[CVE-2026-64290][MODERATE REGULAR] iommufd: Break the loop on failure in iommufd_fault_fops_read()

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

CVE: CVE-2026-64290
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: iommufd: Break the loop on failure in iommufd_fault_fops_read()
Commit: 5539da127d03c1f6c2e2a49fdfbe331a0ccbdea8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5539da127d03c1f6c2e2a49fdfbe331a0ccbdea8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64290
Analysis date: Sat, 25 Jul 2026 06:41:52 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Ordinary Moderate / Low-like with limited Actionable Moderate concern
Manual review required: NO

Summary:
A local iommufd fault queue reader can trigger an infinite retry loop after copy_to_user failure, causing 100 percent CPU usage while holding fault->mutex and resulting in a local denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-64290 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64290

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 REGULAR
Manual review required: NO

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-2026-64290	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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-835;**CWE-400;CWE-755;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_fault_fops_read can enter an infinite retry loop when copy_to_user fails while processing fault events. The inner loop exits, but the outer loop can fetch the same restored fault group again and retry the same failing user copy while fault mutex remains held. This can spin a local reader at 100 percent CPU and block other operations that need the same fault mutex. For the CVSS the PR:L is used because reliable triggering requires a local process with access to the iommufd fault file or a service context that owns such an object, but it does not require full kernel privileges once that interface is available. The issue is not network reachable and the impact is denial of service only. There is no evidence of memory corruption, information disclosure, or privilege escalation from this patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like with limited Actionable Moderate concern (with actual score 3)	YES	SIMPLEFIX HARDWARE  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Low-like with limited Actionable Moderate concern**

## 2. Signal breakdown

Conservative signals:

* Local trigger: +1. A local process that can read the relevant iommufd fault file can trigger the failing copy_to_user path with an invalid or inaccessible userspace buffer.
* Reliable local DoS behavior: +1. The bug can spin the reader at 100 percent CPU while holding fault->mutex.
* Rare or configuration dependent exposure: -1. iommufd fault queues are not a broad default user interface for ordinary processes and are mainly relevant to VFIO or device assignment setups.
* Availability impact realistic: +1. The mutex being held can block other operations on the same fault object, but the impact is still local and object scoped rather than clearly host wide.

Paranoid interpretation:

* Same as above, but without the rare exposure penalty in deployments where iommufd is intentionally exposed to untrusted VM or device-management workloads.
* No memory corruption, UAF, OOB access, info leak, or privilege escalation signal is supported by the patch.

## 3. Reachability analysis

The bug is locally reachable by a process that already has access to the iommufd fault object and can call read with a buffer that makes copy_to_user fail. It is not network reachable. Namespaces or containers matter only if the deployment delegates iommufd or VFIO-like device assignment access to less trusted workloads. In typical systems this is not a default broadly exposed interface, but in virtualization hosts it may be reachable by service processes or VM management components.

Call-site confidence: high, because the affected function and failure loop are visible in the patch context.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like local DoS, not an Important-class vulnerability. The failure mode is an infinite retry loop with a mutex held, causing CPU consumption and possible blocking of related iommufd fault handling. There is no demonstrated memory corruption primitive, no confidentiality impact, no integrity impact, and no plausible privilege escalation path from the patch.

## 5. One-sentence report phrase

A local iommufd fault queue reader can trigger an infinite retry loop after copy_to_user failure, causing 100 percent CPU usage while holding fault->mutex and resulting in a local denial of service.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a local DoS-only logic bug with no evidence of memory corruption, privilege escalation, information disclosure, or network reachability.

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

Patch: iommufd: Break the loop on failure in iommufd_fault_fops_read()
Commit: 5539da127d03c1f6c2e2a49fdfbe331a0ccbdea8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5539da127d03c1f6c2e2a49fdfbe331a0ccbdea8

Commit description:

On a copy_to_user() failure inside the inner list_for_each_entry, only the
inner loop breaks; the outer while re-fetches the just-restored fault group
and retries the failing copy_to_user() forever, spinning the reader at 100%
CPU with fault->mutex held.

Check rc after the inner loop and break the outer while as well.

Fixes: 07838f7 ("iommufd: Add iommufd fault object")
Link: https://patch.msgid.link/r/336a9b6e44fe66a24199d3be777c405c85c98622.1780343944.git.nicolinc@nvidia.com
Cc: stable@vger.kernel.org
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Pranjal Shrivastava <praan@google.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=5539da127d03c1f6c2e2a49fdfbe331a0ccbdea8

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

The original Linux kernel CVE announcement for CVE-2026-64290 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64290

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: 2
  Paranoid ActionableScore: 3

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).