[CVE-2026-68093][MODERATE 7.0] KVM: SVM: Bump asid_generation on CPU online to avoid ASID collision after hotplug

[CVE-2026-68093][MODERATE 7.0] KVM: SVM: Bump asid_generation on CPU online to avoid ASID collision after hotplug

[AL-KERNEL]

CVE: CVE-2026-68093
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: SVM: Bump asid_generation on CPU online to avoid ASID collision after hotplug
Commit: 60283726f2845bd78b95efbd0e50b93944780477
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=60283726f2845bd78b95efbd0e50b93944780477
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68093
Analysis date: Mon, 10 Aug 2026 20:05:08 -0400
ActionableScore: 7
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A KVM SVM ASID generation reset after CPU hotplug can allow two vCPUs from different VMs to reuse the same ASID on one pCPU, causing stale NPT TLB translations, KVM internal errors, and a worst-case VM isolation concern.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-68093 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68093

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-2026-68093	MODERATE	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:L/UI:N/S:C/C:H/I:H/A:H';*CWE-664;*CWE-672;CWE-284;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:C/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'KVM SVM can reuse an ASID generation after a CPU online-offline-online hotplug cycle. A vCPU that was scheduled out before the hotplug can later resume on the same pCPU with an ASID that has already been assigned to a vCPU from another VM, causing stale NPT TLB translations to be used across VM boundaries. The directly observed failure mode is a KVM internal error and denial of service, but the stale translation condition is an isolation sensitive primitive and may allow confidentiality or integrity impact between guests in a worst case review. For the CVSS the PR:L is used for the paranoid score because delegated virtualization services or reduced capability VM management roles may control VM placement and CPU hotplug workflows without being full host root. The issue is not directly network reachable and requires local host virtualization control rather than packet traffic. Impact should be reviewed as more than a simple crash because ASID collision affects address translation isolation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	VIRT HARDWARE  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=7
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The commit describes KVM internal error and emulation failure after stale NPT TLB translations.
* Cross-boundary isolation/policy bypass: +1. The ASID collision can occur between vCPUs from different VMs, so the affected state crosses VM isolation boundaries.
* Privileged kernel/device-management memory corruption path or high-value MMU state issue: +1. The bug affects KVM SVM ASID and NPT TLB translation state, which is security-sensitive virtualization MMU metadata.
* Broad/default/common subsystem exposure: +1. KVM SVM is a common virtualization path on AMD hosts.
* Requires admin/root/CAP-level control in typical deployments: -2. Reliable triggering usually requires CPU hotplug and VM scheduling or placement control.
* Hard or special timing required: -1. The scenario requires a vCPU to remain scheduled out while its previous pCPU goes through an online-offline-online hotplug cycle, followed by ASID reuse.

Paranoid additional interpretation:

* Security-boundary bypass with practical sensitive-object access: +2. Stale NPT TLB translations across VMs are an isolation-sensitive primitive and may expose or corrupt memory mappings across guest boundaries in worst-case review.
* Confidentiality impact plausible: +1. Reused translations across VMs could theoretically expose data belonging to another guest.
* Integrity impact plausible: +1. Incorrect stale translations could theoretically allow writes to the wrong guest physical backing.
* Availability impact realistic: +1. The demonstrated failure mode is a KVM internal error and VM failure path.
* Privilege penalty reduced in paranoid interpretation: 0 instead of -2. Delegated virtualization operators, reduced-capability service accounts, or orchestration components may control CPU hotplug and VM placement workflows without being full host root.

## 3. Reachability analysis

The bug is not network reachable and is not triggered by guest packet traffic. The practical trigger requires local host-side virtualization control, especially CPU hotplug behavior plus vCPU scheduling or placement conditions. In typical deployments this is administrative and therefore the conservative score is much lower.

Namespaces and containers generally do not make this reachable unless a containerized or delegated management component can control KVM workloads and CPU hotplug or equivalent host orchestration. The affected path is common on AMD KVM SVM virtualization hosts, but the exact trigger sequence is special and timing dependent.

Call-site confidence: medium. The patch and commit message give a detailed scenario involving pre_svm_run, new_asid, per-CPU svm_data, and CPU online handling, but the full caller context is not included in the provided diff.

