[CVE-2026-46113][IMPORTANT] KVM: x86: Fix shadow paging use-after-free due to unexpected GFN

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

CVE: CVE-2026-46113
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: KVM: x86: Fix shadow paging use-after-free due to unexpected GFN
Commit: 488e386484ec8c0e558be6e156edf34ed9f4d5c8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=488e386484ec8c0e558be6e156edf34ed9f4d5c8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46113
Analysis date: Mon, 06 Jul 2026 19:59:28 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale rmap entry in KVM x86 shadow paging can survive shadow page zap after an unexpected GFN mismatch, allowing later dirty logging or MMU notifier walks to dereference a freed `kvm_mmu_page`, causing host DoS and potentially higher-impact KVM MMU memory corruption.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-46113 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46113

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: IMPORTANT
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-2026-46113	IMPORTANT	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:L/UI:N/S:C/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'A use-after-free can occur in KVM x86 shadow paging when a direct shadow page keeps stale rmap state after the guest page tables change and a later SPTE is installed for a GFN outside the original shadow page range. A later rmap walk such as dirty logging or an MMU notifier invalidation can then dereference a freed kvm_mmu_page. For the CVSS the PR:L value is used for the paranoid score because a practical attacker may only need the ability to run guest code or access a KVM based VM path, not full host administrator privileges. The issue is not a normal remote network bug, but it is relevant to guest-to-host attack surfaces in virtualization deployments. Impact is at least local denial of service via host kernel crash and in worst case may allow confidentiality or integrity impact due to KVM MMU memory corruption, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8) 	YES	VIRT INIT UAF HARDWARE KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: **6**
* Paranoid score: **8**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged trigger: +1**. In many deployments, access to KVM can be available to non-root users through `/dev/kvm` group membership or VM service paths.
* **Generic memory corruption, strong primitive: +2**. The commit explicitly describes a stale rmap walk dereferencing a freed `kvm_mmu_page`, i.e. a real UAF candidate in KVM MMU state.
* **Real lifetime corruption: +1**. The stale rmap entry survives zap/removal and later points to freed shadow MMU memory.
* **Reliable kernel crash / strong DoS: +1**. A stale rmap dereference in host KVM MMU code can crash the host kernel.
* **Cross-boundary isolation concern: +1**. The bug is in a guest-to-host virtualization boundary path.
* **Hard/special sequence required: -1**. Trigger requires a specific shadow paging flow, GFN mismatch, memslot invalidation or rmap walk.
* **Legacy / non-default execution mode: -1**. Shadow paging is less common than EPT/NPT on modern x86 virtualization.

Paranoid additions:

* **Privilege escalation plausible, weak-to-strong KVM/MMU concern: +1 additional**. Stale rmap plus freed shadow page traversal is a high-value KVM MMU UAF pattern even without a public reclaim primitive.
* **Confidentiality impact plausible: +1**. In the paranoid interpretation, UAF in MMU/rmap state may affect host memory isolation.
* **Integrity impact plausible: +1**. In the paranoid interpretation, stale MMU metadata could lead to corruption beyond a simple crash.

Call-site confidence: **high**. The commit message gives the full stale rmap lifecycle and names later rmap walkers such as dirty logging and MMU notifier invalidations.

## 3. Reachability analysis

The most realistic attacker is a local user or VM-management context able to create or influence a KVM VM, or a guest workload in combination with host-side KVM operations such as memslot deletion, dirty logging, or MMU notifier invalidation. This is not a normal remote network vulnerability.

The path depends on **KVM x86 shadow paging**, which is not the common fast path on modern systems using EPT/NPT. That reduces broad default exposure, but does not make the issue low priority because the affected code is a host virtualization boundary and the primitive is a stale rmap UAF.

Namespaces and containers matter mainly through access to `/dev/kvm` or delegated VM services. A reduced-privilege service account, container root with KVM access, or user in a KVM group should not be treated as full host root.

## 4. Severity interpretation

This behaves more like an **Important candidate** than an ordinary Moderate issue. The conservative view is an actionable host-kernel DoS with real lifetime corruption in KVM MMU state. The paranoid view is stronger because stale rmap traversal after freeing a shadow MMU page is a known high-risk pattern for guest-to-host memory corruption analysis.

