[CVE-2026-63940][MODERATE 7.0] KVM: SEV: Ignore Port I/O requests of length '0'

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

CVE: CVE-2026-63940
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: SEV: Ignore Port I/O requests of length '0'
Commit: 3b6035bc6bff20e89752ce4358bc4c9a9d5883f2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3b6035bc6bff20e89752ce4358bc4c9a9d5883f2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63940
Analysis date: Sun, 19 Jul 2026 14:02:38 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
KVM SEV failed to ignore zero-length Port I/O VMGEXIT requests, allowing a malicious SEV-ES guest to drive host-side scratch-area handling into a len 0 underflow-sensitive path that may cause a host warning or denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-63940 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63940

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: NO

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-2026-63940	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:C/C:N/I:N/A:H';CWE-191;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:L';BEST CVSS score: '6.5';DESCR 'KVM SEV did not explicitly ignore zero-length Port I/O VMGEXIT requests. A malicious or compromised SEV-ES guest could present an I/O exit with size 0 or a string I/O request with count 0, causing host side handling to enter paths that were not intended for len 0 and could underflow length accounting in the scratch area setup. For the CVSS the PR:L is used because reliable triggering requires control of code inside the guest or a guest component able to issue the malformed VMGEXIT, but it does not require host administrator privileges. The issue is not directly network reachable from outside the hypervisor. Impact is primarily denial of service against the host KVM path or a host warning that may become a crash with panic_on_warn. No direct confidentiality or integrity impact is claimed because the patch context does not show a UAF, OOB write, arbitrary write, or information leak primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	OOB VIRT INIT SIMPLEFIX HARDWARE KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

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

## 2. Signal breakdown

* Local unprivileged or tenant-controlled trigger: +1
  A malicious or compromised SEV-ES guest can influence VMGEXIT Port I/O state. This does not require host root, but does require control inside the guest.

* Memory corruption, weak or indirect corruption candidate: +1
  The patch prevents len==0 from reaching scratch-area setup paths where length accounting could underflow. The patch does not show UAF, OOB write, arbitrary write, or type confusion.

* Privileged kernel or virtualization-path state corruption concern: +1
  The affected code is host-side KVM SEV VMGEXIT handling. Guest-controlled architectural state reaches trusted host virtualization code.

* Cross-boundary availability impact: +1
  A guest can potentially affect host KVM handling, which is a guest-to-host boundary concern even if the demonstrated impact is DoS-like.

* Reliable crash or strong DoS, paranoid only: +1
  The commit mentions underflow avoidance and WARN-related handling. A host warning may become a crash with panic_on_warn, but the patch does not demonstrate a reliable host crash in all configurations.

* Legacy or non-default execution mode: -1
  The issue depends on AMD SEV or SEV-ES VMGEXIT handling, not a default path for all KVM guests.

No LPE, confidentiality, or integrity points are awarded because the patch does not show a concrete UAF reclaim, OOB write, arbitrary write, info leak, refcount takeover, or object replacement primitive.

## 3. Reachability analysis

The bug is reachable from a guest that can generate malformed SEV-ES VMGEXIT Port I/O requests with zero size or zero byte string I/O. In practical cloud or virtualization threat models this may be tenant-controlled, but it is not remotely reachable over the network from outside the hypervisor. Host administrator privileges are not required by the attacker if they already control the guest workload, but the affected path requires SEV/SEV-ES support and an SEV guest configuration.

Namespaces and containers on the host are not the main factor here. The relevant boundary is guest-to-host virtualization isolation.

Call-site confidence: medium. The patch shows the immediate VMGEXIT and string I/O handling paths, but not the full setup_vmgexit_scratch internals.

## 4. Severity interpretation

This behaves more like an actionable Moderate KVM guest-to-host DoS or state-validation issue than a proven Important-class memory corruption vulnerability. The theoretical concern is that zero-length handling can underflow host-side scratch-area length accounting. The realistic evidence supports host warning or DoS risk, not demonstrated privilege escalation or confidentiality/integrity compromise.

The paranoid score is kept at 5 because KVM guest-to-host bugs with length underflow in trusted host handling deserve manual review, but there is not enough evidence to classify this as Strong Important.

## 5. One-sentence report phrase

KVM SEV failed to ignore zero-length Port I/O VMGEXIT requests, allowing a malicious SEV-ES guest to drive host-side scratch-area handling into a len 0 underflow-sensitive path that may cause a host warning or denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a KVM guest-to-host surface involving length underflow handling in host virtualization code. Current evidence supports DoS rather than LPE or data compromise.

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

Patch: KVM: SEV: Ignore Port I/O requests of length '0'
Commit: 3b6035bc6bff20e89752ce4358bc4c9a9d5883f2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3b6035bc6bff20e89752ce4358bc4c9a9d5883f2

Commit description:

Explicitly ignore Port I/O requests of length '0' (or count '0'), so that
setting up the software scratch area (and other code) doesn't have to
worry about underflowing the length, and to allow for WARNing on trying
to configure the scratch area with len==0.

Fixes: 291bd20 ("KVM: SVM: Add initial support for a VMGEXIT VMEXIT")
Cc: stable@vger.kernel.org
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260501202250.2115252-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  arch/x86/kvm/svm/sev.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=3b6035bc6bff20e89752ce4358bc4c9a9d5883f2

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

The original Linux kernel CVE announcement for CVE-2026-63940 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63940

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:C/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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).