[CVE-2026-89448][MODERATE 7.0] iommu/vt-d: Force requesting ACS when tboot is enabled

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

CVE: CVE-2026-89448
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: iommu/vt-d: Force requesting ACS when tboot is enabled
Commit: aaeb81241e802c86be69394f72d49fde3f861fbb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=aaeb81241e802c86be69394f72d49fde3f861fbb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89448
Analysis date: Sat, 12 Sep 2026 04:38:27 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89448 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89448

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-2026-89448	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89448 CHECK WITH IMPACT FROM ORIG NN MODERATE	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-693;CWE-284;CWE-1189;Other CVSS 'AV:P/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:L';BEST CVSS score: '7';DESCR 'A protection mechanism failure in Intel VT-d setup can leave PCI ACS not requested when tboot later forces IOMMU on. This may weaken DMA and PCIe peer to peer isolation because the system can operate with IOMMU enabled but without the ACS configuration expected for device isolation. For the CVSS the PR:L is used for the paranoid score because practical exploitation may involve a local tenant, service account, or reduced privilege user with delegated access to a PCIe or passthrough device rather than full host administration. The issue is not directly network reachable and depends on tboot, Intel IOMMU, ACS capable topology, and attacker influence over a device path. Impact may include confidentiality and integrity loss through weakened DMA isolation, with possible availability impact if memory or device state is corrupted.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	SIMPLEFIX KPANIC MEMORY 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4. No floor or cap applied.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2. Meaningful protection or security-boundary failure. The patch establishes that ACS might not be requested when tboot later forces Intel IOMMU on, leaving a DMA and PCIe peer-to-peer isolation mechanism weaker than expected.
* +1. Firmware or device-mediated external influence. Exploitation is not direct network input, but a malicious or attacker-influenced PCIe device, passthrough device, or device path could make missing ACS security-relevant.
* +1. Confidentiality plausible. Weakened DMA or PCIe isolation can plausibly expose memory or device state across intended isolation boundaries.
* +1. Integrity plausible. Weakened DMA or PCIe isolation can plausibly allow corruption of memory or device state across intended isolation boundaries.
* -1. Concrete non-default execution prerequisite. The issue specifically depends on tboot/TXT behavior and an Intel VT-d setup where IOMMU is forced while ACS would otherwise not be requested.

Raw conservative score: 2 + 1 + 1 + 1 - 1 = 4.

Paranoid additional signal:

* +1. Availability plausible. If the weakened isolation permits invalid DMA or peer-to-peer effects, host or device availability can plausibly be affected, although a concrete crash primitive is not shown.

Raw paranoid score: 2 + 1 + 1 + 1 + 1 - 1 = 5.

Important non-awarded signals:

* RealUAF not awarded. No stale pointer, freed object use, RCU lifetime bug, or repeated release/refcount operation is established.
* Controlled reclaim not demonstrated.
* Arbitrary kernel read or write not demonstrated.
* Direct remote reachability not established.
* Local unprivileged trigger not established.
* Practical exploit chain not demonstrated.
* Strong LPE or root compromise not demonstrated.
* No race or timing penalty applied. This is a boot or detection logic issue, not a timing race.
* No host-admin penalty applied because host-admin-equivalent privilege is not positively established as required for every plausible security-relevant attacker model. Prior host setup is likely in many deployments, but the complete privilege boundary remains unresolved.

3. Reachability analysis

The vulnerable condition is reached during Intel IOMMU detection at boot or early initialization. The relevant prerequisites are tboot enabled, Intel VT-d present, a PCIe topology where ACS matters, and a configuration where ACS would not be requested by the old detect_intel_iommu logic while IOMMU is later forced by tboot_force_iommu.

This is not directly network reachable. It is also not a normal local syscall or namespace-triggered bug. The practical attacker model is deployment dependent. A meaningful attacker may be a local tenant, service account, or device owner with delegated access to a PCIe or passthrough device, or an attacker controlling a malicious physical or virtualized device path. A separate host administrator may be needed to enable tboot, configure boot parameters, assign devices, or expose the relevant topology.

HostAdminRequired is UNKNOWN because the supplied evidence does not prove that host-admin-equivalent privilege is always required for the complete security-relevant exploitation path. NamespaceScopedPrivilege is NO because no user namespace or network namespace scoped capability path is involved.

There is no repeatable race. There is a configuration and platform dependency rather than a timing dependency.

4. Primitive and severity interpretation

The primitive is a protection mechanism failure in platform device isolation. Intel IOMMU may be forced on by tboot while PCI ACS is not requested, creating a state where the system appears to rely on IOMMU isolation but lacks the expected ACS enforcement for PCIe peer-to-peer paths.

This is not memory corruption in the patched code and not a UAF. It is not fixed stale storage or active structured object lifetime corruption. There is no concrete repeated release or refcount mutation.

PrimitiveClass is CLASS_A because the patch directly restores a security isolation mechanism whose absence may weaken DMA and PCIe boundary enforcement. The demonstrated consequence is misconfiguration of ACS request behavior under tboot-forced IOMMU. The speculative but technically defensible consequence is confidentiality, integrity, or availability impact through weakened device isolation.

ImportantCandidate is YES because platform DMA isolation failures are security-boundary issues whose real impact depends heavily on hardware topology, passthrough configuration, and attacker access to a device path. AutoDowngradeAllowed is NO because automatically treating this as an ordinary low-risk configuration bug could miss a meaningful IOMMU or PCIe isolation bypass.

Likely operational severity is Actionable Moderate, with Strong Important candidacy requiring manual platform-specific review.

5. One-sentence report phrase

A tboot forced Intel VT-d configuration can leave PCI ACS not requested, weakening expected DMA and PCIe peer-to-peer isolation and requiring manual review for device or passthrough based confidentiality and integrity impact.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch fixes a security-boundary protection mechanism around IOMMU and ACS, and the practical impact depends on platform topology, ACS support, tboot behavior, boot options, and whether attackers can influence a PCIe or passthrough device path.

7. Call-site confidence

Call-site confidence: medium

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

Patch: iommu/vt-d: Force requesting ACS when tboot is enabled
Commit: aaeb81241e802c86be69394f72d49fde3f861fbb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=aaeb81241e802c86be69394f72d49fde3f861fbb

Commit description:

Currently the conditions of requesting ACS in detect_intel_iommu()
don't include tboot, leading to a possible misconfiguration with ACS
disabled (e.g. due to user opts) while iommu is later forced on by
tboot_force_iommu().

Fix it by checking tboot in detect_intel_iommu().

Fixes: 5d990b6 ("PCI: add pci_request_acs")
Cc: stable@vger.kernel.org
Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/iommu/intel/dmar.c
  drivers/iommu/intel/iommu.c
  drivers/iommu/intel/iommu.h

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=aaeb81241e802c86be69394f72d49fde3f861fbb

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

The original Linux kernel CVE announcement for CVE-2026-89448 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89448

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:P/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:L
  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: 4
  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).