[CVE-2026-90243][MODERATE 7.0] iommu/vt-d: Clear Present bit before tearing down copied context entry [ Upstream

[CVE-2026-90243][MODERATE 7.0] iommu/vt-d: Clear Present bit before tearing down copied context entry [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90243
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: iommu/vt-d: Clear Present bit before tearing down copied context entry [ Upstream
Commit: 97750a8dce79c1a9b2b2d2a020b61b800141996c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=97750a8dce79c1a9b2b2d2a020b61b800141996c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90243
Analysis date: Fri, 18 Sep 2026 07:37:38 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A VT-d copied context entry can be torn down while the Present bit is still visible to IOMMU hardware, allowing a torn DMA remapping entry to be fetched and causing at least DoS through spurious faults, with unresolved DMA isolation impact requiring manual review.

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

The original Linux kernel CVE announcement for CVE-2026-90243 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90243

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.

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

CVE-2026-90243	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90243 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-362;CWE-662;CWE-693;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A VT-d copied context entry could be torn down by clearing the full 128 bit entry while the Present bit was still visible to hardware. During the window before context cache and IOTLB invalidation, the IOMMU could fetch a partially cleared entry and use inconsistent DMA remapping state. The realistic impact is local denial of service through spurious faults or unpredictable device translation behavior. For the CVSS the PR:L is used in the paranoid view because a reduced capability local actor may control a passed through device or VFIO style device context in some deployments. The issue is not network reachable and requires local device or IOMMU lifecycle interaction. In the worst defensible interpretation, inconsistent IOMMU context state may affect DMA isolation and could expose confidentiality or integrity risk in addition to availability impact.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	INIT SIMPLEFIX HARDWARE  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4	YES	NO	checked

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

ActionableScore=6
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

## 1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

No floor or cap changed the score.

## 2. Signal breakdown

Conservative signals:

* +2: CLASS_B active-state retirement / synchronization violation. A live VT-d context entry is torn down incorrectly while the Present bit can still be visible to IOMMU hardware.
* +1: Active translation synchronization signal. The fix adds an explicit cache flush before invalidation, showing that stale or partially updated translation state can remain operationally visible outside the CPU-side structure.
* +1: Availability impact. Hardware may fetch a torn context entry, causing spurious faults or unpredictable DMA translation behavior.

Paranoid additional signals:

* +1: Active semantic / translation-domain transition. The context entry can be observed as still Present while other fields are already zeroed, so hardware may reinterpret a partially destroyed translation entry as a valid one.
* +1: High-value DMA/IOMMU state. The affected state participates in DMA remapping and device isolation, so unresolved consequences deserve stronger manual review even though arbitrary host memory access is not demonstrated.

Non-awarded signals:

* RealUAF not awarded. The patch does not establish a heap use-after-free or stale object dereference.
* Controlled reclaim not demonstrated.
* Arbitrary read/write not demonstrated.
* Confidentiality and integrity consequences are not scored directly because the patch does not prove a concrete DMA isolation bypass or attacker-directed memory access.
* Remote reachability not established.
* Local unprivileged trigger not established.
* No race penalty was applied merely because a timing window exists.
* No host-admin penalty was applied because host-admin-equivalent privilege is not positively established from the supplied evidence.

## 3. Reachability analysis

The vulnerable operation is in Intel VT-d copied context teardown. It is likely reached during device, domain, passthrough, or IOMMU lifecycle operations, but the supplied patch does not fully establish who can trigger the complete path.

A host administrator can plausibly trigger relevant device/IOMMU lifecycle transitions. A reduced-privilege actor may be relevant in deployments using VFIO or delegated device passthrough, but the supplied evidence does not prove that a below-host-admin user can complete the trigger sequence. Therefore HostAdminRequired is UNKNOWN rather than YES or NO.

Namespace-scoped privilege is not established. This is not a network namespace or ordinary namespaced CAP_NET_ADMIN path, so NamespaceScopedPrivilege is NO.

No network vector is established. No direct remote packet-triggered path is shown. Special hardware is involved in the sense that Intel VT-d/IOMMU behavior matters, but VT-d is common enough that no rare-hardware penalty is applied.

The timing issue depends on hardware observing a torn 128-bit entry or stale cache-visible state. The patch establishes a real ordering and visibility problem, but not enough to apply a hard-race penalty as a negative score.

## 4. Primitive and severity interpretation

The exact primitive is an active IOMMU translation/context-state consistency violation. The old context entry remains live enough for hardware to observe it while software is tearing it down in the wrong order.

This is not a UAF. It is also not merely a warning-only bug, because the entry is active translation state used by IOMMU hardware.

PrimitiveClass is CLASS_B because the bug affects live DMA remapping state and bypasses the required retirement/invalidation handshake, but does not establish a controlled corruption primitive, arbitrary write, controlled reclaim, or demonstrated isolation escape.

The demonstrated consequence is unpredictable behavior or spurious faults, which supports DoS or device/IOMMU disruption. The speculative but technically relevant consequence is DMA isolation impact if a torn Present entry is interpreted in a security-relevant way. That possibility is not proven strongly enough for CLASS_A, but it is important enough to prevent automatic downgrade.

ImportantCandidate is YES because IOMMU context corruption is security-boundary relevant and the full impact is unresolved. AutoDowngradeAllowed is NO because the bug affects active DMA translation state and has unresolved security-boundary implications beyond an ordinary device-local warning.

Likely operational severity is Actionable Moderate, with Strong Important review warranted if a product has VFIO, device passthrough, or delegated device-management exposure.

## 5. One-sentence report phrase

A VT-d copied context entry can be torn down while the Present bit is still visible to IOMMU hardware, allowing a torn DMA remapping entry to be fetched and causing at least DoS through spurious faults, with unresolved DMA isolation impact requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug affects live IOMMU/DMA remapping state, and the patch restores a required Present-bit clear, cache flush, and invalidation ordering protocol. The demonstrated impact is mainly DoS, but DMA isolation consequences are not safely dismissible from the supplied patch alone.

## 7. Call-site confidence

Call-site confidence: medium

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

Patch: iommu/vt-d: Clear Present bit before tearing down copied context entry [ Upstream
Commit: 97750a8dce79c1a9b2b2d2a020b61b800141996c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=97750a8dce79c1a9b2b2d2a020b61b800141996c

Changed files:
  drivers/iommu/intel/iommu.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=97750a8dce79c1a9b2b2d2a020b61b800141996c

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

The original Linux kernel CVE announcement for CVE-2026-90243 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90243

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: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: 6

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).