[CVE-2026-99099][MODERATE 7.0] x86/CPU/AMD: Fix AMD TLBI Erratum #1718

[CVE-2026-99099][MODERATE 7.0] x86/CPU/AMD: Fix AMD TLBI Erratum #1718

[AL-KERNEL]

CVE: CVE-2026-99099
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: x86/CPU/AMD: Fix AMD TLBI Erratum #1718
Commit: e3e6779f77ab6f0a1aa80e24868d2d89f31053fe
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e3e6779f77ab6f0a1aa80e24868d2d89f31053fe
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99099
Analysis date: Mon, 26 Oct 2026 23:00:32 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
Affected AMD Zen 5 CPUs may retain and consume a stale TLB translation after kernel invalidation, creating an unresolved low-privilege memory-access and crash primitive.

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

The original Linux kernel CVE announcement for CVE-2026-99099 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99099

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-2026-99099	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99099 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-672;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'On affected AMD Zen 5 processors, an INVLPGB and TLBSYNC sequence may complete without invalidating the targeted TLB entry. This can leave a stale translation after kernel page table updates and may permit access through an obsolete mapping if the underlying page is unmapped and later reused. The resulting primitive can plausibly cause a kernel crash and may permit confidentiality or integrity impact depending on page reuse, although reliable privilege escalation is not established. For the CVSS the PR:L value is used because reliable triggering requires a local user or process to drive memory mapping and unmapping activity. AC:H reflects dependence on affected hardware, missing fixed microcode, and a difficult processor state or timing condition. The issue is not directly network reachable. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the ActionableScore threshold rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4. The independent weaker Aegis shadow evaluated CVSS 0.0 and ActionableScore 0 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	SIMPLEFIX HARDWARE LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 4.
* Conservative score: 4; no floor or cap applied.
* Raw paranoid score: 5.
* Paranoid score: 5; no floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +1 local low-privilege trigger: supplied triage evidence identifies local mapping/unmapping activity with PR:L; host-admin power is not required.
* +1 active translation-state violation: INVLPGB/TLBSYNC can report completion while leaving the targeted TLB entry operational.
* +1 active translation synchronization failure: the extra TLBSYNC is specifically required to ensure invalidation, proving stale state can survive the original synchronization sequence.
* +1 plausible kernel crash/DoS: later use of an obsolete translation after unmap/reuse can access an invalid or repurposed page.
* -1 difficult hardware/state condition: affected Zen 5 hardware, absent fixed microcode, and difficult processor timing/state are positively identified.
* Paranoid +1 weak exploit leverage: continued CPU consumption of an obsolete translation can affect memory access after page reuse. This is not arbitrary read/write or proven disclosure.
* No C/I points: no protected reused page, read-back, directed write, or cross-owner modification is demonstrated.
* No CLASS_A leverage: no controlled page reuse, selected target, LPE, or arbitrary kernel access is shown.
* No extra availability point: it would duplicate the same possible crash.

3. Reachability analysis

The assessed actor is a below-host-admin local process driving mapping and unmapping activity. No UID 0, capability, namespace, prior privileged setup, or special device assignment is identified. The patch confirms affected Zen 5 CPUs without corrective microcode; reliable triggering remains hardware/state dependent. It is not remotely reachable.

4. Primitive and severity reading

The primitive is a hardware-caused failure to retire an active TLB translation despite the kernel issuing its invalidation and synchronization protocol. The stale entry may subsequently be consumed by CPU memory accesses after page-table removal and possible physical-page reuse. This is CLASS_B active translation-state corruption, not a UAF.

A crash and obsolete-mapping access are plausible; controlled reuse, protected-resource access, confidentiality, integrity, and privilege escalation remain unproven. ImportantCandidate is YES because active stale translation state is consumed across the user/kernel trust boundary with unresolved post-reuse effects. Auto-downgrade is unsafe despite the absence of a completed exploit.

5. One-sentence report phrase

Affected AMD Zen 5 CPUs may retain and consume a stale TLB translation after kernel invalidation, creating an unresolved low-privilege memory-access and crash primitive.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm practical low-privilege triggering, stale-entry lifetime, page-reuse control, and whether accesses can reach host-protected pages.

7. Call-site confidence

Call-site confidence: medium

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

Patch: x86/CPU/AMD: Fix AMD TLBI Erratum #1718
Commit: e3e6779f77ab6f0a1aa80e24868d2d89f31053fe
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e3e6779f77ab6f0a1aa80e24868d2d89f31053fe

Commit description:

The processor may fail to invalidate a TLB entry while executing an
INVLPGB/TLBSYNC instruction pair.

Detect the erratum and apply a software workaround when a microcode fix
is not present.

Fixes: 82378c6 ("x86/mm: Use INVLPGB for kernel TLB flushes")
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Rik van Riel <riel@surriel.com>
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Tested-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Tested-by: Rik van Riel <riel@surriel.com>
Cc: stable@kernel.org
Link: https://patch.msgid.link/20261002211617.1001617-1-bp@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/include/asm/cpufeatures.h
  arch/x86/include/asm/tlb.h
  arch/x86/kernel/cpu/amd.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=e3e6779f77ab6f0a1aa80e24868d2d89f31053fe

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

The original Linux kernel CVE announcement for CVE-2026-99099 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99099

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