[CVE-2026-53359][IMPORTANT] KVM: x86: Fix shadow paging use-after-free due to unexpected role

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

CVE: CVE-2026-53359
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: KVM: x86: Fix shadow paging use-after-free due to unexpected role
Commit: b1337aae5e194324e4810d561764e7793f8b3864
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b1337aae5e194324e4810d561764e7793f8b3864
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53359
Analysis date: Sat, 04 Jul 2026 09:01:10 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
KVM x86 shadow paging may leave a stale rmap entry pointing into a freed shadow page after a role mismatch, allowing later MMU rmap walks to trigger a host use-after-free with at least DoS impact and possible higher impact requiring review.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53359 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53359

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-2026-53359	IMPORTANT	CHECK	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-416;*CWE-672;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'KVM x86 shadow paging can leave a stale rmap entry when kvm_mmu_get_child_sp reuses an existing child shadow page only by gfn and does not verify that the role matches the new path. After a PDE mapping is changed outside the guest and a memslot is later deleted, the shadow page can be freed while an rmap entry still points to an sptep inside that freed page. Later rmap walks such as dirty logging or MMU notifier invalidation may dereference the stale sptep, creating a host kernel use after free. For the CVSS the PR:L is used because reliable triggering requires the ability to run or control a KVM VM or its userspace VMM operations, but not full host administrative privileges. The issue is not directly network reachable. Impact is at least local denial of service and in worst case may allow host memory corruption with confidentiality, integrity, and availability impact.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	VIRT UAF HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	NO	NO	checked

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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

* Local unprivileged trigger: +1. A local user with access to `/dev/kvm` or control over a userspace VMM can potentially reach the affected KVM MMU path without full host root.
* Generic memory corruption, strong primitive: +2. The commit explicitly describes a stale rmap entry pointing to an `sptep` inside a freed shadow page, followed by later dereference.
* Real lifetime corruption: +1. This is a real use-after-free caused by stale reverse mapping state after shadow page free.
* Privilege escalation plausible, weak to moderate: +1 conservative. The primitive is a host kernel UAF in KVM MMU state, but the patch does not demonstrate controlled reclaim or an arbitrary write.
* Privilege escalation plausible, stronger paranoid interpretation: +2 paranoid. KVM rmap UAFs are high-risk because later rmap walkers may operate on freed MMU memory, so guest-to-host or VMM-to-host impact deserves manual review.
* Reliable kernel crash / strong DoS: +1. Later dirty logging, MMU notifier invalidation, or similar rmap walks can dereference the stale `sptep`.
* Broad/common subsystem exposure: +1. KVM is widely deployed on virtualization hosts.
* Legacy / non-default execution mode: -1. The issue is in shadow paging, which is less common than EPT/NPT on modern hardware.

Call-site confidence: high. The commit message explicitly describes the downstream rmap removal failure, memslot deletion, shadow page free, and later rmap traversal.

## 3. Reachability analysis

The issue is local to the host KVM interface and is not directly network reachable. Practical triggering requires the ability to run or control a VM/VMM workflow that can manipulate guest mappings and memslots. In many deployments this is not full root, because `/dev/kvm` may be available to users in a `kvm` group or to virtualization service accounts. Containers or reduced-capability service contexts matter if they can access KVM or control a VMM, so PR should not automatically be treated as full admin. The main limiting factor is that the vulnerable path depends on shadow paging rather than the more common hardware-assisted EPT/NPT mode.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup bug. It is a KVM MMU lifetime bug with an explicit host use-after-free through stale rmap state. Realistic exploitation evidence is stronger for DoS than for privilege escalation, because no controlled reclaim or arbitrary write primitive is shown. However, the affected object is MMU/rmap state in a host virtualization boundary, so the paranoid score should treat it as a Strong Important candidate requiring manual review.

## 5. One-sentence report phrase

KVM x86 shadow paging may leave a stale rmap entry pointing into a freed shadow page after a role mismatch, allowing later MMU rmap walks to trigger a host use-after-free with at least DoS impact and possible higher impact requiring review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: explicit KVM MMU use-after-free, stale rmap pointer after free, and virtualization boundary relevance make this unsuitable for auto-close even though the vulnerable mode is not the default on many modern systems.

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

Patch: KVM: x86: Fix shadow paging use-after-free due to unexpected role
Commit: b1337aae5e194324e4810d561764e7793f8b3864
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b1337aae5e194324e4810d561764e7793f8b3864

Commit description:

Commit 0cb2af2 ("KVM: x86: Fix shadow paging use-after-free due
to unexpected GFN") fixed a shadow paging mismatch between stored and
computed GFNs; the bug could be triggered by changing a PDE mapping from
outside the guest, and then deleting a memslot.  The rmap_remove()
call would miss entries created after the PDE change because the GFN
of the leaf SPTE does not match the GFN of the struct kvm_mmu_page.

A similar hole however remains if the modified PDE points to a non-leaf
page.  In this case the gfn can be made to match, but the role does not
match: the original large 2MB page creates a kvm_mmu_page with direct=1,
while the new 4KB needs a kvm_mmu_page with direct=0.  However,
kvm_mmu_get_child_sp() does not compare the role, and therefore reuses
the page.

The next step is installing a leaf (4KB) SPTE on the new path which
records an rmap entry under the gfn resolved by the walk.  But when
that child is zapped its parent kvm_mmu_page has direct=1 and
kvm_mmu_page_get_gfn() computes the gfn for the 4KB page as
sp->gfn + index instead of using sp->shadowed_translation[] (or sp->gfns[]
in older kernels).  It therefore fails to remove the recorded entry.

When the memslot is dropped the shadow page is freed but the rmap
entry survives, as in the scenario that was already fixed.  Code that
later walks that gfn (dirty logging, MMU notifier invalidation, and
so on) dereferences an sptep that lies in the freed page, causing the
use-after-free.

Fixes: 2032a93 ("KVM: MMU: Don't allocate gfns page for direct mmu pages")
Reported-by: Hyunwoo Kim <imv4bel@gmail.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=b1337aae5e194324e4810d561764e7793f8b3864

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

The original Linux kernel CVE announcement for CVE-2026-53359 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53359

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: 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).