: 948b701 ("binfmt_misc: add persistent opened binary handler for containers")
Cc: stable@vger.kernel.org
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=fdc1d702bf3001586221fa07e598e876a0a854c5

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

The original Linux kernel CVE announcement for CVE-2026-74487 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74487

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:H/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: 0
  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).