[CVE-2026-68428][MODERATE 7.0] KVM: x86/mmu: Fix use-after-free on vendor module reload

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

CVE: CVE-2026-68428
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: x86/mmu: Fix use-after-free on vendor module reload
Commit: 6f4be73880302d5642c83a0813fdfe1f5fd4b6e3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6f4be73880302d5642c83a0813fdfe1f5fd4b6e3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68428
Analysis date: Mon, 10 Aug 2026 11:12:13 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
KVM x86 MMU cache teardown can leave stale `kmem_cache` pointers across vendor module reloads, causing a privileged reload failure path to hit a slab use after free and potentially crash the host kernel.

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

The original Linux kernel CVE announcement for CVE-2026-68428 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68428

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.

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

CVE-2026-68428	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';*CWE-416;*CWE-672;CWE-665;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.2';DESCR 'KVM x86 MMU cache teardown can leave stale global pointers after a vendor module is unloaded while kvm.ko remains loaded. A later vendor module reload can enter an initialization failure path where mmu_destroy_caches tries to destroy a cache that was already destroyed, causing a slab use after free. For the CVSS the PR:H is used even for the paranoid score because reliable triggering requires administrative control over KVM vendor module load and unload operations, typically CAP_SYS_MODULE or an equivalent root level service context. AC:H is used because the vulnerable path also depends on a later initialization failure such as an allocation failure during reload. The issue is not network reachable. Impact is at least local denial of service via kernel crash, and the paranoid score keeps limited confidentiality and integrity impact because the patch context shows a real slab use after free but not a clear arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	DANGER VIRT ERRORPATH UAF HARDWARE TEST 	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Triggered positive signals:

* Generic memory corruption: +2
  The commit explicitly reports a slab use after free caused by passing a stale `mmu_page_header_cache` pointer to `kmem_cache_destroy()`.

* Real lifetime corruption: +1
  The bug is a stale pointer lifetime issue. Cache objects are destroyed, but global pointers in `kvm.ko` remain live across vendor module unload and reload.

* Reliable kernel crash / strong DoS: +1
  KASAN reports a reproducible slab use after free in the reload error path. Without KASAN this can plausibly become a kernel crash.

* Privileged kernel/device-management memory corruption path: +1
  The corruption occurs in KVM MMU cache lifecycle management during vendor module initialization and teardown.

* Broadly deployed subsystem exposure: +1
  KVM is widely deployed, although this specific trigger path is narrow and requires modular KVM vendor module reload behavior.

Negative signals:

* Requires admin/root/CAP_SYS_MODULE in typical deployments: -2
  Reliable triggering requires loading and unloading KVM vendor modules, normally requiring full administrative privileges.

* Hard or special failure condition required: -1
  The demonstrated path requires a later vendor module reload plus an initialization failure, such as forced or real `-ENOMEM`.

* Conservative constrained primitive adjustment: -1
  The stale pointer is passed to `kmem_cache_destroy()`. The patch does not show attacker-controlled reclaim, arbitrary write, callback control, or controlled object replacement.

Paranoid-only impact additions:

* Confidentiality impact plausible: +1
  Counted only in the paranoid score because this is real slab UAF in kernel memory management context, but no concrete disclosure primitive is shown.

* Integrity impact plausible: +1
  Counted only in the paranoid score because UAF can sometimes become corruption-relevant, but the provided patch does not demonstrate an attacker-controlled overwrite or LPE primitive.

## 3. Reachability analysis

The bug is locally reachable only by an actor that can manage KVM kernel modules, typically host root or a process with `CAP_SYS_MODULE`. It is not reachable from a guest, not network reachable, and not triggerable by ordinary `/dev/kvm` users.

Namespaces and containers usually do not lower the privilege requirement, because loading and unloading host KVM modules is not delegated to normal containers. The realistic trigger also depends on KVM being modular and on a reload failure path, so exploitation conditions are uncommon.

