[CVE-2026-98642][MODERATE 7.0] bpf: Fix UAF due to concurrent consumption of ttrace lists in alloc_bulk [ Upstream

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

CVE: CVE-2026-98642
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bpf: Fix UAF due to concurrent consumption of ttrace lists in alloc_bulk [ Upstream
Commit: a0e150a118025a05c8662c536ba2a865da86419c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a0e150a118025a05c8662c536ba2a865da86419c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98642
Analysis date: Mon, 19 Oct 2026 05:26:28 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A synchronization race in the BPF memory allocator allows `alloc_bulk()` to read a freed RCU Tasks Trace list node, producing a confirmed slab use-after-free but no demonstrated controlled corruption or privilege-escalation primitive.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98642 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98642

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-2026-98642	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98642 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (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-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in the BPF memory allocator allows alloc_bulk to consume waiting_for_gp_ttrace while an RCU Tasks Trace callback removes and frees the same list. The confirmed result is a use-after-free read of entry next in llist_del_first, which can cause a kernel crash and may propagate stale list state if the freed node is reused before the read. A related race can affect free_by_rcu_ttrace during allocator draining when IRQ work and cross-CPU frees overlap. For the CVSS the PR:L value is used because a local user or process with access to the BPF interface may create the required allocation and free activity. Systems that disable unprivileged BPF may require CAP_BPF or equivalent capabilities and therefore have lower practical exposure. AC:H is used because successful triggering depends on a concurrent timing window involving RCU callbacks and list consumption. The issue is not directly network reachable. Impact is at least a local denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact due to reuse of a freed list node and possible allocator state corruption. Privilege escalation is plausible as a worst-case consequence of the memory corruption class but is not demonstrated by the available evidence. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 4 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. 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. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	MAYBE	READ LOCK INIT BPF RACE UAF HARDWARE LINUS  KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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

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

1. ActionableScore

* Raw conservative score: 2.
* Conservative score: 4.
* Raw paranoid score: 3.
* Paranoid score: 4.
* Conservative adjustment: REAL UAF FLOOR 4 changes 2 to 4 because a freed list node is actually dereferenced and host-admin-equivalent privilege is not established.
* Paranoid adjustment: REAL UAF FLOOR 4 changes 3 to 4.
* The BPF control-plane exception was checked. It prevents an unsupported FLOOR 5 but does not suppress FLOOR 4.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF. `llist_del_first()` reads `entry->next` after `free_all()` has freed the same list node.
* +0 crash. KASAN reliably detects the stale read, but the supplied evidence does not establish a reliable production-kernel panic.
* +0 local-unprivileged reachability. Syzkaller reproduced the bug, but the supplied material does not establish that the complete BPF operation works for an ordinary unprivileged user.
* +0 race penalty. The issue is a race, but repeated Syzkaller reports do not support classifying it as a materially hard or one-shot race.
* No negative privilege penalty was applied because host-admin-equivalent privilege is not positively established.

Paranoid signals:

* +2 Real UAF.
* +1 plausible fatal fault. A stale slab read can become fatal with KASAN panic settings or related hardening, but a host crash is not demonstrated for a normal production configuration.
* +0 structured stale-object leverage. The stale `next` pointer is read, but attacker-controlled replacement contents and a successful stale update of the list head are not established.

Important non-awarded signals:

* Controlled reclaim, attacker-shaped replacement data, arbitrary read/write, disclosure, callback control, and privilege escalation are not demonstrated.
* In the documented interleaving, `llist_del_all()` changes the list head before freeing the nodes. The later `try_cmpxchg()` should therefore fail rather than install the stale `next` value. An ABA-based alternative is not established.
* No concrete repeated release or refcount operation exists. The stale path reads `entry->next`; it does not perform a second `free`, `put`, or refcount mutation on the same object.
* Concrete repeated release/refcount +1 was not awarded.
* Paranoid repeated-refcount leverage +1 was withheld.
* FLOOR 5 was not applied.
* Extra Availability was not awarded for the same possible crash.
* NEW52 B5-R does not apply because there is no confirmed retry-accumulated OOB write.
* NEW53 B3-PENDING does not apply because no shared-backing write is involved.
* V32, V33, and V34 do not apply because this is not checksum-metadata or cross-boundary packet-state corruption.
* B4-PENDING does not apply because no authorization decision changes from DENY to ALLOW.

3. Reachability analysis

The vulnerable operations occur in the BPF memory allocator during refill, RCU Tasks Trace callback processing, and allocator draining. Syzkaller can repeatedly create the required concurrency, but the supplied evidence does not identify its effective BPF privileges or establish an ordinary-user trigger.

BPF loading and related object creation may require `CAP_BPF`, `CAP_SYS_ADMIN`, or other product-specific permissions when unprivileged BPF is disabled. The complete chain is therefore not proven to require host administration, but it is also not proven to work below host-admin-equivalent privilege. `HostAdminRequired` and `NamespaceScopedPrivilege` remain `UNKNOWN`.

There is no direct network trigger, no device prerequisite, and no special hardware requirement. PREEMPT_RT is relevant to the additional theoretical `free_by_rcu_ttrace` race, but the confirmed `waiting_for_gp_ttrace` race is independently documented.

4. Primitive and severity interpretation

The exact primitive is a slab use-after-free read of a freed `llist_node` field. The affected node is part of an active allocator list rather than fixed DMA storage, but the demonstrated stale operation is shallow: it reads `entry->next` while retrying an atomic list-head removal.

This is CLASS_B because a real lifetime violation exists. It is not CLASS_A because the evidence does not establish controlled reclaim, a successful attacker-influenced list-head overwrite, arbitrary kernel access, callback influence, or another strong exploitation primitive.

This is category A, an ordinary stale dereference, rather than category B, a second lifetime/refcount mutation through a dangling object. The freeing path ends the node lifetime, but the stale path does not release or mutate the lifetime of that node again.

The demonstrated consequence is a KASAN UAF report. A kernel crash is plausible under diagnostic or hardened configurations. List corruption, information disclosure, and privilege escalation remain unresolved hypotheses without sufficient supporting mechanics.

`ImportantCandidate=NO` because meaningful exploitation leverage beyond the stale read is not established. `AutoDowngradeAllowed=YES` because this is presently a constrained CLASS_B issue without a repeated lifetime mutation, controlled reuse, boundary bypass, or demonstrated non-crash primitive. Operationally it is a borderline Moderate issue that still merits analyst review because the UAF occurs in allocator code.

5. One-sentence report phrase

A synchronization race in the BPF memory allocator allows `alloc_bulk()` to read a freed RCU Tasks Trace list node, producing a confirmed slab use-after-free but no demonstrated controlled corruption or privilege-escalation primitive.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm product-specific BPF privilege gating and whether non-KASAN execution can produce an ABA condition, return a freed object, corrupt allocator state, or reliably crash the host.

7. Call-site confidence

Call-site confidence: high

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

Patch: bpf: Fix UAF due to concurrent consumption of ttrace lists in alloc_bulk [ Upstream
Commit: a0e150a118025a05c8662c536ba2a865da86419c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a0e150a118025a05c8662c536ba2a865da86419c

Changed files:
  lib/llist.c
  kernel/bpf/memalloc.c
  kernel/rcu/tasks.h

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=a0e150a118025a05c8662c536ba2a865da86419c

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

The original Linux kernel CVE announcement for CVE-2026-98642 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98642

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: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: 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: 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).