[CVE-2026-63937][MODERATE 7.0] KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer

[CVE-2026-63937][MODERATE 7.0] KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer

[AL-KERNEL]

CVE: CVE-2026-63937
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer
Commit: bd232801ef1d1fd985d2d4ca3cd1d888303ca86f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bd232801ef1d1fd985d2d4ca3cd1d888303ca86f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63937
Analysis date: Sun, 19 Jul 2026 16:12:46 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A TOCTOU race in KVM SEV SNP PSC handling allows a malicious guest to modify guest accessible PSC indices or entries while the host validates and consumes them, potentially causing inconsistent page state processing and host side denial of service or limited isolation impact.

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

The original Linux kernel CVE announcement for CVE-2026-63937 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63937

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
======================================================================

CVE-2026-63937	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:C/C:L/I:L/A:H';CWE-367;CWE-362;*CWE-20;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A TOCTOU race exists in the KVM/SEV SNP Page State Change handling path because KVM reads guest accessible PSC header indices and PSC entries without READ_ONCE(). A malicious guest can modify the PSC buffer while the host is validating and consuming it, which can make KVM process inconsistent entry data or index values during page state change handling. For the CVSS the PR:L is used because the attacker needs the ability to execute code in a guest or otherwise control a guest PSC buffer, but does not need host administrative privileges. The issue is not directly network reachable and is a guest to host virtualization interface issue. Impact is at least host denial of service risk and in the paranoid interpretation may allow limited confidentiality or integrity impact through inconsistent page state processing, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	VIRT SIMPLEFIX HARDWARE  INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local guest controlled trigger: +1. A malicious guest can control the guest accessible PSC buffer and trigger the KVM SEV SNP VMGEXIT handling path without host administrative privileges.
* Weak or indirect corruption candidate: +1. The patch prevents TOCTOU use of guest controlled PSC indices and entries, but does not show a concrete arbitrary write, UAF, double free, or controlled overwrite primitive.
* Security sensitive KVM page state path: +1. The affected code processes SEV SNP Page State Change data, including GFN ranges, page size, and operation selection in a host virtualization boundary path.
* Availability impact realistic: +1. Inconsistent PSC processing can plausibly cause failed page state handling or host side disruption, although the patch does not demonstrate a reliable host crash primitive.
* Rare or configuration dependent exposure: -1. The issue requires KVM with AMD SEV SNP support and a guest using PSC VMGEXITs, so it is important in cloud virtualization deployments but not a universal default kernel path.

Paranoid additions:

* Confidentiality impact plausible: +1. Incorrect page state processing in a confidential computing boundary could theoretically affect isolation semantics, though no direct disclosure primitive is shown.
* Integrity impact plausible: +1. Guest controlled TOCTOU changes could make KVM consume inconsistent page state data, creating a limited integrity concern in the guest host transition logic.

Not awarded:

* No remote reachable signal. This is not a network packet parsing issue.
* No strong LPE signal. The patch does not show reclaim, arbitrary write, callback control, refcount takeover, or object replacement.
* No strong generic memory corruption signal. READ_ONCE hardening prevents inconsistent reads, but the diff does not demonstrate UAF, OOB write, double free, or type confusion.

## 3. Reachability analysis

The realistic attacker is a malicious guest or guest kernel component that can modify the PSC buffer while KVM is processing a Page State Change VMGEXIT. This is a guest to host virtualization interface issue, not a remotely reachable network issue. A normal local host user generally needs access to run or control a VM using SEV SNP, so namespace effects are less relevant than virtualization tenant boundaries. The path is hardware and configuration dependent, but in SEV SNP hosting environments it is part of a security sensitive guest host boundary.

Call-site confidence: medium. The patch shows the affected PSC processing sites, but not a concrete downstream crash or corruption trace.

## 4. Severity interpretation

This is more than an ordinary cleanup because the buffer is guest accessible and consumed by KVM in a confidential virtualization path. Conservative handling is borderline Moderate because the visible primitive is a TOCTOU consistency bug without demonstrated memory corruption. Paranoid handling reaches Actionable Moderate because inconsistent page state processing across a guest host boundary can affect availability and may have limited confidentiality or integrity implications, even though strong exploit evidence is not shown.

## 5. One-sentence report phrase

A TOCTOU race in KVM SEV SNP PSC handling allows a malicious guest to modify guest accessible PSC indices or entries while the host validates and consumes them, potentially causing inconsistent page state processing and host side denial of service or limited isolation impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should be reviewed manually because it affects KVM guest to host SEV SNP page state handling and the patch explicitly defends against malicious guest controlled TOCTOU behavior, even though the concrete exploit primitive is not demonstrated in the diff.

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

Patch: KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer
Commit: bd232801ef1d1fd985d2d4ca3cd1d888303ca86f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bd232801ef1d1fd985d2d4ca3cd1d888303ca86f

Commit description:

Use READ_ONCE() when reading entries/indices from the guest-accessible
Page State Change buffer to defend against TOCTOU bugs.

Don't bother with READ_ONCE()/WRITE_ONCE() for cases where KVM is writing
(and not consuming the result!), as the guest isn't supposed to touch the
buffer while it's being processed.  I.e. using READ_ONCE() is all about
protecting against misbehaving guests.

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>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260501202250.2115252-11-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=bd232801ef1d1fd985d2d4ca3cd1d888303ca86f

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

The original Linux kernel CVE announcement for CVE-2026-63937 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63937

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:C/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:H
  The Best / paranoid CVSS score: 7

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: 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: NO
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).