Realistic exploitation is not proven by the patch alone. However, the patch and commit message explicitly support UAF, stale reverse mapping, later traversal, and host MMU object lifetime corruption, so this should not be auto-closed as a simple crash.

## 5. One-sentence report phrase

A stale rmap entry in KVM x86 shadow paging can survive shadow page zap after an unexpected GFN mismatch, allowing later dirty logging or MMU notifier walks to dereference a freed `kvm_mmu_page`, causing host DoS and potentially higher-impact KVM MMU memory corruption.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is a KVM/MMU stale-rmap UAF affecting a guest-to-host boundary. The trigger is specialized and shadow paging dependent, but the primitive is serious enough to require manual review rather than automatic Moderate handling.

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

Patch: KVM: x86: Fix shadow paging use-after-free due to unexpected GFN
Commit: 488e386484ec8c0e558be6e156edf34ed9f4d5c8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=488e386484ec8c0e558be6e156edf34ed9f4d5c8

Commit description:

The shadow MMU computes GFNs for direct shadow pages using sp->gfn plus
the SPTE index. This assumption breaks for shadow paging if the guest
page tables are modified between VM entries (similar to commit
aad885e , "KVM: x86/mmu: Drop/zap existing present SPTE even
when creating an MMIO SPTE", 2026-03-27).  The flow is as follows:

- a PDE is installed for a 2MB mapping, and a page in that area is
  accessed.  KVM creates a kvm_mmu_page consisting of 512 4KB pages;
  the kvm_mmu_page is marked by FNAME(fetch) as direct-mapped because
  the guest's mapping is a huge page (and thus contiguous).

- the PDE mapping is changed from outside the guest.

- the guest accesses another page in the same 2MB area.  KVM installs
  a new leaf SPTE and rmap entry; the SPTE uses the "correct" GFN
  (i.e. based on the new mapping, as changed in the previous step) but
  that GFN is outside of the [sp->gfn, sp->gfn + 511] range; therefore
  the rmap entry cannot be found and removed when the kvm_mmu_page
  is zapped.

- the memslot that covers the first 2MB mapping is deleted, and the
  kvm_mmu_page for the now-invalid GPA is zapped.  However, rmap_remove()
  only looks at the [sp->gfn, sp->gfn + 511] range established in step 1,
  and fails to find the rmap entry that was recorded by step 3.

- any operation that causes an rmap walk for the same page accessed
  by step 3 then walks a stale rmap and dereferences a freed kvm_mmu_page.
  This includes dirty logging or MMU notifier invalidations (e.g., from
  MADV_DONTNEED).

The underlying issue is that KVM's walking of shadow PTEs assumes that
if a SPTE is present when KVM wants to install a non-leaf SPTE, then the
existing kvm_mmu_page must be for the correct gfn.  Because the only way
for the gfn to be wrong is if KVM messed up and failed to zap a SPTE...
which shouldn't happen, but *actually* only happens in response to a
guest write.

That bug dates back literally forever, as even the first version of KVM
assumes that the GFN matches and walks into the "wrong" shadow page.
However, that was only an imprecision until 2032a93 ("KVM: MMU:
Don't allocate gfns page for direct mmu pages") came along.

Fix it by checking for a target gfn mismatch and zapping the existing
SPTE.  That way the old SP and rmap entries are gone, KVM installs
the rmap in the right location, and everyone is happy.

Fixes: 2032a93 ("KVM: MMU: Don't allocate gfns page for direct mmu pages")
Fixes: 6aa8b73 ("kvm: userspace interface")
Reported-by: Alexander Bulekov <bkov@amazon.com>
Reported-by: Fred Griffoul <fgriffo@amazon.co.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://patch.msgid.link/20260503201029.106481-1-pbonzini@redhat.com/
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.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=488e386484ec8c0e558be6e156edf34ed9f4d5c8

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

The original Linux kernel CVE announcement for CVE-2026-46113 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46113

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:C/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 8

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).