[CVE-2026-64291][LOW] iommufd: Set veventq_depth upper bound

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

CVE: CVE-2026-64291
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: iommufd: Set veventq_depth upper bound
Commit: f565297edf316016be4a1a9e2eb9f39359313f43
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f565297edf316016be4a1a9e2eb9f39359313f43
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64291
Analysis date: Sat, 25 Jul 2026 07:26:31 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like. Not an Actionable Moderate candidate
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-2026-64291 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64291

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64291	LOW	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-400;CWE-770;CWE-789;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'iommufd_veventq_alloc accepted any nonzero veventq_depth from userspace up to U32_MAX, allowing a local process to request excessively large virtual event queues and consume kernel memory reserves. For the CVSS the PR:L is used because triggering requires local code execution and access to the iommufd interface, but it does not necessarily require full administrator privileges in environments where device assignment or VFIO style access is delegated. The issue is not network reachable and is triggered through a local control interface rather than packet traffic. Impact is denial of service through memory pressure or exhaustion. There is no evidence of UAF, OOB access, arbitrary write, information disclosure, or privilege escalation in this patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like. Not an Actionable Moderate candidate (with actual score 2)	YES	SIMPLEFIX  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2

1. ActionableScore
* Conservative score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like. Not an Actionable Moderate candidate**::

2. Signal breakdown
* Local unprivileged trigger: +1. A local process with access to the iommufd interface can supply an excessive veventq_depth value.
* Reliable kernel crash / strong DoS: +1. The commit explicitly states that userspace can allocate excessively large queues and exhaust kernel memory reserves.
* No generic memory corruption: +0. The patch only adds an upper bound check and does not indicate UAF, OOB access, double free, type confusion, arbitrary write, or stale pointer use.
* No privilege escalation plausible: +0. There is no described primitive beyond memory exhaustion.
* No confidentiality or integrity impact: +0. The issue is resource exhaustion only.
* No remote reachability: +0. This is a local iommufd control interface, not a network path.
* No rare subsystem penalty applied. iommufd is specialized, but reachable through an ordinary local device-management interface when configured.

3. Reachability analysis
Triggering requires local code execution and access to the iommufd uAPI, likely in environments using VFIO, device assignment, or virtual IOMMU features. It does not require network reachability. Namespace or delegated device-management setups may make the interface reachable to non-root service users or container-like workloads, so PR:L is reasonable for CVSS-style analysis. The practical effect is memory pressure or kernel memory reserve exhaustion from oversized virtual event queues.

Call-site confidence: high. The allocation validation site and exact rejected parameter are present in the patch.

4. Severity interpretation
This behaves like an ordinary Moderate or Low-like resource exhaustion issue, not an Important-class vulnerability. The theoretical worst case is system-wide DoS via memory exhaustion, but the patch does not support memory corruption, information disclosure, privilege escalation, or cross-boundary policy bypass. It should not be auto-escalated solely because the allocation happens in kernel memory.

5. One-sentence report phrase
iommufd_veventq_alloc allowed userspace to request excessively large virtual event queues, which could exhaust kernel memory reserves and cause a local denial of service.

6. Manual review recommendation
NO MANUAL CHECK NEEDED. This is a bounded resource-exhaustion validation fix with no evidence of memory corruption, privilege escalation, network reachability, or sensitive data exposure.

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

Patch: iommufd: Set veventq_depth upper bound
Commit: f565297edf316016be4a1a9e2eb9f39359313f43
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f565297edf316016be4a1a9e2eb9f39359313f43

Commit description:

iommufd_veventq_alloc() accepts any !0 veventq_depth from userspace, with
an upper bound at U32_MAX.

This leaves a vulnerability where userspace can allocate excessively large
queues to exhaust kernel memory reserves.

Cap the veventq_depth (maximum number of entries) to 1 << 19, matching the
maximum number of entries in the SMMUv3 EVTQ (the largest use case today).

Fixes: e36ba5a ("iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ_ALLOC")
Link: https://patch.msgid.link/r/8426cbaa5e8294472ec7f076ef427cc473be5985.1779408671.git.nicolinc@nvidia.com
Cc: stable@vger.kernel.org
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/iommu/iommufd/eventq.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=f565297edf316016be4a1a9e2eb9f39359313f43

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

The original Linux kernel CVE announcement for CVE-2026-64291 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64291

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:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).