[CVE-2026-74356][MODERATE 7.0] vhost: fix vhost_get_avail_idx for a non empty ring [ Upstream

[CVE-2026-74356][MODERATE 7.0] vhost: fix vhost_get_avail_idx for a non empty ring [ Upstream

[AL-KERNEL]

CVE: CVE-2026-74356
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: vhost: fix vhost_get_avail_idx for a non empty ring [ Upstream
Commit: e115471008111f894c6528d9ab2ce7d0ce306f35
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e115471008111f894c6528d9ab2ce7d0ce306f35
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74356
Analysis date: Sat, 15 Aug 2026 19:17:06 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-74356 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74356

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: NO

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

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

CVE-2026-74356	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-670;BEST CVSS score: '5.5';DESCR 'vhost_get_avail_idx() used the wrong condition to report whether vq avail_idx was updated for a non empty virtqueue. With vhost net and mergeable buffers, the host can see available entries that are still too small for an incoming packet and can re enable notifications while entries remain in the ring. The incorrect return value can make vhost_enable_notify immediately disable notifications again and retry the same entries, causing a host livelock if the guest does not make progress. For the CVSS the PR:L is used because reliable triggering requires access to a guest or control over the guest virtqueue state relative to the host. The issue is not directly network reachable from outside the virtualization boundary. Impact is denial of service through host CPU livelock or stalled vhost processing, with no supported confidentiality or integrity impact.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	IMPROVEONLY LINUS PACKET  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

1. ActionableScore
* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

2. Signal breakdown
* Local or guest-controlled trigger: +1. The issue is triggered through guest-controlled virtqueue state rather than direct host admin actions.
* Reliable kernel or host-side DoS: +1. The commit explicitly describes host livelock when the guest is not making progress.
* Availability impact realistic: +1. The failure can stall vhost processing and consume host CPU, affecting VM networking and potentially host responsiveness.
* Broad/common virtualization path: +1. vhost-net and virtio-net are common in Linux virtualization deployments.
* Paranoid additional cross-boundary impact: +1. A guest influencing host vhost worker availability crosses the VM-to-host isolation boundary, even though this is availability-only.

No memory corruption, UAF, OOB write, information leak, or privilege-escalation primitive is supported by the patch.

3. Reachability analysis
The realistic attacker is a guest or guest-controlled workload able to influence the virtio/vhost-net receive ring state. The bug is not directly reachable from the external network without the VM and vhost-net setup. Mergeable buffers are a relevant condition, but they are common enough in virtio-net deployments that this should not be treated as a rare hardware-only path. Host-side creation of vhost devices is privileged, but the security-relevant trigger is guest-to-host availability impact, so a full host-root privilege penalty is not appropriate.

Call-site confidence: high. The commit explicitly names drivers/vhost/net.c, vhost_enable_notify(), mergeable buffers, and the livelock condition.

4. Severity interpretation
This behaves as an actionable Moderate issue rather than an Important-class memory corruption bug. The realistic impact is host-side livelock or stalled vhost networking, not confidentiality loss, integrity loss, or privilege escalation. Theoretical escalation is not supported because the patch only fixes return-value semantics and cached index handling, with no stale pointer, reclaim, overwrite, type confusion, or refcount primitive.

5. One-sentence report phrase
An incorrect return condition in vhost_get_avail_idx() can make vhost-net repeatedly disable notifications and retry unchanged virtqueue entries, allowing a guest-controlled non-empty ring state to cause host-side livelock and denial of service.

6. Manual review recommendation
MANUAL CHECK RECOMMENDED.
This is availability-only and lacks memory corruption, but it affects a guest-to-host virtualization path and should not be auto-closed without confirming product exposure to vhost-net with mergeable buffers.

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

Patch: vhost: fix vhost_get_avail_idx for a non empty ring [ Upstream
Commit: e115471008111f894c6528d9ab2ce7d0ce306f35
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e115471008111f894c6528d9ab2ce7d0ce306f35

Changed files:
  drivers/vhost/net.c
  drivers/vhost/vhost.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=e115471008111f894c6528d9ab2ce7d0ce306f35

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

The original Linux kernel CVE announcement for CVE-2026-74356 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74356

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: Not available
  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: 4
  Paranoid ActionableScore: 5

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 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).