[CVE-2026-46014][LOW] KVM: SVM: Add missing save/restore handling of LBR MSRs

[CVE-2026-46014][LOW] KVM: SVM: Add missing save/restore handling of LBR MSRs

[AL-KERNEL]

CVE: CVE-2026-46014
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: SVM: Add missing save/restore handling of LBR MSRs
Commit: 2b922a42b531a82d7881add14a7698dcdc5e1f0a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2b922a42b531a82d7881add14a7698dcdc5e1f0a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46014
Analysis date: Wed, 27 May 2026 16:46:11 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-46014 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46014

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-46014	LOW	CHECK	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:L';CWE-665;*CWE-664;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.5';DESCR 'KVM SVM did not enumerate DEBUGCTL and LBR MSRs for save\/restore and did not fully restore LBR state for nested SVM transitions. This can break VM migration or nested state restore by losing or corrupting guest LBR debug state, especially when LBR virtualization is enabled. For the CVSS the PR:L is used because triggering requires access to KVM userspace ioctls or control of VM save\/restore flow, but not full host root in deployments where VM execution is delegated. The issue is not network reachable by itself and does not provide a host memory corruption primitive. Impact is mainly guest state correctness and limited guest availability or integrity, with only limited confidentiality concern for possible guest branch history state confusion.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2) 	MAYBE	DANGER VIRT INIT HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2

1. ActionableScore
* Conservative score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

2. Signal breakdown
* Local delegated KVM control: +1. Triggering requires access to KVM userspace ioctls or control over VM save/restore or migration state.
* Availability or correctness impact: +1. Broken LBR save/restore can corrupt guest debug/branch-record state and break migration or nested restore correctness.
* Rare/configuration-dependent condition: -1. Meaningful impact depends on LBR virtualization, SVM or nested SVM state, and VM save/restore flows.
* No memory corruption signal. The patch handles MSR enumeration and architectural state migration, not UAF, OOB write, arbitrary write, or host memory lifetime corruption.
* No host privilege escalation or network trigger. The issue affects guest state correctness, not a demonstrated guest-to-host escape or host compromise path.

3. Reachability analysis
A local actor with permission to manage a KVM VM, perform save/restore, or run nested virtualization can trigger the affected state transitions. In many deployments this may be a non-root user if `/dev/kvm` access is delegated, but it still requires VM control rather than ordinary local code execution. Containers matter only if they are granted KVM device access. The issue is not network reachable by itself.

4. Severity interpretation
This behaves like a Low-like or ordinary Moderate KVM state correctness bug. The realistic impact is VM migration failure, nested restore misbehavior, or loss/corruption of guest LBR debug state. There is no evidence of host memory corruption, host confidentiality impact, host integrity impact, or privilege escalation.

5. One-sentence report phrase
KVM SVM failed to save and restore DEBUGCTL and LBR MSRs across VM migration or nested SVM restore, causing guest branch-debug state corruption without evidence of host compromise.

6. Manual review recommendation
NO MANUAL CHECK NEEDED. This is a guest state save/restore correctness issue with no supported memory corruption, security-boundary bypass, or host privilege-escalation primitive.

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

Patch: KVM: SVM: Add missing save/restore handling of LBR MSRs
Commit: 2b922a42b531a82d7881add14a7698dcdc5e1f0a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2b922a42b531a82d7881add14a7698dcdc5e1f0a

Commit description:

MSR_IA32_DEBUGCTLMSR and LBR MSRs are currently not enumerated by
KVM_GET_MSR_INDEX_LIST, and LBR MSRs cannot be set with KVM_SET_MSRS. So
save/restore is completely broken.

Fix it by adding the MSRs to msrs_to_save_base, and allowing writes to
LBR MSRs from userspace only (as they are read-only MSRs) if LBR
virtualization is enabled.  Additionally, to correctly restore L1's LBRs
while L2 is running, make sure the LBRs are copied from the captured
VMCB01 save area in svm_copy_vmrun_state().

Note, for VMX, this also fixes a flaw where MSR_IA32_DEBUGCTLMSR isn't
reported as an MSR to save/restore.

Note #2, over-reporting MSR_IA32_LASTxxx on Intel is ok, as KVM already
handles unsupported reads and writes thanks to commit b5e2fec ("KVM:
Ignore DEBUGCTL MSRs with no effect") (kvm_do_msr_access() will morph the
unsupported userspace write into a nop).

Fixes: 24e09cb ("KVM: SVM: enable LBR virtualization")
Cc: stable@vger.kernel.org
Reported-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260303003421.2185681-4-yosry@kernel.org
[sean: guard with lbrv checks, massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kvm/svm/nested.c
  arch/x86/kvm/svm/svm.c
  arch/x86/kvm/x86.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=2b922a42b531a82d7881add14a7698dcdc5e1f0a

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

The original Linux kernel CVE announcement for CVE-2026-46014 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46014

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 4.5

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: unknown
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).