[CVE-2026-64247][MODERATE 7.0] KVM: x86: hyper-v: Bound the bank index when querying sparse banks

[CVE-2026-64247][MODERATE 7.0] KVM: x86: hyper-v: Bound the bank index when querying sparse banks

[AL-KERNEL]

CVE: CVE-2026-64247
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: x86: hyper-v: Bound the bank index when querying sparse banks
Commit: d18756b12aab30d07794446445c93112e5c69a2e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d18756b12aab30d07794446445c93112e5c69a2e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64247
Analysis date: Fri, 24 Jul 2026 11:51:48 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A guest-controlled VP ID in the KVM Hyper-V sparse bank lookup can exceed the 4096 VP limit, causing an out-of-bounds read and KASAN use-after-free read in host KVM context during nested paravirtual TLB flush handling.

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

The original Linux kernel CVE announcement for CVE-2026-64247 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64247

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-2026-64247	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-125;CWE-129;*CWE-416;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.6';DESCR 'KVM Hyper-V sparse bank handling can perform an out of bounds read because hv_is_vp_in_sparse_set() uses a VP ID copied from the enlightened VMCS without first checking that it is below the 4096 VP limit. A malicious or buggy nested guest path can provide an out of range VP ID during a paravirtual TLB flush, causing KVM to read past the valid sparse bank mask and triggering a KASAN use after free read in host kernel context. For the CVSS the PR:L is used because the attacker needs local ability to run or control a KVM guest or a process with access to /dev/kvm, but full host administrator privileges are not required in common KVM group deployments. The issue is not network reachable and is triggered through the local virtualization interface. Impact is at least denial of service via host kernel crash or warning. In the paranoid score, limited confidentiality or integrity impact is kept because the primitive is an out of bounds or use after free read in KVM host context, but no direct arbitrary write primitive is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	READ OOB DANGER VIRT UAF HARDWARE LINUS TEST  KPANIC INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS	YES	NO	guess

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

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

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A user with access to /dev/kvm or the kvm group can run or control a guest that reaches the Hyper-V enlightened nested path.
* Memory corruption, weak or indirect corruption candidate: +1. The bug is an out-of-bounds read from a VP ID derived from guest-controlled enlightened VMCS state.
* Reliable kernel crash or strong DoS concern: +1. The provided trace shows KASAN reporting a use-after-free read in host KVM context during KVM execution.
* Broad deployed subsystem exposure: +1. KVM is a broadly deployed virtualization subsystem, even though this exact path depends on Hyper-V nested virtualization behavior.

Paranoid additional signal:

* Limited confidentiality or integrity concern: +1. The primitive is read-only and no disclosure path or write primitive is shown, but it is a guest-influenced OOB or UAF read in host KVM control flow, so limited manual-review impact is defensible.

Not counted:

* Remote reachable: +0. This is not network-triggerable.
* Strong LPE plausibility: +0. No reclaim, overwrite, callback control, type confusion, or arbitrary write is shown.
* Strong memory corruption primitive: +0. The patch supports OOB read / KASAN UAF read, not OOB write or controlled overwrite.
* Admin-only penalty: +0. Full host root is not necessarily required when /dev/kvm is delegated to local users.

## 3. Reachability analysis

The likely attacker is a local user or workload able to create or control a KVM VM with Hyper-V enlightenment support and nested L2 behavior. The problematic VP ID is copied from enlightened VMCS state during a paravirtual TLB flush path, so the trigger is guest-controlled through the virtualization interface rather than through ordinary host syscalls alone.

Namespaces and containers matter if /dev/kvm is exposed into a container or to a reduced-privilege service account. In such deployments this should still be treated as PR:L rather than PR:H, because the attacker does not need full host administrator privileges.

The path is not remotely reachable by packets. It requires KVM availability and a Hyper-V nested virtualization configuration, so exploitation is narrower than a generic KVM ioctl bug, but still relevant for virtualization hosts.

