[CVE-2026-89587][MODERATE 7.0] ACPI: pfr_update: fix stack buffer overflow in query_capability()

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

CVE: CVE-2026-89587
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ACPI: pfr_update: fix stack buffer overflow in query_capability()
Commit: a2151624b55029dad0ca7efd17fb83a0461c8843
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a2151624b55029dad0ca7efd17fb83a0461c8843
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89587
Analysis date: Fri, 11 Sep 2026 20:41:25 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A stack based buffer overflow in ACPI pfr_update query_capability allows oversized firmware _DSM buffers to overwrite kernel stack memory during the PFRU ioctl path, causing at least local DoS and requiring manual review for possible stronger corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-89587 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89587

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 7.0
Manual review required: YES

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
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CVE-2026-89587	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89587 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:H/I:H/A:H';CWE-121;*CWE-787;*CWE-20;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A stack based buffer overflow exists in the ACPI pfr_update query_capability path because firmware supplied _DSM buffer lengths are used for memcpy into fixed u8(16) fields without checking the destination size. The affected structure is allocated on the kernel stack in pfru_ioctl, so an oversized firmware buffer can corrupt adjacent stack memory and may cause a kernel crash or potentially more serious memory corruption effects. For the CVSS the PR:L value is used for the paranoid score because a local process with access to the PFRU ioctl path may be enough on systems where the device node or a firmware management service is delegated below full administrator control. The issue is not network reachable and depends on local device access plus ACPI firmware returning oversized buffers. Impact is at least local denial of service via kernel memory corruption, and worst case may include confidentiality and integrity impact due to a kernel stack overwrite.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	WRITE KASAN OOB HARDWARE KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

## 1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

Conservative floor applied: 4 -> 5 because the patch establishes a concrete kernel stack out-of-bounds write, not a crash-only validation bug.

## 2. Signal breakdown

Conservative signals:

* +2: Strong kernel corruption primitive. `query_capability()` copies firmware-supplied ACPI `_DSM` buffers into fixed `u8[16]` stack fields without length validation, producing a stack out-of-bounds write.
* +1: Firmware-mediated external influence. The unsafe copy length and source buffer come from firmware `_DSM` output, so firmware or platform-controlled data is material to triggering the overwrite.
* +1: Reliable kernel crash / strong DoS. KASAN confirms stack-out-of-bounds writes with 64-byte buffers, and this can plausibly crash or corrupt kernel stack state.

Paranoid additional interpretation:

* +1: Weak exploitation concern from active kernel stack overwrite. The affected object is stack-allocated in `pfru_ioctl()`, and the overwrite reaches adjacent stack redzones, so consequences beyond a clean crash are technically defensible enough for manual review.

Non-awarded signals:

* RealUAF not awarded. No use-after-free, stale pointer, RCU lifetime issue, or repeated release/refcount operation is established.
* Concrete repeated release/refcount not awarded. There is no first release, surviving exact reference, and second lifetime mutation.
* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Direct remote reachability not established.
* Local unprivileged trigger not established because access permissions for the complete PFRU ioctl path are not supplied.
* Host-admin penalty not applied because host-admin-equivalent privilege is not positively established.
* Namespace-scoped privilege not awarded because ACPI firmware/PFRU ioctl is not a namespace-scoped operation.
* Extra availability not awarded because the same crash/corruption path is already counted.
* Confidentiality and integrity bonuses are not independently awarded. They remain possible paranoid consequences of stack corruption, but no concrete disclosure, credential overwrite, arbitrary write, or privilege-escalation chain is supplied.

## 3. Reachability analysis

The vulnerable operation is reached through the ACPI PFRU update driver, specifically the `pfru_ioctl()` path calling `query_capability()`. The overflow requires ACPI firmware `_DSM` output with one or more oversized buffers for `code_type`, `drv_type`, `platform_id`, or `oem_id`.

The attacker model is local plus firmware-mediated. A local process must be able to reach the PFRU ioctl path, and the platform firmware must return oversized `_DSM` buffers. The supplied input does not establish whether ordinary users can access the device node or whether host administrator privileges are required. Therefore `HostAdminRequired=UNKNOWN`.

