[CVE-2026-53281][IMPORTANT] iommu/vt-d: Avoid NULL pointer dereference or refcount corruption

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

CVE: CVE-2026-53281
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: iommu/vt-d: Avoid NULL pointer dereference or refcount corruption
Commit: 9022cb9ac0c2a72a57fa8ebf92ac74f953ca0153
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9022cb9ac0c2a72a57fa8ebf92ac74f953ca0153
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53281
Analysis date: Fri, 26 Jun 2026 16:14:06 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
Intel VT-d PASID detach may continue teardown after failing to find dev_pasid, causing a NULL pointer dereference or refcount corruption that can potentially lead to use-after-free in shared IOMMU domain state.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53281 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53281

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: IMPORTANT
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-2026-53281	IMPORTANT	CHECK	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-476;CWE-911;*CWE-416;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'Intel VT d PASID detach can run teardown even when no matching dev_pasid entry was found in the domain list. In that state cache tag unassignment and domain_detach_iommu can operate on a domain that was not actually attached for that PASID, which may cause a NULL pointer dereference or unbalanced refcount decrement. The refcount corruption can prematurely drop shared domain state and is a plausible use after free candidate for other active devices that still share the domain. For the CVSS the PR:L is used for the paranoid score because reliable triggering normally needs local access to a device or management interface that can exercise PASID attach and detach paths, but not necessarily full host administrator privileges in delegated VFIO or device assignment environments. The issue is not directly network reachable. Impact is at least local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to refcount based memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	LOCK DANGER NULLPTR UAF HARDWARE DEBUGFS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Generic memory corruption: +2. The commit explicitly describes refcount corruption that can prematurely drop shared domain state and potentially cause use-after-free for remaining active devices.
* Real lifetime corruption: +1. The bug is a refcount lifetime imbalance in IOMMU domain teardown.
* Reliable kernel crash / strong DoS: +1. If info is NULL, the path can immediately dereference it and crash.
* Privileged kernel/device-management memory corruption path: +1. The corruption occurs in Intel VT-d PASID and IOMMU domain management, a trusted device-management path.
* Hard or unreliable condition: -1. The vulnerable state requires a missing dev_pasid during PASID detach, described as an unlikely situation.
* Privilege/delegation uncertainty: -1. Typical triggering likely requires device-management access, but delegated VFIO/iommufd or assigned-device environments may reduce this below full host-root.

Paranoid additions:

* Local delegated trigger concern: +1. A local user or service with assigned-device or PASID-management access may be able to exercise the affected attach/detach path.
* Confidentiality impact plausible: +1. Refcount-driven UAF in shared IOMMU domain state can plausibly expose cross-device or stale kernel state in worst case.
* Integrity impact plausible: +1. Refcount lifetime corruption in IOMMU domain management can plausibly affect mappings or shared device-domain state.

## 3. Reachability analysis

This is not network reachable. The affected path is local and tied to Intel VT-d PASID removal in IOMMU domain management. In typical deployments, direct triggering requires privileged device-management operations, but practical privilege may be lower in systems using VFIO, iommufd, SVA, device assignment, or delegated device access. The path is not universally default-exposed, but it exists in security-sensitive IOMMU/device isolation code where refcount mistakes can affect shared domain lifetime.

## 4. Severity interpretation

This is stronger than an ordinary Moderate NULL dereference because the commit explicitly mentions refcount corruption and a potential use-after-free affecting other active devices sharing the domain. Realistic exploitation evidence is limited, and the trigger appears specialized, so the conservative score remains Actionable Moderate. The paranoid interpretation is a Strong Important candidate because refcount lifetime corruption in IOMMU domain state is a plausible memory-safety and isolation-risk class requiring manual review.

## 5. One-sentence report phrase

Intel VT-d PASID detach may continue teardown after failing to find dev_pasid, causing a NULL pointer dereference or refcount corruption that can potentially lead to use-after-free in shared IOMMU domain state.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch context explicitly identifies refcount corruption and potential UAF in IOMMU domain lifetime management, which is security-sensitive even though the trigger is local and likely specialized.

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

Patch: iommu/vt-d: Avoid NULL pointer dereference or refcount corruption
Commit: 9022cb9ac0c2a72a57fa8ebf92ac74f953ca0153
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9022cb9ac0c2a72a57fa8ebf92ac74f953ca0153

Commit description:

Commit 60f030f ("iommu/vt-d: Avoid use of NULL after WARN_ON_ONCE")
fixed a NULL pointer dereference in an unlikely situation partly.

If dev_pasid is not found in the dev_pasids list, it remains NULL.
However, the teardown operations are executed unconditionally, this lead
to a NULL pointer dereference or refcount corruption.

If the domain was never attached to this IOMMU, info will be NULL, which
would cause an immediate dereference when checking --info->refcnt.

Even if info is not NULL, decrementing the refcount without having removed
a valid PASID might unbalance the count. This could lead to premature
dropping of the refcount to 0, potentially causing a use-after-free for the
remaining active devices sharing the domain.

Fix it by returning early if dev_pasid is NULL, before executing the
teardown operations.

Issue found by AI review and suggested by Kevin Tian.
https://sashiko.dev/#/patchset/20260421031347.1408890-1-zhenzhong.duan%40intel.com

Fixes: 60f030f ("iommu/vt-d: Avoid use of NULL after WARN_ON_ONCE")
Cc: stable@vger.kernel.org
Suggested-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20260422033538.95000-1-zhenzhong.duan@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/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=9022cb9ac0c2a72a57fa8ebf92ac74f953ca0153

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

The original Linux kernel CVE announcement for CVE-2026-53281 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53281

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:L/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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).