[CVE-2026-98262][MODERATE REGULAR] ata: libahci: clear PxCLBU and PxFBU for AHCI_HFLAG_32BIT_ONLY

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

CVE: CVE-2026-98262
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ata: libahci: clear PxCLBU and PxFBU for AHCI_HFLAG_32BIT_ONLY
Commit: 0191c9ff23ef1678dbfb5ad253a2dee92feb5d71
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0191c9ff23ef1678dbfb5ad253a2dee92feb5d71
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98262
Analysis date: Fri, 09 Oct 2026 03:47:09 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98262 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98262

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.

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

CVE-2026-98262	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98262 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (independent Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';CWE-665;CWE-754;CWE-823;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.2';DESCR 'An AHCI controller marked with AHCI_HFLAG_32BIT_ONLY can retain implementation specific nonzero values in PxCLBU and PxFBU because the hardware supports 64 bit addressing even though the driver exposes only 32 bit DMA. The HBA can therefore combine stale upper address bits with the programmed lower bits and access an incorrect physical address for the command list or received FIS area. The demonstrated result is an unusable JMicron JMB585 controller during driver probe, which can make attached storage unavailable and may prevent normal boot. A misdirected PxFBU DMA write also creates a limited and hardware dependent memory corruption concern, although the available information does not establish attacker control over the destination or transferred data. For the CVSS the PR:H reflects that reliable triggering normally requires administrative control over the driver, device reset, or hardware configuration. The issue is not network reachable. Impact is primarily denial of service, with limited confidentiality and integrity impact retained only in the paranoid assessment because an incorrect DMA target should receive manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	WRITE OOB INIT SIMPLEFIX DMAorINTERRUPT LINUS BOOT MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Availability impact realistic. The demonstrated failure makes the affected AHCI controller unusable during probe and may make attached storage or a boot volume unavailable.
* +1 Weak or indirect corruption candidate. A nonzero PxFBU upper address can make the HBA form an incorrect DMA destination for received FIS data.
* -1 No attacker-controlled payload or target. The upper address comes from an implementation-specific register reset value, while the transferred data is generated by the controller. No attacker-directed destination is established.
* B5-R was not awarded. There is no confirmed heap or slab OOB write, no retry-accumulated index, no missing retry reset, and no attacker-invocable retry sequence.
* A constrained overwrite bonus was not awarded because the supplied evidence does not confirm that the incorrect DMA destination resolves to writable host memory or that memory corruption actually occurred.
* Reliable kernel crash was not awarded. The observed result is device initialization failure rather than a kernel panic.
* Confidentiality and integrity were not awarded. No disclosure, protected-object modification, or attacker-controlled DMA corruption is established.
* Remote reachability, arbitrary write, controlled target selection, controlled payload, UAF, repeated lifetime mutation, and privilege escalation are not demonstrated.
* No rare-hardware penalty was applied merely because the issue is in a storage driver.

3. Reachability analysis

The failure occurs when libahci initializes an affected physical HBA carrying the AHCI_HFLAG_32BIT_ONLY quirk. The supplied evidence does not establish an ordinary local user, delegated service, or namespace actor that can cause the implementation-specific upper-register state or complete the relevant probe or reset sequence.

Physical PCI controller probing and AHCI register programming are not operations available through attacker-created user or network namespaces, so NamespaceScopedPrivilege is NO. HostAdminRequired remains UNKNOWN because no complete attacker-triggered operation is established. Administrative control would normally be relevant to reprobe or reset operations, but the initial failure can also occur automatically during boot.

Prior presence of affected hardware and the applicable quirk is required. There is no race, retry accumulation, network path, or repeatable userspace-controlled sequence in the supplied evidence.

4. Primitive and severity interpretation

The exact established defect is incomplete hardware initialization. The driver programs the low 32 bits of the command-list and received-FIS DMA addresses but does not clear writable upper address registers when 64-bit capability is hidden by AHCI_HFLAG_32BIT_ONLY.

The HBA later consumes the combined address. PxCLBU affects a controller read, while PxFBU describes a controller DMA write destination. This supports a technically plausible incorrect-DMA-address concern, but not a confirmed heap OOB write or independently useful corruption primitive. The demonstrated consequence is an unusable storage controller.

This is neither stale storage nor an active-object UAF. No repeated release or refcount operation exists. PrimitiveClass is CLASS_C because the demonstrated behavior is device failure and invalid hardware state, while useful host-memory corruption remains unconfirmed.

ImportantCandidate is NO because B5-R is not established and there is no confirmed memory-corruption or security-boundary primitive. AutoDowngradeAllowed is YES because the available evidence supports device-local availability failure without meaningful unresolved attacker-controlled leverage. Manual review is still recommended to determine whether specific affected controllers can DMA into valid host RAM before failing.

5. One-sentence report phrase

Failure to clear writable AHCI upper DMA address registers can make an affected controller use an incorrect command-list or received-FIS address, causing storage initialization failure and a hardware-dependent but unconfirmed risk of misdirected DMA.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm the reset values and DMA behavior of affected controllers, especially whether PxFBU can direct received-FIS writes into valid host RAM. The supplied NEW52 B5-R preservation profile does not apply.

7. Call-site confidence

Call-site confidence: medium

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

Patch: ata: libahci: clear PxCLBU and PxFBU for AHCI_HFLAG_32BIT_ONLY
Commit: 0191c9ff23ef1678dbfb5ad253a2dee92feb5d71
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0191c9ff23ef1678dbfb5ad253a2dee92feb5d71

Commit description:

A user reported that commit 105c425 ("ata: ahci: force 32-bit DMA for
JMicron JMB582/JMB585") made the JMicron JMB585 unusable on his board.

The failure is seen as soon as the ahci driver is probed, and booting with
iommu=off does not solve the problem.

Looking at the AHCI specification, PxCLBU and PxFBU are both read only '0'
for HBAs that do not support 64-bit addressing.

For HBAs that do support 64-bit addressing, the registers are read write,
with a reset value that is Implementation Specific.

When using the AHCI_HFLAG_32BIT_ONLY flag, the HBA does support 64-bit
addressing, and a 32-bit DMA mask is set by simply clearing HOST_CAP_64.
Thus, in this case, we need to explicitly clear the registers to 0.

Fixes: 105c425 ("ata: ahci: force 32-bit DMA for JMicron JMB582/JMB585")
Fixes: c7a4215 ("ahci: disable 64bit dma on sb600")
Cc: stable@vger.kernel.org
Reported-by: Roland Waltersson <roland.waltersson@netinsight.net>
Closes: https://lore.kernel.org/linux-ide/IA0PR17MB668730A4ECCD65F7A1DC3EDC9EB62@IA0PR17MB6687.namprd17.prod.outlook.com/
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20260904134310.1465051-2-cassel@kernel.org
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/ata/libahci.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=0191c9ff23ef1678dbfb5ad253a2dee92feb5d71

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

The original Linux kernel CVE announcement for CVE-2026-98262 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98262

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.2

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: 1

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).