## 4. Severity interpretation

This is not ordinary memory corruption in the classic heap UAF or OOB write sense. The directly demonstrated impact is a KVM internal error and denial of service.

However, the bug affects ASID reuse and stale NPT TLB translations across different VMs. That makes it more serious than a simple crash because it touches a core VM isolation invariant. Realistic exploitation beyond DoS is not demonstrated, but the worst-case isolation impact is credible enough for manual review and Important-candidate handling.

## 5. One-sentence report phrase

A KVM SVM ASID generation reset after CPU hotplug can allow two vCPUs from different VMs to reuse the same ASID on one pCPU, causing stale NPT TLB translations, KVM internal errors, and a worst-case VM isolation concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the observed impact is DoS, but the root condition is stale address translation state across VM boundaries. That is an isolation-sensitive MMU bug where confidentiality or integrity impact cannot be safely auto-closed without expert KVM review.

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

Patch: KVM: SVM: Bump asid_generation on CPU online to avoid ASID collision after hotplug
Commit: 60283726f2845bd78b95efbd0e50b93944780477
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=60283726f2845bd78b95efbd0e50b93944780477

Commit description:

If a vCPU stays scheduled out (or blocked) while the last pCPU it ran
on goes through a hotplug cycle (online->offline->online), and the vCPU
then resumes execution on the same pCPU, then it is possible for it to
run with an ASID that has now been assigned to a different vCPU,
resulting in stale TLB translations being used.

svm_enable_virtualization_cpu() resets asid_generation to 1 and sets
next_asid to max_asid + 1 on every CPU online event, including hotplug
cycles.  Because next_asid starts beyond the pool boundary, the first
call to new_asid() after an online event always wraps the pool,
incrementing asid_generation to 2 and assigning ASIDs starting from
min_asid.

Consider two vCPUs from different VMs, vCPU-A pinned to CPU-X holding
asid_generation=2 and ASID=N from before the hotplug event:

  1. CPU-X goes offline and back online: asid_generation resets to 1,
     next_asid = max_asid + 1.

  2. One or more vCPUs migrate to CPU-X and call new_asid(), wrapping
     the pool and consuming ASIDs starting from min_asid.  Eventually
     vCPU-B from a different VM is assigned asid_generation=2, ASID=N
     — the same ASID that vCPU-A held before the hotplug.

  3. vCPU-A enters pre_svm_run() on CPU-X: current_vmcb->cpu is
     unchanged so the migration branch is skipped.  Its saved
     asid_generation=2 matches sd->asid_generation=2, so the generation
     check silently passes and vCPU-A continues running with ASID=N —
     the same ASID just freshly assigned to vCPU-B.

Both vCPUs from different VMs now run on CPU-X with the same ASID,
causing them to share NPT TLB entries and producing stale translations.

The collision manifests as a KVM internal error (Suberror: 1, emulation
failure).  The NPT page fault reports a faulting GPA far outside the
VM's physical memory range — a sign of stale TLB translations being
used.  KVM falls back to instruction emulation, which fails on
FPU/XSave instructions (XRSTOR, STMXCSR) that the emulator does not
implement.

Fix this by incrementing asid_generation instead of resetting it to 1
in svm_enable_virtualization_cpu().  On module load, asid_generation
starts at 0 (memset) and the increment produces 1, identical to the
old behaviour.  On subsequent hotplug cycles the generation advances
beyond any value a vCPU previously observed on this CPU, so the
generation check in pre_svm_run() reliably forces new_asid() on every
vCPU after every hotplug cycle.

Fixes: 774c47f ("[PATCH] KVM: cpu hotplug support")
Reported-by: Chandrakanth Silveru <Chandrakanth.Silveru@amd.com>
Tested-by: Srikanth Aithal <Srikanth.Aithal@amd.com>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Message-ID: <20260715063506.672432-1-nikunj@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kvm/svm/svm.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=60283726f2845bd78b95efbd0e50b93944780477

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

The original Linux kernel CVE announcement for CVE-2026-68093 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68093

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:H/UI:N/S:C/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 7

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).