[CVE-2026-74580][MODERATE 7.0] vhost: reset the vring metadata cache on vring reconfiguration

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

CVE: CVE-2026-74580
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: vhost: reset the vring metadata cache on vring reconfiguration
Commit: 5224bd37e37d36076a550d99b2aebba33939fd95
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5224bd37e37d36076a550d99b2aebba33939fd95
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74580
Analysis date: Fri, 21 Aug 2026 12:53:18 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale vhost vring metadata IOTLB cache can survive live vring reconfiguration, allowing later metadata accesses to translate through an old mapping and read or write outside the intended IOTLB region, with local vhost control plane access required and potential confidentiality, integrity, and availability impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74580 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74580

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: 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-2026-74580	MODERATE	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-664;CWE-823;*CWE-787;CWE-125;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A stale vhost vring metadata IOTLB cache can remain valid after VHOST_SET_VRING_ADDR or VHOST_SET_VRING_NUM replaces desc avail used addresses or changes ring size. Because iotlb_access_ok accepts a cache hit as already validated, later metadata prefetch can translate a new GIOVA through an old mapping and make vhost_copy_to_user or vhost_copy_from_user access memory outside the intended IOTLB region. For the CVSS the PR:L is used for the paranoid score because a local process with access to vhost ioctls or a reduced capability virtualization service account can reconfigure a live vring. The issue is not directly network reachable and is triggered through the vhost control plane rather than packet traffic. Impact includes local denial of service and potential confidentiality or integrity impact from out of region reads or writes. Direct kernel privilege escalation is not proven, but stale mapping based memory access is stronger than a pure warning or resource leak.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	OOB HARDWARE LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	guess

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Local low privileged or service account trigger: +1
  Trigger requires a local process able to issue vhost ioctls, typically QEMU or a virtualization service account with access to `/dev/vhost-*`.

* Memory corruption, weak/indirect corruption candidate: +1
  The stale metadata cache can make a new GIOVA translate through an old `vhost_iotlb_map`, producing out of region access relative to the intended IOTLB mapping.

* Memory layout or mapping invariant restoration: +1
  The patch restores consistency between vring metadata addresses, vring size, and the cached IOTLB mapping.

* Cross boundary isolation or policy bypass: +1
  The bug bypasses the intended device IOTLB validation boundary for vring metadata access.

* Privileged kernel/device management corruption path: +1
  The corruption occurs in the trusted vhost virtualization data path and affects device IOTLB backed metadata access.

* Confidentiality impact plausible: +1
  `vhost_copy_from_user()` may read from a userspace address outside the intended IOTLB region.

* Integrity impact plausible: +1
  `vhost_copy_to_user()` may write to a userspace address outside the intended IOTLB region.

* Broad virtualization exposure: +1
  vhost is common on KVM virtualization hosts, especially for virtio networking/storage acceleration.

Subtracting signals:

* Requires vhost control plane access: -1
  This is not a remote packet path and not generally reachable by an arbitrary local user. Full `-2` is not used because QEMU or a reduced capability virtualization service account is not equivalent to full host root in practical risk terms.

* Primitive constrained and not direct kernel arbitrary write: -1
  The demonstrated access is out of the intended IOTLB region through `copy_to_user()` / `copy_from_user()`, not a proven arbitrary kernel memory write or direct code execution primitive.

Conservative total: 6
Paranoid total: 7

## 3. Reachability analysis

The bug is triggered through vhost control plane ioctls, specifically live vring reconfiguration through `VHOST_SET_VRING_ADDR` or `VHOST_SET_VRING_NUM` while a device IOTLB is attached. It is not directly network reachable and is not triggered merely by packet traffic.

In typical deployments, the actor is a local VMM process such as QEMU or a virtualization service account with access to vhost device nodes. Ordinary unprivileged local users usually cannot trigger this unless device permissions or container delegation expose vhost ioctls. Namespaces and containers matter because container root or a reduced capability service account with delegated vhost access should not always be treated as full host root.

