[CVE-2026-63938][MODERATE 7.0] KVM: SEV: Check PSC request indices against the actual size of the buffer

[CVE-2026-63938][MODERATE 7.0] KVM: SEV: Check PSC request indices against the actual size of the buffer

[AL-KERNEL]

CVE: CVE-2026-63938
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: SEV: Check PSC request indices against the actual size of the buffer
Commit: 5198f70c09a5f6e9e5f5a0a2c6b388f24294b176
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5198f70c09a5f6e9e5f5a0a2c6b388f24294b176
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63938
Analysis date: Sun, 19 Jul 2026 20:01:46 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
KVM SEV-SNP PSC validation used the maximum GHCB entry count instead of the effective scratch area size, allowing a malicious guest to make host KVM parse PSC entries beyond the valid buffer with realistic DoS impact and possible limited integrity impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63938 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63938

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-63938	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:L/I:L/A:H';CWE-125;CWE-129;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.9';DESCR 'KVM SEV-SNP PSC handling validates request indices against a maximum count derived from the full GHCB shared buffer, but the effective scratch area can be smaller when the guest supplied scratch pointer is not at the start of the GHCB buffer. A malicious guest can provide PSC metadata where end_entry is below VMGEXIT_PSC_MAX_COUNT but beyond the actual scratch area, causing KVM to read PSC entries outside the effective buffer during VMGEXIT handling. For the CVSS the PR:L is used because an attacker needs the ability to run or control a guest that can issue SEV-SNP PSC VMGEXIT requests, but no host administrator privileges are required. The issue is not directly network reachable. Impact is at least host denial of service, and the paranoid score treats the host-side out-of-bounds PSC parsing as possible limited confidentiality or integrity impact across the guest to host boundary.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	VIRT INIT SIMPLEFIX HARDWARE  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	YES	NO	guess

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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

Triggered signals:

* +1 Local or guest controlled trigger: a malicious SEV-SNP guest can issue PSC VMGEXIT requests without host administrator privileges.
* +1 Memory corruption, weak or indirect corruption candidate: KVM may parse PSC entries beyond the effective GHCB scratch area due to validation against VMGEXIT_PSC_MAX_COUNT instead of the actual buffer size. This is best treated as OOB read or invalid struct interpretation, not a strong overwrite primitive.
* +1 Availability impact realistic: malformed PSC metadata can affect host-side KVM PSC handling and plausibly cause VM or host KVM failure.
* +1 Cross-boundary isolation concern: the bug is in a guest-to-host KVM boundary path.
* +1 Privileged kernel state-management path: PSC handling affects SEV-SNP page state management in trusted host kernel code.
* -1 Rare or configuration-dependent exposure: requires KVM SEV-SNP and PSC/GHCB handling, not a broad default kernel path.

Paranoid-only signal:

* +1 Integrity impact plausible: if out-of-bounds parsed PSC entries are interpreted as valid page-state requests, unintended host-side handling of guest page state is possible. This is not enough to claim strong LPE, but it is enough to avoid auto-close.

Not awarded:

* No +2 remote reachable: this is not network-triggerable.
* No +2 strong memory corruption: no OOB write, UAF reclaim, arbitrary write, type confusion, or callback control is shown.
* No +2 strong privilege escalation: guest-to-host impact is concerning, but the patch does not demonstrate a practical host code execution or LPE primitive.
* No confidentiality point in the conservative score: the likely OOB read is around GHCB scratch/shared guest-controlled data, not clearly host-secret disclosure.

Call-site confidence: medium. The changed function and VMGEXIT context are visible, but the full downstream PSC processing path is not included in the provided patch.

## 3. Reachability analysis

The likely attacker is a malicious SEV-SNP guest or guest-controlled kernel component capable of issuing PSC VMGEXIT requests with crafted GHCB scratch metadata. This does not require host root privileges, but it does require the attacker to control a guest running under KVM SEV-SNP. Namespaces and ordinary local containers on the host do not directly expose this path. The path is not default-enabled on all systems because it depends on AMD SEV-SNP virtualization support and a guest using PSC requests. Realistic exploitation is therefore configuration-dependent, but the guest-to-host boundary makes it security-relevant.

## 4. Severity interpretation

This is stronger than an ordinary Moderate validation bug because the malformed input crosses the guest-to-host KVM boundary and can make host kernel code parse PSC entries outside the effective buffer. Realistically, the demonstrated impact is closer to host-side DoS or incorrect page-state handling than proven host privilege escalation. Theoretical memory corruption potential exists as OOB parsing or invalid object interpretation, but there is no evidence of attacker-controlled overwrite, UAF reclaim, arbitrary write, or reliable host code execution.

## 5. One-sentence report phrase

KVM SEV-SNP PSC validation used the maximum GHCB entry count instead of the effective scratch area size, allowing a malicious guest to make host KVM parse PSC entries beyond the valid buffer with realistic DoS impact and possible limited integrity impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a guest-to-host KVM boundary bug involving bounds validation and host-side parsing of guest-controlled PSC metadata. It should not be auto-closed even though the currently supported primitive is weaker than a demonstrated host memory corruption or LPE exploit.

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

Patch: KVM: SEV: Check PSC request indices against the actual size of the buffer
Commit: 5198f70c09a5f6e9e5f5a0a2c6b388f24294b176
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5198f70c09a5f6e9e5f5a0a2c6b388f24294b176

Commit description:

When processing Page State Change (PSC) requests, validate the PSC buffer
against the effective size of the scratch area, which could be less than
the maximum size if the guest provided a pointer that isn't exactly at the
start of the GHCB shared buffer.

Fixes: 9b54e24 ("KVM: SEV: Add support to handle Page State Change VMGEXIT")
Cc: stable@vger.kernel.org
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260501202250.2115252-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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=5198f70c09a5f6e9e5f5a0a2c6b388f24294b176

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

The original Linux kernel CVE announcement for CVE-2026-63938 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63938

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:C/C:L/I:L/A:H
  The Best / paranoid CVSS score: 7.9

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).