## 4. Severity interpretation

This behaves above ordinary Moderate because it is a guest-influenced host KVM memory-safety bug with a demonstrated KASAN use-after-free read. Realistic evidence supports host DoS or warning/crash behavior. Theoretical escalation is weaker because the primitive is read-only and the patch does not show attacker-controlled reuse, object replacement, write-after-free, or function pointer control.

The best practical classification is Actionable Moderate at minimum, with manual review recommended or required for virtualization products that expose KVM to non-root users or containers.

## 5. One-sentence report phrase

A guest-controlled VP ID in the KVM Hyper-V sparse bank lookup can exceed the 4096 VP limit, causing an out-of-bounds read and KASAN use-after-free read in host KVM context during nested paravirtual TLB flush handling.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is KVM guest-to-host memory-safety behavior with a demonstrated KASAN UAF read. Even though the current evidence suggests DoS and limited read-only impact rather than a strong LPE primitive, the virtualization boundary and host-kernel context make auto-closing inappropriate.

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

Patch: KVM: x86: hyper-v: Bound the bank index when querying sparse banks
Commit: d18756b12aab30d07794446445c93112e5c69a2e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d18756b12aab30d07794446445c93112e5c69a2e

Commit description:

When checking if a VP ID is included in a sparse bank set, explicitly check
that the ID can actually be contained in a sparse bank (the TLFS allows for
a maximum of 64 banks of 64 vCPUs each).  When handling a paravirtual TLB
flush for L2, the VP ID is copied verbatim from the enlightened VMCS,
without any bounds check, i.e. isn't guaranteed to be under the limit of
4096.

Failure to check the bounds of the VP ID leads to an out-of-bounds read
when testing the sparse bank, and super strictly speaking could lead to KVM
performing an unnecessary TLB flush for an L2 vCPU.

  ==================================================================
  BUG: KASAN: use-after-free in hv_is_vp_in_sparse_set+0x85/0x100 [kvm]
  Read of size 8 at addr ffff88811ba5f598 by task hyperv_evmcs/2802

  CPU: 12 UID: 1000 PID: 2802 Comm: hyperv_evmcs Not tainted 7.1.0-rc2 #7 PREEMPT
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  Call Trace:
   <TASK>
   dump_stack_lvl+0x51/0x60
   print_report+0xcb/0x5d0
   kasan_report+0xb4/0xe0
   kasan_check_range+0x35/0x1b0
   hv_is_vp_in_sparse_set+0x85/0x100 [kvm]
   kvm_hv_flush_tlb+0xe9e/0x16c0 [kvm]
   kvm_hv_hypercall+0xe6b/0x1e60 [kvm]
   vmx_handle_exit+0x485/0x1b60 [kvm_intel]
   kvm_arch_vcpu_ioctl_run+0x22e3/0x5070 [kvm]
   kvm_vcpu_ioctl+0x5d0/0x10c0 [kvm]
   __x64_sys_ioctl+0x129/0x1a0
   do_syscall_64+0xb9/0xcf0
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
  RIP: 0033:0x7f0e62d1a9bf
   </TASK>

  The buggy address belongs to the physical page:
  page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffffffffffffffff pfn:0x11ba5f
  flags: 0x4000000000000000(zone=1)
  raw: 4000000000000000 0000000000000000 00000000ffffffff 0000000000000000
  raw: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000000
  page dumped because: kasan: bad access detected

  Memory state around the buggy address:
   ffff88811ba5f480: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
   ffff88811ba5f500: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
  >ffff88811ba5f580: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                              ^
   ffff88811ba5f600: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
   ffff88811ba5f680: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
  ==================================================================
  Disabling lock debugging due to kernel taint

Opportunistically add a compile time assertion to ensure the maximum number
of sparse banks exactly matches the number of possible bits in the passed
in mask.

Cc: stable@vger.kernel.org
Fixes: c58a318 ("KVM: x86: hyper-v: L2 TLB flush")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://patch.msgid.link/aiQyZIJtO-2Aj_xN@v4bel
[sean: add KASAN splat, drop comment, add assert, massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kvm/hyperv.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=d18756b12aab30d07794446445c93112e5c69a2e

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

The original Linux kernel CVE announcement for CVE-2026-64247 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64247

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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.6

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: 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).