[CVE-2026-98540][MODERATE 7.0] fprobe: Terminate the fgraph_data list when the reservation is not filled

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-98540
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: fprobe: Terminate the fgraph_data list when the reservation is not filled
Commit: b7459fddcb27a4b7d01067678172afa362cf660d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b7459fddcb27a4b7d01067678172afa362cf660d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98540
Analysis date: Sat, 24 Oct 2026 07:41:14 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
An unterminated fprobe shadow-stack record list can reinterpret stale entries as live records and incorrectly execute privileged exit handlers twice or after an explicit skip.

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

The original Linux kernel CVE announcement for CVE-2026-98540 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98540

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-2026-98540	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98540 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 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:N/I:L/A:H';CWE-908;CWE-665;CWE-670;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.3';DESCR 'An unterminated fgraph_data list can cause fprobe_return to parse stale shadow stack words as valid headers. This may invoke an exit handler twice or invoke it even though its entry handler requested that processing be skipped. No OOB access, UAF, arbitrary write, or direct information disclosure is established by the change. However, a duplicated non-idempotent kernel callback can corrupt handler state or cause a kernel crash depending on the registered fprobe. For the CVSS the PR:L value in the paranoid vector represents a local process triggering functions covered by probes that were already installed by a privileged tracing component. Direct remote reachability is not indicated. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.3 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 manual review requirement 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_FROM_LOW_BASED_ON_REQUIREMANUALCHECK. The independent weaker Aegis shadow evaluated CVSS 0.0 and ActionableScore 1 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	TRACE SIMPLEFIX HARDWARE LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	checked

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

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

1. ActionableScore

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

2. Signal breakdown

* +2 — Specific Class B active-state corruption: an unterminated `fgraph_data` list lets `fprobe_return()` decode stale shadow-stack words as current headers and invoke a stale-record-selected exit handler.
* +1 — Practical active-consumer chain: unfilled reservation tail → stale header parsing → exit handler executes twice or despite an entry-handler skip.
* +1 — Integrity plausible: duplicate execution of a non-idempotent kernel tracing callback can mutate or corrupt handler-maintained state.
* +1 — Plausible kernel crash/strong DoS from corrupted handler state; no separate availability point was added.
* No OOB read/write, UAF, controlled reclaim, arbitrary write, attacker-selected callback pointer, disclosure, or LPE is shown.
* No local-unprivileged bonus: the supplied material does not establish a complete below-host-admin trigger.
* B5-R and B3-PENDING do not apply: there is neither a confirmed heap OOB write nor a shared-backing write.
* No authorization decision changes from DENY to ALLOW, so B4-PENDING is not applicable.

3. Reachability analysis

A privileged tracing component must first register relevant fprobes and exit handlers. A local process may subsequently invoke a probed function, but the supplied call path does not identify which functions are exposed or prove that an unprivileged actor can drive the required entry-handler outcomes and stale-tail sequence. Thus effective host-admin requirements and namespace capability scope remain unknown. Prior privileged probe installation is credible; no special hardware, remote path, or narrow timing race is shown.

4. Primitive and severity reading

This is not memory OOB or UAF. The primitive is stale structured shadow-stack data being accepted as live fprobe records and consumed by privileged kernel callback dispatch. The proven effect is incorrect duplicate or skip-violating callback execution; crash and handler-state corruption are plausible but callback-dependent. A credible local-trigger-to-privileged-consumer boundary remains unresolved, satisfying the active-state trust-boundary review rule. Therefore it remains a Class B Important candidate, but not proof of arbitrary execution, privilege escalation, or Important severity. Auto-downgrade is disallowed because the active callback-state consequence and boundary need manual resolution.

5. One-sentence report phrase

An unterminated fprobe shadow-stack record list can reinterpret stale entries as live records and incorrectly execute privileged exit handlers twice or after an explicit skip.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm which probed functions low-privilege callers can invoke and whether affected exit handlers perform non-idempotent security- or lifetime-sensitive operations.

7. Call-site confidence

Call-site confidence: medium

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

Patch: fprobe: Terminate the fgraph_data list when the reservation is not filled
Commit: b7459fddcb27a4b7d01067678172afa362cf660d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b7459fddcb27a4b7d01067678172afa362cf660d

Commit description:

fprobe_fgraph_entry() reserves shadow stack space for every fprobe with
an exit handler, but only fills it for those whose entry handler returns
0. fgraph_reserve_data() does not clear the area, so fprobe_return()
parses the unused tail as headers left over from an earlier call, and an
exit handler can run twice or despite its entry handler asking to skip
it.

Write a zero word after the last entry to terminate the walk. A zeroed
slot does not decode to a NULL fprobe on the arches that encode the
header into one unsigned long, since arch_decode_fprobe_header_fp() ORs
in FPROBE_HEADER_MSB_PATTERN, so make read_fprobe_header() return NULL
for a zeroed slot.

Link: https://lore.kernel.org/all/20260917212407.384468-1-devnexen@gmail.com/

Fixes: e0a3844 ("tracing: fprobe: do not zero out unused fgraph_data")
Cc: stable@vger.kernel.org
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: David Carlier <devnexen@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/trace/fprobe.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=b7459fddcb27a4b7d01067678172afa362cf660d

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

The original Linux kernel CVE announcement for CVE-2026-98540 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98540

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H
  The Best / paranoid CVSS score: 5.3

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: 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).