[CVE-2026-98861][MODERATE 7.0] fprobe: Use guard(rcu_sched_notrace) and check rcu_is_watching() [ Upstream

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

CVE: CVE-2026-98861
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: fprobe: Use guard(rcu_sched_notrace) and check rcu_is_watching() [ Upstream
Commit: a85d0f00671b4d4527317c10ba018680f7d8d673
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a85d0f00671b4d4527317c10ba018680f7d8d673
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98861
Analysis date: Mon, 19 Oct 2026 03:35:45 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A race between fprobe handler execution and unregistration can free an active fprobe while RCU is not watching, causing a local use after free with kernel crash potential and unresolved exploitation leverage in the BPF kprobe-multi path.

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

The original Linux kernel CVE announcement for CVE-2026-98861 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98861

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
======================================================================

CVE-2026-98861	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98861 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 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-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in fprobe unregistration can cause a use after free because standard RCU grace periods do not wait for an in flight handler that runs while RCU is not watching. An fprobe object may therefore be freed after unregister_fprobe or unregister_fprobe_async while a handler executing in an idle loop or nohz_full state still references it. The demonstrated and most likely consequence is temporal memory corruption followed by a kernel crash. A controlled read or write primitive is not established, but the UAF class leaves a defensible worst case risk of privilege escalation and confidentiality or integrity impact that requires manual review. For the CVSS the PR:L value in the paranoid score reflects a restricted tracing or BPF capable local process that can manage multi kprobes without full system administration rights. Typical systems restrict these operations to root or processes with CAP_BPF, CAP_PERFMON, or CAP_SYS_ADMIN, so the realistic base score uses PR:H. The race requires a narrow timing condition and handler execution while RCU is not watching, so AC:H is appropriate. The issue is local and is not 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 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 2.0 and ActionableScore 4 with primitive class CLASS_B 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	OOB LOCK TRACE BPF UAF IMPROVEONLY LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	checked

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

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

1. ActionableScore

* Raw conservative score: 3.
* Conservative score: 4.
* Raw paranoid score: 5.
* Paranoid score: 5.
* Conservative floor applied: 3 -> 4 under the real UAF floor because the stale fprobe is actively consumed and host-admin-equivalent privilege is not positively established.
* No paranoid floor or BPF cap changed the score.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF. `unregister_fprobe()` or `unregister_fprobe_async()` can complete an ordinary RCU grace period and free the fprobe while a handler executing with RCU not watching still uses it.
* +1 Strong DoS. Continued traversal and handler processing through the stale fprobe can realistically produce an Oops or kernel crash.
* Raw conservative total: 3.
* Floor 4 applies because this is active handler execution rather than fixed stale storage or a shallow cleanup-only dereference.

Additional paranoid signals:

* +1 Attacker-influenced lifetime manipulation. A BPF kprobe-multi management process can plausibly register a probe, exercise selected probe points, and initiate link teardown or unregistration repeatedly. The exact capability boundary remains unresolved.
* +1 Structured stale-object leverage. The stale `struct fprobe` is obtained from the lookup table and passed into active handler processing, where handler-related state is consumed. This is meaningfully deeper than an immediate stale-pointer fault, although controlled replacement is not demonstrated.
* Raw paranoid total: 5.

Not awarded:

* No remote-reachability point. This is a local tracing and BPF control path.
* No local-unprivileged point. Complete access below host-admin privilege is not established.
* No privilege penalty. Host-admin-equivalent privilege is also not positively established, so uncertainty cannot justify `-2`.
* No race penalty. The required RCU-not-watching state is specific, but the evidence does not establish that repeated attempts are prohibitively narrow or externally uncontrollable.
* No controlled-reclaim or arbitrary-write points.
* No confidentiality or integrity points. The UAF does not establish disclosure, directed writes, credential modification, or privilege escalation.
* No repeated-release/refcount point. The patch establishes premature free followed by stale use, not two concrete lifetime mutations through the same reference.
* No extra availability point. The possible kernel crash is counted once.
* NEW52 B5-R does not apply because no retry-accumulated heap OOB write exists.
* NEW53 B3-PENDING does not apply because no shared-backing write exists.
* NEW54 B4-PENDING does not apply because no authorization decision is bypassed.

3. Reachability analysis

The direct kernel API is used by fprobe clients, and the commit explicitly identifies BPF kprobe-multi as one user. A capable local BPF or tracing process can plausibly create probes and initiate unregistration by destroying or detaching the associated link.

The supplied evidence does not resolve whether the complete BPF load, attach, trigger, and detach sequence requires host-admin-equivalent powers or can be performed by a process with delegated `CAP_BPF`, `CAP_PERFMON`, or another restricted tracing permission. Therefore `HostAdminRequired=UNKNOWN`.

No evidence establishes that every required stage is available through attacker-created user or network namespaces, so `NamespaceScopedPrivilege=UNKNOWN`. No special physical hardware is required. Triggering does require an in-flight handler to execute while RCU is not watching, such as during idle or `nohz_full`, concurrently with unregistration. Repeated probe execution and teardown may improve race reliability, but reliable attacker control of that timing is not demonstrated.

4. Primitive and severity interpretation

The exact primitive is an RCU lifetime violation. Standard RCU can consider a grace period complete while a handler running in an extended quiescent state remains active. The fprobe can then be freed before that handler finishes using it.

This is an active structured-object UAF, not fixed stale storage. The stale use occurs in normal fprobe entry, graph-entry, or return processing and can reach handler processing. It is not merely a destructor reading a stale field.

There is no concrete repeated release or refcount mutation. There is also no demonstrated controlled reclaim, attacker-shaped replacement object, arbitrary kernel read or write, or callback hijack. Consequently the primitive remains `CLASS_B`, not `CLASS_A`.

The demonstrated mechanism supports temporal memory corruption and a realistic kernel crash. Privilege escalation is an unresolved worst-case possibility rather than a demonstrated consequence. `ImportantCandidate=YES` and `AutoDowngradeAllowed=NO` are based on the active callback-oriented stale-object use and unresolved BPF lifecycle control, not on UAF classification alone.

Operationally, this is at least an actionable Moderate issue. Important severity is not proven, but automatic downgrade would discard a concrete active-object lifetime violation that requires call-site and privilege-boundary review.

5. One-sentence report phrase

A race between fprobe handler execution and unregistration can free an active fprobe while RCU is not watching, causing a local use after free with kernel crash potential and unresolved exploitation leverage in the BPF kprobe-multi path.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The UAF is concrete and reaches active handler processing, while the complete BPF privilege boundary, stale-field consumption, reclaim opportunities, and practical impact beyond a crash remain unresolved.

7. Call-site confidence

Call-site confidence: medium

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

Patch: fprobe: Use guard(rcu_sched_notrace) and check rcu_is_watching() [ Upstream
Commit: a85d0f00671b4d4527317c10ba018680f7d8d673
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a85d0f00671b4d4527317c10ba018680f7d8d673

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=a85d0f00671b4d4527317c10ba018680f7d8d673

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

The original Linux kernel CVE announcement for CVE-2026-98861 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98861

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: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).