The issue is not network reachable. It is not namespace scoped. There is likely a platform prerequisite: the system must expose the ACPI PFRU interface and the relevant firmware `_DSM` method. This limits exposure, but it does not erase the established stack overwrite primitive.

No timing race is involved.

## 4. Primitive and severity interpretation

The exact primitive is a stack-based out-of-bounds write in kernel context. Four firmware-supplied buffers are copied into fixed-size 16-byte fields without checking whether the ACPI buffer length exceeds the destination field size.

This is not fixed stale storage and not cleanup-only behavior. It is an active stack corruption primitive during an ioctl-triggered query path. The demonstrated consequence is KASAN-confirmed stack-out-of-bounds writes and likely DoS. The speculative but technically plausible consequence is broader kernel stack corruption, potentially affecting control/data state depending on layout, compiler options, stack protections, and exact overwritten bytes.

PrimitiveClass is `CLASS_A` because the patch establishes a concrete kernel stack overwrite. It is not downgraded to Class C because this is not merely a NULL dereference, warning, or passive invalid state.

ImportantCandidate is `YES` because kernel stack memory corruption may have security impact beyond DoS and should not be automatically closed based only on the observed KASAN symptom.

AutoDowngradeAllowed is `NO` because a concrete kernel corruption primitive remains unresolved. The absence of a public exploit or demonstrated LPE does not make this safe to autoclose.

Likely operational severity is Actionable Moderate at minimum, with Strong Important review warranted if the PFRU ioctl path is reachable by a below-host-admin user or by a delegated firmware management service.

## 5. One-sentence report phrase

A stack based buffer overflow in ACPI pfr_update query_capability allows oversized firmware _DSM buffers to overwrite kernel stack memory during the PFRU ioctl path, causing at least local DoS and requiring manual review for possible stronger corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is confirmed kernel stack memory corruption with unresolved exploitability and unclear local privilege boundary for the complete PFRU ioctl trigger path.

## 7. Call-site confidence

Call-site confidence: medium

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

Patch: ACPI: pfr_update: fix stack buffer overflow in query_capability()
Commit: a2151624b55029dad0ca7efd17fb83a0461c8843
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a2151624b55029dad0ca7efd17fb83a0461c8843

Commit description:

query_capability() copies four ACPI buffer objects returned by the
firmware _DSM into fixed-size u8[16] fields in struct
pfru_update_cap_info using memcpy with the firmware-supplied length:

  memcpy(&cap_hdr->code_type,
         elements[CAP_CODE_TYPE_IDX].buffer.pointer,
         elements[CAP_CODE_TYPE_IDX].buffer.length);

The same pattern repeats for drv_type, platform_id, and oem_id.
If the firmware returns buffer.length > 16 for any of these fields,
memcpy writes past the destination array.

struct pfru_update_cap_info is stack-allocated in pfru_ioctl().

Confirmed with KASAN on 7.2-rc6: three stack-out-of-bounds reports
are generated when a DSM returns 64-byte buffers, with writes reaching
44 bytes past the end of cap_hdr's [64, 156) frame window into
adjacent stack redzones.

Introduce a helper pointer to out_obj->package.elements and use it
to validate each buffer length against its destination field size
before copying, returning -EINVAL if the firmware supplies an
oversized buffer.

Fixes: 0db89fa ("ACPI: Introduce Platform Firmware Runtime Update device driver")
Cc: All applicable <stable@vger.kernel.org>
Signed-off-by: Anirudh Prasad <icarus@a0rg.com>
Link: https://patch.msgid.link/1a001e1fee9.637da6dc3533246.238498880682901704@a0rg.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/acpi/pfr_update.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=a2151624b55029dad0ca7efd17fb83a0461c8843

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

The original Linux kernel CVE announcement for CVE-2026-89587 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89587

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:H/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: unknown
  Paranoid ActionableScore: 5

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).