[CVE-2026-98728][MODERATE REGULAR] tpm: fix off-by-four bounds check in tpm2_get_random() [ Upstream

[CVE-2026-98728][MODERATE REGULAR] tpm: fix off-by-four bounds check in tpm2_get_random() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98728
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: tpm: fix off-by-four bounds check in tpm2_get_random() [ Upstream
Commit: be54101992c2c5218be0e0a87827ccc96e3089af
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=be54101992c2c5218be0e0a87827ccc96e3089af
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98728
Analysis date: Sat, 24 Oct 2026 03:31:39 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate
Manual review required: YES

Summary:
An off-by-four TPM response check can copy up to four stale bytes into returned random data, creating a limited OOB read and unresolved entropy-integrity risk.

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

The original Linux kernel CVE announcement for CVE-2026-98728 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98728

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 REGULAR
Manual review required: YES

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

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

CVE-2026-98728	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98728 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: 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:L/I:L/A:N';CWE-125;CWE-193;CWE-345;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '3.6';DESCR 'An off by four bounds check in tpm2_get_random can accept a truncated TPM2_ST_SESSIONS response and make memcpy read up to four bytes past the logical response end. The stale bytes become part of the random output, creating a limited OOB read and an RNG integrity problem rather than a demonstrated kernel crash or arbitrary memory write. For the CVSS the PR:H value reflects that reliable triggering normally requires control of the TPM, vTPM, emulator, transport, or another privileged response source. For the CVSS the PR:L paranoid value covers deployments where a less privileged service or delegated virtual TPM backend can influence responses. The issue is not directly network reachable. Confidentiality and integrity are rated Low because stale buffer contents can replace requested entropy, but compromised keys or direct disclosure of specific secrets are not proven. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 3.6 and the production ActionableScore returned 4 with recommendation Actionable Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage IMPORTANT to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. KP and FD specialist checks stage MODERATE_KPANIC to MODERATE because KP specialist did not establish a qualifying panic. 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 LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 3.0 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 4)	MAYBE	OOB SIMPLEFIX IMPROVEONLY LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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

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

1. ActionableScore

* Raw base score: 2.
* Conservative score: 4. V30 successful-but-invalid entropy-output safety floor applied: 2 -> 4.
* Raw paranoid score: 2.
* Paranoid score: 4. No additional supported exploit or reachability signal.
* Final recommended bucket: **Actionable Moderate**::

2. Signal breakdown

* +1 limited OOB read: the sessions offset is four bytes beyond `TPM_HEADER_SIZE`, allowing `memcpy()` to read up to four bytes past the logical TPM response.
* +1 entropy-integrity primitive: those stale bytes are returned as TPM random output rather than causing a guaranteed rejection.
* V30 floor to 4: a kernel RNG source can report successful output containing invalid/stale entropy, requiring manual security review.
* No confidentiality point: stale bytes reach the kernel caller, but no userspace disclosure or identifiable secret is shown.
* No integrity point: weak keys, predictable Linux CRNG state, or modification of protected state is not demonstrated.
* No crash/availability point, write primitive, UAF, controlled target, or arbitrary read.
* NEW52/B5-R does not apply: this is a bounded read, not a retry-accumulated heap OOB write.
* NEW53 does not apply: no shared-backing write exists.
* B4 does not apply: no authorization decision or omitted security guard is involved.

3. Reachability analysis

The malformed length and size values originate in a TPM response. Triggering therefore requires influence over a physical TPM, vTPM/emulator, transport, or equivalent response source. The supplied evidence does not establish whether a normal user, delegated backend, or only a host administrator can provide such responses; accordingly, host-admin and UID-0 requirements remain unknown. No namespace-scoped complete trigger, direct network path, special retry sequence, or timing race is shown.

4. Primitive and severity reading

The exact primitive is a bounded, up-to-four-byte OOB read whose stale contents contaminate RNG output. It is CLASS_B for security-relevant entropy integrity, not memory corruption. The patch supports invalid random output reaching a caller, but not compromised keys, Linux CRNG predictability, protected-state corruption, or disclosure to an attacker. ImportantCandidate is NO because no vulnerable cryptographic consumer or compromise chain is identified. Auto-downgrade remains disallowed under V30 because successful invalid entropy output requires human assessment.

5. One-sentence report phrase

An off-by-four TPM response check can copy up to four stale bytes into returned random data, creating a limited OOB read and unresolved entropy-integrity risk.

6. Manual review advice

MANUAL CHECK REQUIRED

Determine who can control TPM/vTPM responses and whether affected output directly feeds security-sensitive key generation or entropy consumers.

7. Call-site confidence

Call-site confidence: medium

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

Patch: tpm: fix off-by-four bounds check in tpm2_get_random() [ Upstream
Commit: be54101992c2c5218be0e0a87827ccc96e3089af
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=be54101992c2c5218be0e0a87827ccc96e3089af

Changed files:
  drivers/char/tpm/tpm2-cmd.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=be54101992c2c5218be0e0a87827ccc96e3089af

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

The original Linux kernel CVE announcement for CVE-2026-98728 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98728

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:L/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N
  The Best / paranoid CVSS score: 3.6

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: 4

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 REGULAR

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).