[CVE-2026-98390][LOW] x86/amd_node: Avoid divide by zero on virtualized systems

[CVE-2026-98390][LOW] x86/amd_node: Avoid divide by zero on virtualized systems

[AL-KERNEL]

CVE: CVE-2026-98390
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: x86/amd_node: Avoid divide by zero on virtualized systems
Commit: 9273a90465d58b82aca4bea71451ef6bee084928
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9273a90465d58b82aca4bea71451ef6bee084928
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98390
Analysis date: Wed, 14 Oct 2026 22:57:23 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98390 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98390

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-2026-98390	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98390 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT LOW AEGIS_AS=0 LEGACY=LOW RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both LOW)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';CWE-369;CWE-682;CWE-754;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.4';DESCR 'A divide by zero can occur during x86 AMD node initialization because num_roots may be smaller than num_nodes on a virtualized system, producing roots_per_node equal to zero. A later modulo operation using roots_per_node can raise a fatal exception and prevent the affected kernel from completing initialization. Xen PVH dom0 is a confirmed configuration, although other virtualized platforms with a similar topology may also be affected. For the CVSS the PR:H reflects that reliable triggering requires control over the virtual hardware or NUMA topology, which normally belongs to a hypervisor administrator rather than an ordinary user inside the affected system. The issue is not network reachable. Impact is denial of service through a boot time kernel crash, with no supported memory corruption, information disclosure, or privilege escalation primitive. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.4 and the production ActionableScore returned 0 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE to LOW because the ActionableScore threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3. The independent weaker Aegis shadow evaluated CVSS 4.5 and ActionableScore 0 with primitive class CLASS_C and manual review NO. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)	MAYBE	DANGER UAF KERNEL_PANIC_PLUS_UAF HARDWARE  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=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: -1
* Conservative score: 0
* Raw paranoid score: -1
* Paranoid score: 0
* Applied score lower bound: -1 -> 0
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel crash. `roots_per_node` becomes zero when `num_roots < num_nodes`, and the subsequent modulo operation causes a divide-by-zero exception during kernel initialization.
* -2 Proven host-admin-equivalent prerequisite. Deliberately controlling the virtual hardware or NUMA topology presented to Xen PVH dom0 requires control of the hypervisor or equivalent host configuration.
* No additional rare-configuration penalty was applied because the Xen topology prerequisite overlaps substantially with the host-administrator restriction.
* No memory-corruption signal. The primitive is an arithmetic exception, not an OOB access, UAF, overwrite, or stale-object operation.
* No extra Availability point. The boot failure and kernel crash are the same consequence.
* No confidentiality, integrity, authorization, ownership, or security-boundary signal.
* B5-R does not apply because there is no heap OOB write, retry accumulation, or later consumption of corrupted state.
* B3-PENDING and B4-PENDING do not apply because there is no shared-backing write or authorization decision.

3. Reachability analysis

The confirmed affected configuration is Xen PVH dom0 with `num_nodes` equal to 2 and `num_roots` equal to 1. The failure occurs in `amd_smn_init()` during kernel initialization and is deterministic once that topology is present.

No ordinary local-user, namespace-capability, device ioctl, or network trigger is established. The complete operation is not namespace-scoped. An intentional attacker would need effective control over the hypervisor or host topology configuration. The evidence does not establish that numeric host UID 0 specifically is mandatory, because equivalent authority could be exercised through a privileged virtualization management service.

There is no race or retry requirement. Other virtualization environments might be affected, but no additional attacker-accessible path is established.

4. Primitive and severity interpretation

The exact primitive is integer division followed by modulo using a zero divisor. It causes a synchronous exception and boot-time denial of service.

This is `CLASS_C` crash behavior. It is neither active structured-object corruption nor fixed stale storage, and no repeated release or refcount operation exists. There is no leverage beyond preventing initialization of the affected kernel.

`ImportantCandidate=NO` because no memory corruption, authorization bypass, cross-boundary effect, or below-host-admin trigger is supported. `AutoDowngradeAllowed=YES` because the demonstrated result is a configuration-dependent crash with no unresolved exploitation primitive.

The likely operational severity is Low-like to ordinary Moderate, despite CVSS availability scoring, because the party able to intentionally construct the triggering topology already controls the affected host virtualization environment.

5. One-sentence report phrase

A virtualized AMD system can calculate roots_per_node as zero and crash during kernel initialization when the value is later used as a modulo divisor, but intentional triggering requires control over the host virtual topology and exposes no memory corruption or privilege escalation primitive.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a deterministic divide-by-zero DoS with a host-administrator-controlled prerequisite and no unresolved corruption or security-boundary primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: x86/amd_node: Avoid divide by zero on virtualized systems
Commit: 9273a90465d58b82aca4bea71451ef6bee084928
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9273a90465d58b82aca4bea71451ef6bee084928

Commit description:

On a virtualized system, the number of nodes does not have a relationship to
the number of roots.  A Xen PVH dom0 can calculate roots_per_node as 0, which
crashes with a divide by zero in:

  if (count++ % roots_per_node)

because the underlying topology code on Xen ends up making num_nodes
2 and num_roots 1 and the integer division result is 0.

The issue is seen with Xen, but it could affect other systems.

Set roots_per_node to 1 in this case. Print a firmware bug when this is
performed for non-virtualized systems.

  [ bp: Massage commit message. ]

Fixes: 0a4b61d ("x86/amd_node: Fix AMD root device caching")
Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Yazen Ghannam <yazen.ghannam@amd.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260825214805.39148-2-jason.andryuk@amd.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kernel/amd_node.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=9273a90465d58b82aca4bea71451ef6bee084928

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

The original Linux kernel CVE announcement for CVE-2026-98390 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98390

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:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.4

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: 0

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).