[CVE-2026-74486][LOW] binfmt_misc: use exe_file_deny_write_access() for the interpreter clone

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

CVE: CVE-2026-74486
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: binfmt_misc: use exe_file_deny_write_access() for the interpreter clone
Commit: 255a758697da87a205e072e0cfc35897b8f743b1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=255a758697da87a205e072e0cfc35897b8f743b1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74486
Analysis date: Sat, 15 Aug 2026 18:03:34 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-74486 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74486

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74486	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:N/I:N/A:H';CWE-667;CWE-404;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'binfmt_misc can leave an interpreter inode permanently write denied because load_misc_binary used plain deny_write_access on an interpreter clone while the exec cleanup path uses exe_file_allow_write_access. When pre-content watches are involved, the deny and allow sides may disagree and i_writecount can be decremented without a matching increment on each exec through the registered entry. This can prevent later writes or updates to the interpreter even after the binfmt_misc entry and users are gone. For the CVSS the PR:L is used for the paranoid score because a local user may only need to execute through an existing vulnerable binfmt_misc entry once the watched interpreter setup exists, although creating that entry or watch is normally administrator controlled. The issue is not network reachable and the impact is denial of service against file write availability rather than memory corruption or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-74486 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 0357ef03c94ef835bd44a0658b8edb672a9dbf51 YES  NO NO unknown 	MAYBE	LOCK DISK LINUS 	-	-	checked

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

ActionableScore=2
ActionableScoreLower=1

1. ActionableScore
* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**

2. Signal breakdown
* Local unprivileged trigger: +1 in the paranoid interpretation. A local user may be able to repeatedly execute through an already configured vulnerable binfmt_misc entry after the relevant pre-content watch exists.
* Availability impact realistic: +1. Repeated execs can drive the interpreter inode write-denial accounting negative and leave the interpreter unwritable even after the binfmt_misc entry and users are gone.
* Rare or configuration-dependent setup: -1 for the conservative score. The issue needs MISC_FMT_OPEN_FILE binfmt_misc entries plus pre-content watch interaction, and creating this setup is normally administrator controlled.
* No remote reachable signal. This is not packet-triggered or network-exposed.
* No memory corruption signal. The patch fixes mismatched write-deny/write-allow accounting, not UAF, OOB access, double free, or type confusion.
* No privilege escalation signal. The demonstrated consequence is interpreter write availability loss, not credential change or code execution.
* Call-site confidence: medium-high. The changed load_misc_binary path and the paired exe_file_allow_write_access cleanup behavior are described clearly, but the full cleanup call site is not shown in the diff.

3. Reachability analysis
A local user can plausibly trigger the repeated exec side if a vulnerable binfmt_misc entry and pre-content watch are already present. Creating or modifying binfmt_misc entries and configuring such watches is normally administrator controlled, so conservative scoring treats this as a special deployment condition. Namespaces may make binfmt_misc exposure more variable, but ordinary container users typically do not get unrestricted host binfmt_misc configuration. The path is not network reachable and exploitation is realistic only on systems using this specific interpreter registration and watch combination.

4. Severity interpretation
This behaves like an ordinary Moderate or Low-like availability issue. The practical impact is a persistent write-denial state for the interpreter inode, which may block updates or replacement of that interpreter. There is no supported memory corruption primitive, no sensitive data exposure, and no credible privilege escalation path from the supplied patch context. The paranoid score keeps the local-user trigger in scope but still remains below Actionable Moderate.

5. One-sentence report phrase
A binfmt_misc interpreter clone used mismatched write-deny and write-allow helpers, allowing repeated local execs under a pre-content watch setup to leave the interpreter inode unwritable and cause a local availability issue.

6. Manual review recommendation
NO MANUAL CHECK NEEDED. The bug is configuration-dependent, local-only, and limited to write-denial accounting without evidence of memory corruption, privilege escalation, or sensitive-object access.

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

Patch: binfmt_misc: use exe_file_deny_write_access() for the interpreter clone
Commit: 255a758697da87a205e072e0cfc35897b8f743b1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=255a758697da87a205e072e0cfc35897b8f743b1

Commit description:

For MISC_FMT_OPEN_FILE entries load_misc_binary() clones the
registered interpreter file and denies write access to the clone via
plain deny_write_access(). The clone is installed as
bprm->interpreter and later released by the exec machinery through
exe_file_allow_write_access() which skips the i_writecount increment
for files with FMODE_FSNOTIFY_HSM set.

The deny and allow side can therefore come to different conclusions
when pre-content watches are in play: if a pre-content watch is added
to the interpreter after registration every subsequent exec through
that entry takes a write denial on the clone that is never paired
with a write allowance, driving the interpreter inode's i_writecount
further down with each exec and leaving the interpreter unwritable
even after the entry and all its users are gone.

Take the write denial via exe_file_deny_write_access() so both sides
of the pairing base their decision on the same file mode, and
propagate failure instead of silently ignoring it: an interpreter
that is concurrently open for writing now fails the exec with
ETXTBSY, exactly like an interpreter freshly opened via open_exec()
would.

Link: https://patch.msgid.link/20260710-work-binfmt_misc-locking-v3-2-a162f7cb58d6@kernel.org
Fixes: 0357ef0 ("fs: don't block write during exec on pre-content watched files")
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=255a758697da87a205e072e0cfc35897b8f743b1

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

The original Linux kernel CVE announcement for CVE-2026-74486 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74486

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:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1
  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).