[CVE-2026-72285][MODERATE REGULAR] KVM: TDX: Reject concurrent change to CPUID entry count

[CVE-2026-72285][MODERATE REGULAR] KVM: TDX: Reject concurrent change to CPUID entry count

[AL-KERNEL]

CVE: CVE-2026-72285
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: TDX: Reject concurrent change to CPUID entry count
Commit: d6b5aba65e99531c97b146622a406c75653819d5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d6b5aba65e99531c97b146622a406c75653819d5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72285
Analysis date: Sat, 15 Aug 2026 21:53:04 -0400
ActionableScore: 4
ActionableScore lower bound: 2
Actionable bucket: Borderline Moderate. Manual review recommended
Manual review required: YES

Summary:
A local KVM TDX userspace process can race cpuid.nent during TD initialization so that CPUID parsing reads beyond the copied entries, causing at least a local DoS risk and limited OOB-read based state influence.

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

The original Linux kernel CVE announcement for CVE-2026-72285 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72285

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 REGULAR
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
======================================================================

CVE-2026-72285	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:L/UI:N/S:U/C:L/I:L/A:H';CWE-367;CWE-125;*CWE-20;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A race in KVM TDX TD initialization can occur because userspace may change cpuid nent between the initial size calculation and the later copy of the initialization data. If the copied cpuid nent is larger than the entry count used for allocation, later CPUID parsing can use the larger value as an array bound and read beyond the entries that were actually copied. For the CVSS the PR:L is used because an attacker needs local access to KVM ioctls and the ability to create or initialize a TDX VM, but full host administrator privileges are not necessarily required in common KVM group deployments. The issue is not network reachable and is triggered through a local virtualization control interface. Impact is at least local denial of service via invalid kernel memory access, and in the paranoid case may allow limited confidentiality or integrity impact due to out-of-bounds read based CPUID parsing. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Moderate. Manual review recommended (with actual score 3)  SKIP CVE-2026-72285 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 0bd0a4a1428baaf4447e95f0832492d9e3d64961 YES  NO NO unknown 	MAYBE	VIRT INIT SIMPLEFIX HARDWARE 	-	-	checked

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

ActionableScore=4
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 4
* Final recommended bucket: **Borderline Moderate. Manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. Trigger requires local userspace access to KVM ioctls and KVM_TDX_INIT_VM. This is not remote, but it may be reachable by users in the kvm group without full host root.
* Memory corruption, weak or indirect candidate: +1. The bug can make cpuid.nent larger than the number of entries actually copied, causing kvm_find_cpuid_entry2() to read beyond the copied flexible array. This is an OOB read, not an OOB write or UAF.
* Reliable kernel crash / strong DoS: +1. A large or mismatched entry count can drive kernel CPUID parsing beyond the allocated object and plausibly cause invalid memory access or crash.
* Hard or unreliable race / special timing required: -1. Userspace must modify cpuid.nent between the initial read and the later copy.

Paranoid-only signals:

* Confidentiality impact plausible: +1. The OOB read may allow kernel heap data to influence CPUID parsing or observable behavior, but no direct disclosure primitive is shown.
* Integrity impact plausible: +1. Bogus out-of-bounds data may be interpreted as CPUID entries and influence TD initialization state, but no write primitive or host privilege escalation path is demonstrated.

Not awarded:

* No remote reachable signal. This is a local KVM ioctl path.
* No strong LPE signal. The patch shows OOB read, not reclaim, arbitrary write, type confusion, callback control, or refcount takeover.
* No strong memory corruption signal. This is read-only over-indexing of a copied userspace structure.

## 3. Reachability analysis

The bug is triggered by a local process invoking KVM TDX VM initialization and racing modification of the userspace cpuid.nent field. In typical deployments this requires access to /dev/kvm and TDX-capable KVM functionality. That may be available to non-root users in the kvm group, so PR:L is more appropriate than PR:H for paranoid triage.

Namespaces and containers can matter if /dev/kvm is delegated into a container or VM-management sandbox. The path is not network reachable by itself. TDX is hardware and configuration dependent, but once exposed through KVM it is reachable through a normal local virtualization control interface.

Call-site confidence: high. The commit message directly names the later consumer, setup_tdparams_cpuids(), and the bound-using helper, kvm_find_cpuid_entry2().

## 4. Severity interpretation

This behaves more like a borderline actionable Moderate than an Important-class vulnerability. The theoretical concern is an OOB read during KVM TDX initialization, with possible crash and limited state influence. The realistic evidence supports local DoS and limited OOB-read risk, but not privilege escalation or arbitrary kernel memory corruption.

The paranoid score is raised to 4 because this is KVM initialization code with a concrete array-bound mismatch and OOB read. It should not be auto-closed without review, but the patch does not justify Important severity by itself.

## 5. One-sentence report phrase

A local KVM TDX userspace process can race cpuid.nent during TD initialization so that CPUID parsing reads beyond the copied entries, causing at least a local DoS risk and limited OOB-read based state influence.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the bug is in KVM TDX initialization and contains a concrete OOB read primitive, but current evidence does not show UAF, OOB write, arbitrary write, or a practical LPE chain.

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

Patch: KVM: TDX: Reject concurrent change to CPUID entry count
Commit: d6b5aba65e99531c97b146622a406c75653819d5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d6b5aba65e99531c97b146622a406c75653819d5

Commit description:

Reject KVM_TDX_INIT_VM if userspace changes cpuid.nent between the
initial read and the subsequent copy of the initialization data.

tdx_td_init() first reads user_data->cpuid.nent to size the flexible
kvm_tdx_init_vm copy.  The copied structure also contains cpuid.nent,
and that field can differ from the value used to size the allocation if
userspace modifies the input concurrently.  setup_tdparams_cpuids() later
passes init_vm->cpuid.nent to kvm_find_cpuid_entry2(), which uses it as
the array bound for the copied entries.

Require the copied count to match the value used to size the allocation
so that CPUID parsing cannot access beyond the entries actually copied.

Fixes: 0bd0a4a ("KVM: TDX: Replace kmalloc + copy_from_user with memdup_user in tdx_td_init()")
Reported-by: Sashiko:gemini-3.1-pro-preview
Cc: <stable@vger.kernel.org>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://patch.msgid.link/20260710035324.3170534-1-binbin.wu@linux.intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kvm/vmx/tdx.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=d6b5aba65e99531c97b146622a406c75653819d5

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

The original Linux kernel CVE announcement for CVE-2026-72285 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72285

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 2
  Paranoid ActionableScore: 4

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 REGULAR

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).