Call-site confidence: high. The commit message describes the exact lifecycle sequence and the diff directly fixes stale cache pointers in `mmu_destroy_caches()`.

## 4. Severity interpretation

This behaves as a privileged local KVM teardown UAF with reliable DoS potential, not as a demonstrated LPE. The theoretical memory corruption potential exists because the bug is an explicit slab use after free, but realistic exploitation evidence is limited by `PR:H`, the narrow module reload path, and the required initialization failure.

Conservatively this is ordinary Moderate / low-like operational risk. Paranoid triage should still keep it as Actionable Moderate at minimum because explicit UAF in KVM MMU lifecycle code should not be auto-closed without review.

## 5. One-sentence report phrase

KVM x86 MMU cache teardown can leave stale `kmem_cache` pointers across vendor module reloads, causing a privileged reload failure path to hit a slab use after free and potentially crash the host kernel.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.
YES REQUIRES MANUAL CHECK.

Reason: this is an explicit slab use after free in KVM code, even though practical triggering appears local, privileged, and dependent on a narrow module reload error path.

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

Patch: KVM: x86/mmu: Fix use-after-free on vendor module reload
Commit: 6f4be73880302d5642c83a0813fdfe1f5fd4b6e3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6f4be73880302d5642c83a0813fdfe1f5fd4b6e3

Commit description:

mmu_destroy_caches() destroys pte_list_desc_cache and
mmu_page_header_cache, but leaves both pointers unchanged.  The pointers
live in kvm.ko, and therefore survive when a vendor module is unloaded
while kvm.ko remains loaded.

If creation of pte_list_desc_cache fails during a subsequent vendor
module load, its assignment sets pte_list_desc_cache to NULL and the
error path calls mmu_destroy_caches().  mmu_page_header_cache still
points to the cache destroyed during the preceding vendor module
unload.  Passing that stale pointer to kmem_cache_destroy() causes a
slab use-after-free.

Reproduce the issue on a v7.1.3 kernel with CONFIG_KASAN=y,
CONFIG_KASAN_GENERIC=y, CONFIG_KVM=m, and CONFIG_KVM_INTEL=m.  A
one-shot test hook forces pte_list_desc_cache to NULL on the second
invocation of kvm_mmu_vendor_module_init():

  1. Load kvm.ko and kvm-intel.ko, creating both caches.
  2. Unload only kvm_intel, leaving kvm.ko loaded.
  3. Reload kvm_intel and force initialization through the -ENOMEM path.

KASAN reports:

  BUG: KASAN: slab-use-after-free in
  kvm_mmu_vendor_module_init+0x5b/0x170 [kvm]
  ...
  kmem_cache_destroy+0x21/0x1d0
  kvm_mmu_vendor_module_init+0x5b/0x170 [kvm]
  ...
  Allocated by task 16817:
  __kmem_cache_create_args+0x12c/0x3b0
  __kmem_cache_create.constprop.0+0xb6/0xf0 [kvm]
  kvm_mmu_vendor_module_init+0x13b/0x170 [kvm]
  ...
  Freed by task 16820:
  kmem_cache_destroy+0x117/0x1d0
  kvm_mmu_vendor_module_exit+0x21/0x30 [kvm]

Clear both pointers immediately after destroying their caches so that
the stored state reflects the caches' lifetime and repeated cleanup is
safe.

With the fix applied, the same injected vendor module reload fails with
-ENOMEM as expected and produces no KASAN report.

Fixes: cb498ea ("KVM: Portability: Combine kvm_init and kvm_init_x86")
Cc: stable@vger.kernel.org
Signed-off-by: Phil Rosenthal <phil@phil.gs>
Message-ID: <20260718-kvm-mmu-cache-uaf-v3-1-e103b93c74e1@phil.gs>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kvm/mmu/mmu.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=6f4be73880302d5642c83a0813fdfe1f5fd4b6e3

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

The original Linux kernel CVE announcement for CVE-2026-68428 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68428

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.2

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: 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).