[CVE-2026-89604][LOW] efivarfs: Rate limit statfs() handler

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

CVE: CVE-2026-89604
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: efivarfs: Rate limit statfs() handler
Commit: 8953bdd1b9c0e748d5408d3d0b8e7aeee5c9cd41
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8953bdd1b9c0e748d5408d3d0b8e7aeee5c9cd41
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89604
Analysis date: Sat, 12 Sep 2026 01:43:34 -0400
ActionableScore: 2
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-2026-89604 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89604

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-2026-89604	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89604 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-770;CWE-799;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'efivarfs statfs() could call the UEFI QueryVariableInfo() runtime service on every request, and statfs() on the efivarfs mount point can be reached by an unprivileged local user. On x86 systems this runtime service may be backed by SMM, where each call can require a rendezvous of all CPUs and can be disproportionately expensive. A local user can repeatedly call statfs() to create excessive firmware runtime activity and degrade system availability. For the CVSS the PR:L is selected because reliable triggering requires a local user or local process context, but no administrative privileges are needed. The issue is not network reachable and does not indicate memory corruption, information disclosure, or privilege escalation. Impact is denial of service through CPU and firmware runtime overhead, with base A:L and paranoid A:H for sustained abuse on affected firmware.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	LOCK DANGER DISK HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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

ActionableScore=2
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2. No floor or cap applied.
* Raw paranoid score: 2
* Paranoid score: 2. No floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive signals:

* +1 Local unprivileged trigger. The commit explicitly states that statfs() may be called by unprivileged users on the efivarfs mount point.
* +1 Availability degradation. Repeated statfs() calls can flood QueryVariableInfo() runtime service calls, which are expensive on x86 systems backed by SMM because each entry may rendezvous all CPUs.

Non-awarded signals:

* No Real UAF. The patch does not show stale object use, double release, refcount misuse, or post-lifetime access.
* No memory corruption primitive. There is no OOB access, arbitrary write, controlled overwrite, type confusion, or kernel object corruption.
* No reliable kernel crash signal. The demonstrated issue is CPU and firmware runtime overhead, not panic or fatal fault.
* No concrete persistent resource or reference leak. The bug is excessive repeated calls, not a missing put or unrecoverable resource leak.
* No confidentiality or integrity signal. The patch does not expose data and does not modify protected state.
* No security-boundary bypass. No authorization decision changes from deny to allow.
* No remote reachability. Triggering is via local statfs() on efivarfs, not network input.
* No extra Availability point. The same SMM induced slowdown is already counted as the availability degradation signal.
* No host-admin penalty. Below-host-admin reachability is positively established by the commit.

Negative signals:

* 0 No rare hardware penalty. The strongest impact is x86 SMM-backed firmware behavior, but this is a performance amplification condition rather than a rare physical-access prerequisite.
* 0 No privilege penalty. The vulnerable operation is explicitly reachable by unprivileged local users.

3. Reachability analysis

The trigger is a local filesystem metadata query against the efivarfs mount point. The supplied commit states that unprivileged users can call statfs() there, so HostAdminRequired=NO. No namespace capability is required or relevant, so NamespaceScopedPrivilege=NO.

There is prior system configuration in the sense that efivarfs must be mounted and EFI runtime QueryVariableInfo() must be supported, but the attacker does not need to create the mount or obtain administrative privileges once the mount is available. The strongest practical impact depends on firmware behavior, especially x86 systems where variable storage operations enter SMM and rendezvous all CPUs. There is no race requirement and the operation can be repeated cheaply.

4. Primitive and severity interpretation

The exact primitive is local resource consumption through repeated expensive firmware runtime service calls from efivarfs_statfs(). This is not active structured stale storage, not fixed stale storage, and not a lifetime bug. There is no concrete repeated release or refcount operation.

PrimitiveClass=NONE because the patch establishes a DoS style resource consumption issue only. The demonstrated consequence is degraded availability due to CPU and firmware runtime overhead. The speculative stronger consequence would be broader host responsiveness loss under sustained abuse on affected firmware, but that remains the same DoS class and does not create C or I impact.

ImportantCandidate=NO because there is no memory corruption, ownership violation, authorization bypass, UAF, or unresolved exploitation primitive. AutoDowngradeAllowed=YES because the issue is suitable for ordinary Moderate or Low-like handling and does not require Important-vs-Moderate preservation.

5. One-sentence report phrase

efivarfs statfs() could be abused by an unprivileged local user to repeatedly invoke expensive UEFI QueryVariableInfo() runtime calls, causing CPU and firmware runtime overhead and local denial of service on affected x86 SMM backed systems.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a local resource-consumption DoS without memory corruption, privilege escalation, information disclosure, authorization bypass, or a persistent resource leak.

7. Call-site confidence

Call-site confidence: high

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

Patch: efivarfs: Rate limit statfs() handler
Commit: 8953bdd1b9c0e748d5408d3d0b8e7aeee5c9cd41
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8953bdd1b9c0e748d5408d3d0b8e7aeee5c9cd41

Commit description:

Ravi reports that statfs() may be called by unprivileged users on the
efivarfs mount point, which may result in a flood of calls to the
QueryVariableInfo() runtime service. These calls are disproportionately
costly on x86 systems where the variable store is backed by SMM, as each
SMM entry requires a rendez-vous of all the CPUs.

So rate limit the calls to QueryVariableInfo() at twice per second, and
return the most recently obtained value for calls that are elided.

Cc: <stable@vger.kernel.org>
Reported-by: Ravi Bangoria <ravi.bangoria@amd.com>
Fixes: d86ff33 ("efivarfs: expose used and total size")
Reviewed-by: Anisse Astier <anisse@astier.eu>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/efivarfs/super.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=8953bdd1b9c0e748d5408d3d0b8e7aeee5c9cd41

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

The original Linux kernel CVE announcement for CVE-2026-89604 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89604

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: unknown
  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).