[CVE-2026-89751][MODERATE REGULAR] x86/tdx: Fix off-by-one in port I/O handling

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

CVE: CVE-2026-89751
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: x86/tdx: Fix off-by-one in port I/O handling
Commit: 222b7005e4519f633b4cd666226256abbab46a1a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=222b7005e4519f633b4cd666226256abbab46a1a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89751
Analysis date: Sat, 12 Sep 2026 05:42:54 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89751 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89751

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.

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

CVE-2026-89751	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89751 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';CWE-193;CWE-682;*CWE-200;Other CVSS 'AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '6.2';DESCR 'An off by one mask in TDX port I/O handling makes handle_in() and handle_out() use one extra bit for IN and OUT emulation. For INB the mask covers 9 bits instead of 8, and the same pattern applies to wider I/O sizes. A malicious or compromised VMM can therefore influence one extra bit of guest register state on port reads, while port writes can expose one extra bit from the guest register to the VMM. For the CVSS the PR:N is used because the attacker does not need privileges inside the guest once the VMM or host side I/O emulation is under attacker control. The issue is not network reachable and requires the same host or VMM attack position. Impact is mainly limited confidentiality and integrity impact across the TDX guest boundary, with possible denial of service if the corrupted bit affects sensitive guest kernel or driver control flow.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	SIMPLEFIX HARDWARE  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3. No floor or cap applied.
* Raw paranoid score: 4
* Paranoid score: 4. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative positive signals:

* +1 Confidentiality plausible. `handle_out()` used a mask that was one bit too wide, so an OUT operation could expose one extra bit from guest register state to the VMM.
* +1 Integrity plausible. `handle_in()` used a mask that was one bit too wide, so a malicious or compromised VMM could influence one extra bit of guest register state on an emulated IN operation.
* +1 Security boundary concern. The bug is in TDX guest port I/O hypercall handling, where the VMM is part of the relevant adversarial boundary.

Paranoid additional signal:

* +1 Availability concern. Corrupting one extra guest register bit during port I/O can plausibly destabilize guest kernel or driver behavior, although a reliable crash is not demonstrated by the patch.

Negative signals:

* No host-admin penalty applied despite `HostAdminRequired=YES`, because the relevant attacker is the host or VMM against a TDX guest, and that is the intended confidential-computing security boundary.
* No race or timing penalty. This is a deterministic off-by-one mask bug.
* No rare-hardware penalty applied. TDX-capable hardware is a deployment prerequisite, but the affected feature is specifically TDX, and no physical attacker interaction is required.

Non-awarded signals:

* No RealUAF. The patch does not establish stale lifetime use.
* No controlled reclaim, arbitrary write, OOB kernel memory access, callback control, or refcount corruption.
* No strong LPE or strong exploitation leverage.
* No remote network reachability.
* No local unprivileged guest trigger as the attacker model. The attacker position is malicious or compromised host/VMM-side I/O emulation.
* No Class A authorization bypass. The bug weakens a TDX boundary in a limited one-bit way but does not establish broad protected-resource access or root compromise.
* No extra availability point for a reliable crash, because a crash is not demonstrated.

3. Reachability analysis

The practical attacker is a malicious or compromised VMM or host-side component that services TDX port I/O exits. The guest itself does not need to be malicious to be affected, but guest execution must reach the vulnerable port I/O paths.

This is not network reachable. It is not a namespace-scoped issue. It is not reachable by an ordinary local user inside the guest unless that user can induce relevant guest port I/O and the VMM is already adversarial.

`HostAdminRequired=YES` is set because controlling the VMM or host-side I/O emulation is host-admin-equivalent in ordinary deployment terms. However, this is also the security boundary that TDX is meant to defend against, so it is not used as a reducing penalty.

Prior setup requires a TDX guest running on TDX-capable hardware. The issue is deterministic once the affected IN or OUT emulation path is exercised. There is no race.

4. Primitive and severity interpretation

The exact primitive is an off-by-one bit mask in TDX port I/O emulation. For `size=1`, the old mask used `GENMASK(8, 0)` instead of `GENMASK(7, 0)`, covering 9 bits instead of 8. The same one-extra-bit pattern applies to wider I/O sizes.

This is not memory corruption in the normal kernel heap or object-lifetime sense. It is not an active structured UAF, fixed stale storage, or repeated release/refcount case.

PrimitiveClass is `CLASS_C` because the established behavior is limited invalid register-value exposure or modification across a security boundary, without a stronger independently useful corruption primitive.

Demonstrated consequence is limited confidentiality and integrity impact on guest register state at the TDX guest/VMM boundary. The paranoid consequence is possible guest DoS if the extra corrupted bit affects sensitive guest kernel or driver behavior.

ImportantCandidate is `NO` because the patch does not establish a strong Class A primitive, controlled corruption, broad boundary compromise, or demonstrated privilege escalation.

AutoDowngradeAllowed is `NO` because this is still a meaningful confidential-computing boundary issue and should not be silently treated as an ordinary low-value correctness bug.

Likely operational severity: borderline to actionable Moderate, mainly due to the TDX security boundary and limited one-bit C/I primitive.

5. One-sentence report phrase

A TDX port I/O mask off by one allows a malicious or compromised VMM to observe or influence one extra bit of guest register state during emulated IN and OUT operations, creating limited confidentiality and integrity impact across the guest/VMM boundary.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the primitive is small and not memory-corruption based, but it affects the TDX guest/VMM trust boundary and may have context-dependent impact depending on which guest port I/O paths consume the extra bit.

7. Call-site confidence

Call-site confidence: medium

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

Patch: x86/tdx: Fix off-by-one in port I/O handling
Commit: 222b7005e4519f633b4cd666226256abbab46a1a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=222b7005e4519f633b4cd666226256abbab46a1a

Commit description:

handle_in() and handle_out() in arch/x86/coco/tdx/tdx.c use:

    u64 mask = GENMASK(BITS_PER_BYTE * size, 0);

GENMASK(h, l) includes bit h. For size=1 (INB), this produces
GENMASK(8, 0) = 0x1FF (9 bits) instead of GENMASK(7, 0) = 0xFF (8
bits). The mask is one bit too wide for all I/O sizes.

Fix the mask calculation.

Fixes: 0314994 ("x86/tdx: Port I/O: Add runtime hypercalls")
Reported-by: Borys Tsyrulnikov <tsyrulnikov.borys@gmail.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Link: https://lore.kernel.org/all/CAKw_Dz96rfSQc6Rn+9QBcUFHhmkK+9zu+P=bxowfZwxrATCBRg@mail.gmail.com/
Cc:stable@vger.kernel.org
Link: https://patch.msgid.link/20260713133753.223947-2-kirill@shutemov.name
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/coco/tdx/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=222b7005e4519f633b4cd666226256abbab46a1a

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

The original Linux kernel CVE announcement for CVE-2026-89751 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89751

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:N/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.2

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: 3
  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).