Realistic exploitation requires a live vhost queue, device IOTLB usage, prior metadata cache population, and subsequent vring address or size reconfiguration. This is configuration dependent but plausible on virtualization hosts.

Call-site confidence: high. The commit message explicitly names the relevant validation path, stale cache behavior, translation formula, and consumers `vhost_copy_to_user()` and `vhost_copy_from_user()`.

## 4. Severity interpretation

This behaves stronger than an ordinary Moderate correctness bug because stale IOTLB metadata can cause out of region reads or writes relative to the intended device memory boundary. It is not just a warning, leak, or local crash cleanup.

At the same time, realistic LPE is not proven. The patch does not show a direct arbitrary kernel write, attacker controlled kernel object replacement, callback control, or UAF reclaim primitive. The main demonstrated risk is bypass of vhost IOTLB validation with possible confidentiality and integrity impact in the virtualization memory boundary.

Conservative interpretation: Actionable Moderate, should not be auto closed.
Paranoid interpretation: Strong Important candidate due to stale IOTLB mapping and out of region copy access in a common virtualization subsystem.

## 5. One-sentence report phrase

A stale vhost vring metadata IOTLB cache can survive live vring reconfiguration, allowing later metadata accesses to translate through an old mapping and read or write outside the intended IOTLB region, with local vhost control plane access required and potential confidentiality, integrity, and availability impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch describes a concrete stale mapping validation bypass with out of region `copy_to_user()` and `copy_from_user()` access in vhost. Even without proven kernel code execution, this is a virtualization boundary and IOTLB correctness issue that needs manual review for guest to host, service account, and container delegated vhost exposure scenarios.

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UPSTREAM PATCH SUMMARY
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Patch: vhost: reset the vring metadata cache on vring reconfiguration
Commit: 5224bd37e37d36076a550d99b2aebba33939fd95
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5224bd37e37d36076a550d99b2aebba33939fd95

Commit description:

vq->meta_iotlb[] caches the vhost_iotlb_map that backs each vring
metadata region, and iotlb_access_ok() returns early on a cache hit,
taking the hit as proof that the region has already been validated:

	if (vhost_vq_meta_fetch(vq, addr, len, type))
		return true;

The cache is reset on VHOST_IOTLB_UPDATE and VHOST_IOTLB_INVALIDATE, on
device IOTLB (re)initialisation and on vq reset, but not when
VHOST_SET_VRING_ADDR replaces vq->desc, vq->avail and vq->used, nor when
VHOST_SET_VRING_NUM changes the region sizes.

With a device IOTLB attached both ioctls are accepted while the vq is
live, and neither validates the addresses at ioctl time: vq_access_ok()
and vq_log_used_access_ok() return true early because the addresses are
GIOVAs, deferring validation to prefetch time.  Once the cache has been
populated that deferred validation no longer runs -- vq_meta_prefetch()
hits the stale entry and returns true -- and vhost_vq_meta_fetch() keeps
translating through the old mapping as

	map->addr + addr - map->start

for an address the mapping no longer covers.  vhost_copy_to_user() and
vhost_copy_from_user() consume the result with __copy_to_user() and
__copy_from_user(), which do not check it either, so a subsequent used
ring update or descriptor fetch accesses memory outside the region the
IOTLB actually maps.

Reset the metadata cache whenever the vring is reconfigured, so the new
addresses are pushed back through iotlb_access_ok()'s slow path.

Fixes: f889491 ("vhost: introduce O(1) vq metadata cache")
Cc: stable@vger.kernel.org
Assisted-by: tencentos-corvus-ai:kimi-k3
Signed-off-by: Jun Yang <junvyyang@tencent.com>
Message-ID: <20260803014823.68623-1-juny24602@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  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=5224bd37e37d36076a550d99b2aebba33939fd95

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

The original Linux kernel CVE announcement for CVE-2026-74580 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74580

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: 6
  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: MODERATE
KPANIC detected for this report: YES
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).