[CVE-2026-74573][IMPORTANT] iommu/arm-smmu-v3-iommufd: Require exactly one Stream ID for a vDEVICE [ Upstream

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

CVE: CVE-2026-74573
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: iommu/arm-smmu-v3-iommufd: Require exactly one Stream ID for a vDEVICE [ Upstream
Commit: 3808bab5d95ae79e333e11f6a73d178e084c645d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3808bab5d95ae79e333e11f6a73d178e084c645d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74573
Analysis date: Sat, 15 Aug 2026 13:42:18 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
ARM SMMU v3 IOMMUFD vDEVICE setup accepted devices without exactly one Stream ID, allowing missed ATC/IOTLB invalidation for multi-stream devices or an OOB read for zero-stream devices, with potential DoS and DMA isolation impact.

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

The original Linux kernel CVE announcement for CVE-2026-74573 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74573

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.

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

CVE-2026-74573	IMPORTANT	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-20;CWE-125;CWE-284;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'arm SMMU v3 IOMMUFD vDEVICE setup accepted devices that did not have exactly one physical Stream ID even though arm_vsmmu_vsid_to_sid maps a guest vSID only to master streams 0. For devices with multiple streams, guest invalidation may miss ATC/IOTLB entries for the remaining streams and stale DMA translations can remain active after they should have been invalidated. For devices with zero streams, the same assumption can read from a ZERO_SIZE_PTR like stream array and create an out of bounds read condition. For the CVSS the PR:L is used for the paranoid score because a local guest, VMM process, or delegated virtualization service with access to IOMMUFD and assigned devices may be sufficient in practical deployments, even if full device assignment is administratively controlled. The issue is not network reachable and requires a specific ARM SMMU v3 IOMMUFD nested or vDEVICE configuration. Impact is at least local denial of service via invalid read or inconsistent invalidation and in worst case may allow confidentiality or integrity impact through stale DMA translations, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	OOB INIT SIMPLEFIX HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6	YES	NO	checked

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

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:

* Local delegated trigger: +1. The path is local through IOMMUFD vDEVICE or nested virtualization device assignment, not network.
* Memory corruption, weak or indirect candidate: +1. The zero stream case can read `master->streams[0]` from a `ZERO_SIZE_PTR` like array, which is an OOB read candidate, but not an OOB write.
* Privileged kernel/device-management memory corruption path: +1. The bug is in ARM SMMU v3 IOMMUFD device management and affects DMA translation invalidation semantics.
* Cross-boundary isolation/policy bypass: +1. Missed ATC/IOTLB invalidation for additional Stream IDs can leave stale DMA translations active.
* Confidentiality impact plausible: +1. Stale DMA translations may allow device access to memory mappings that should no longer be valid.
* Integrity impact plausible: +1. Stale DMA translations may allow writes through outdated mappings.
* Availability impact realistic: +1. The zero stream case can plausibly crash, and stale invalidation can break device isolation state.
* Rare and configuration-dependent path: -1. Requires ARM SMMU v3, IOMMUFD, vDEVICE or nested invalidation support, and a device with zero or multiple Stream IDs.
* Privilege/delegation constraint: -1. Typical setup requires a VMM, device assignment, or delegated virtualization service access rather than a normal local shell user.

Paranoid additions:

* Weak LPE concern: +1. Stale DMA translations are a plausible isolation-break primitive even though no direct arbitrary write or exploit chain is demonstrated.
* Reduced privilege penalty in paranoid mode: 0 instead of -1. A VMM process, guest-controlled invalidation path, or delegated virtualization service may be less than full host-root in practical deployments.

Call-site confidence: medium. The patch provides the affected helper semantics and the vDEVICE init path, but not the full invalidation call chain.

## 3. Reachability analysis

The issue is not remotely reachable over the network. Triggering requires a local virtualization or device-assignment context using ARM SMMU v3 IOMMUFD vDEVICE support. In typical deployments, creating or managing such objects is restricted to a VMM or privileged management service, but containerized or delegated virtualization setups may make this closer to PR:L than PR:H. The affected configuration is specialized, but the impacted object is security-sensitive because IOMMU invalidation controls DMA isolation.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because the bug is not only a validation mistake. The multiple Stream ID case can leave stale ATC/IOTLB entries, which may preserve DMA access beyond the intended invalidation boundary. The zero Stream ID case is closer to a limited OOB read or crash condition. Realistic exploitation is configuration-dependent and no arbitrary write primitive is shown, so the conservative score remains Actionable Moderate. The paranoid score reaches Strong Important candidate because stale DMA translations can plausibly affect confidentiality and integrity across an isolation boundary.

## 5. One-sentence report phrase

ARM SMMU v3 IOMMUFD vDEVICE setup accepted devices without exactly one Stream ID, allowing missed ATC/IOTLB invalidation for multi-stream devices or an OOB read for zero-stream devices, with potential DoS and DMA isolation impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: the patch affects IOMMUFD and DMA translation invalidation semantics, and stale device TLB entries can cross security boundaries even without a demonstrated generic kernel memory corruption primitive.

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

Patch: iommu/arm-smmu-v3-iommufd: Require exactly one Stream ID for a vDEVICE [ Upstream
Commit: 3808bab5d95ae79e333e11f6a73d178e084c645d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3808bab5d95ae79e333e11f6a73d178e084c645d

Changed files:
  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.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=3808bab5d95ae79e333e11f6a73d178e084c645d

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

The original Linux kernel CVE announcement for CVE-2026-74573 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74573

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).