[CVE-2026-98800][MODERATE 7.0] bpf: Wait for an RCU tasks grace period before freeing trampoline progs [ Upstream

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

CVE: CVE-2026-98800
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bpf: Wait for an RCU tasks grace period before freeing trampoline progs [ Upstream
Commit: 40955f6512a03f0e046f5256b0706b26ba191018
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=40955f6512a03f0e046f5256b0706b26ba191018
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98800
Analysis date: Mon, 19 Oct 2026 03:32:25 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A preempted task can resume in a BPF trampoline and call a program after it has been freed, causing a confirmed active-object use-after-free with kernel crash potential and unresolved post-free execution leverage.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98800 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98800

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-2026-98800	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98800 CHECK WITH IMPACT FROM ORIG NN MODERATE | 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 BPF trampoline program teardown can free a program while a task preempted inside the trampoline is still able to call it. The task may be interrupted before the program enter helper acquires its RCU read lock, so the previous RCU synchronization does not protect this execution path. When the task resumes it can execute or access the freed BPF program, producing a confirmed use-after-free symptom reported by KASAN as a vmalloc-out-of-bounds read. Impact is at least a local denial of service via kernel crash. Because the freed object is a trampoline linked BPF program and may be reused before the preempted task resumes, the worst case may include confidentiality and integrity impact or privilege escalation and should be reviewed manually. For the CVSS the PR:L in the paranoid score represents environments where BPF tracing capabilities are delegated to a restricted service account, container root, or local BPF operator without full host administration. AC:H applies because successful triggering depends on a preemption race window. The issue is local and is not directly network reachable. Automated impact decision path. The original SimpleNet classifier selected MODERATE. 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 MODERATE to MODERATE_KPANIC because the CVSS score, vector or bug class 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_MODERATE_BASED_ON_GUESSCVSS. The independent weaker Aegis shadow evaluated CVSS 1.9 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	READ OOB BPF UAF HARDWARE LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
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: 5. REAL UAF FLOOR 5 applied because the stale program is called from a live trampoline execution path rather than merely passed through cleanup.
* Raw paranoid score: 3.
* Paranoid score: 5. No speculative reclaim, disclosure, or privilege-escalation points were added.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Real UAF. A task can resume in the trampoline and call `prog` after `bpf_prog_free()` has ended its lifetime.
* +1 Reliable kernel crash or strong DoS. KASAN confirms an invalid post-free access reported as a vmalloc out-of-bounds read.
* +1 Structured stale-object leverage. The stale path actively calls the freed BPF program and enters `__bpf_prog_enter_recur()`. This is broader than a cleanup-only dereference, although controlled replacement is not demonstrated.
* -1 Hard timing condition. Triggering requires preemption inside a narrow interval before the enter helper acquires its RCU read-side protection.
* Raw total: 2 + 1 + 1 - 1 = 3.
* Floor 5 applies because host-admin-equivalent privilege is not positively established and stale execution continues through an active BPF trampoline path.
* The BPF control-plane exception was checked but does not suppress the floor. The stale use occurs during live syscall trampoline execution, not solely in a destructor or cleanup helper.
* Controlled reclaim, attacker-shaped replacement contents, arbitrary kernel read or write, callback hijacking, and demonstrated LPE are not established.
* No additional availability point was awarded for the same KASAN fault.
* No confidentiality or integrity points were awarded merely from theoretical reuse.
* No repeated release or refcount signal exists. The stale operation calls the freed program but does not perform a second concrete put or refcount mutation.
* NEW52 B5-R does not apply because no retry-accumulated heap OOB write is shown.
* NEW53 B3-PENDING and the V32 to V34 checksum metadata rules do not apply.
* V33 active-state trust-boundary corruption is not independently established.
* B4 authorization-state preservation does not apply because no authorization decision changes from DENY to ALLOW.

3. Reachability analysis

The demonstrated reproducer runs as UID 0, but the supplied evidence does not prove that host UID 0 or full host-administrator powers are strictly required. Loading and attaching tracing BPF programs normally involves BPF-specific capabilities and policy controls, while detaching the program and invoking the instrumented syscall must also be available to the same coordinated trigger.

The complete trigger is not shown to work for an ordinary unprivileged user. It is also not established that all required BPF load, attach, detach, and trigger operations are available inside an attacker-created user or network namespace. Therefore both host-admin and namespace-scoped privilege remain unknown.

No network trigger or special hardware dependency is present. A preemptible kernel is required. Repeated attempts may improve reproducibility, but the decisive preemption interval remains narrow enough to retain the timing penalty.

4. Primitive and severity interpretation

The exact primitive is a real post-lifetime call of a freed `struct bpf_prog` from an active BPF trampoline. This is an active structured-object UAF rather than fixed stale storage or an ordinary cleanup dereference.

The demonstrated consequence is a KASAN-detected invalid read with realistic kernel crash or DoS potential. The patch does not establish controlled memory reuse, attacker-controlled post-free contents, arbitrary read or write, information disclosure, or privilege escalation.

The issue is CLASS_B because a concrete active-object lifetime violation exists, but no CLASS_A exploitation primitive is demonstrated. ImportantCandidate is YES because execution reaches a freed program through a live trampoline and the security usefulness of post-free reuse remains unresolved. AutoDowngradeAllowed is NO because reducing this to an ordinary crash-only defect would discard the active post-free execution primitive. This classification preserves mandatory review without claiming proven Important severity.

The patch also states that sleeping tasks inside a trampoline require a following commit. That adjacent fix should be reviewed before considering the vulnerable lifetime family fully addressed.

5. One-sentence report phrase

A preempted task can resume in a BPF trampoline and call a program after it has been freed, causing a confirmed active-object use-after-free with kernel crash potential and unresolved post-free execution leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch establishes active execution of a freed BPF program, while privilege scope, reclaim behavior, and the related sleeping-task lifetime case remain unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: bpf: Wait for an RCU tasks grace period before freeing trampoline progs [ Upstream
Commit: 40955f6512a03f0e046f5256b0706b26ba191018
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=40955f6512a03f0e046f5256b0706b26ba191018

Changed files:
  include/linux/bpf.h
  kernel/bpf/syscall.c
  kernel/bpf/trampoline.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=40955f6512a03f0e046f5256b0706b26ba191018

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

The original Linux kernel CVE announcement for CVE-2026-98800 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98800

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: 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).