[CVE-2026-98644][MODERATE REGULAR] bpf: Avoid soft lockup in __htab_map_lookup_and_delete_batch() [ Upstream

[CVE-2026-98644][MODERATE REGULAR] bpf: Avoid soft lockup in __htab_map_lookup_and_delete_batch() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98644
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: bpf: Avoid soft lockup in __htab_map_lookup_and_delete_batch() [ Upstream
Commit: 5c8b1869cb99a32a962388848cd19604fb41263e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5c8b1869cb99a32a962388848cd19604fb41263e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98644
Analysis date: Fri, 23 Oct 2026 21:12:57 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A BPF hash map batch operation can traverse a very large bucket array without adequate rescheduling, causing prolonged CPU soft lockups and stalling Tasks RCU users until the syscall completes.

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

The original Linux kernel CVE announcement for CVE-2026-98644 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98644

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-2026-98644	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98644 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE AEGIS_AS=1 LEGACY=MODERATE RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-400;*CWE-835;CWE-770;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A BPF hash map batch lookup can walk the complete bucket array without reaching a scheduling point because the batch count limits copied entries rather than visited buckets. Long runs of empty buckets remain inside one RCU read side and instrumentation disabled section, which can keep a CPU busy for more than 77 seconds on a sufficiently large map and trigger the soft lockup watchdog. The same behavior can delay Tasks RCU grace periods and stall ftrace or BPF trampoline teardown until the syscall returns. For the CVSS the PR:L value in the paranoid score represents a local process that already has access to a delegated BPF map file descriptor and can invoke the batch operation without full administrative control. The issue is not network reachable. Impact is denial of service through CPU starvation, soft lockup, and delayed RCU operations. No UAF, OOB access, information disclosure, or write primitive is supported by the supplied source change. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. 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. ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold 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 INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	MAYBE	LOCK DEADLOCK INIT BPF HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
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: 2
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap changed either score.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative signals:

* +1 Reliable strong DoS. A single batch operation was demonstrated to occupy a CPU for more than 77 seconds and trigger the soft-lockup watchdog.
* +1 Distinct availability effect. The missing Tasks RCU quiescent state can stall `synchronize_rcu_tasks()` users, including ftrace and BPF trampoline teardown, until the syscall returns.
* Total: 2.

Paranoid additional signal:

* +1 Local unprivileged trigger under delegation. A process that receives an already-created suitable BPF map FD can invoke the batch operation without necessarily possessing the privileges originally used to create the map.
* Total: 3.

Not awarded:

* No UAF, OOB access, heap overwrite, stale-object use, or refcount corruption is established.
* No confidentiality, integrity, authorization-bypass, or privilege-escalation primitive is established.
* No remote or namespace-complete trigger is demonstrated.
* No additional crash point is awarded for repeatability of the same CPU stall.
* NEW52 B5-R does not apply because there is no retry-accumulated OOB write or subsequent consumption of corrupted structured state.
* V32, V33, V34, B3-PENDING, and B4-PENDING do not apply.

3. Reachability analysis

The direct caller must possess a suitable BPF map FD and invoke `BPF_MAP_LOOKUP_BATCH` or the corresponding lookup-and-delete operation. Creation of a 16,777,216-bucket LRU hash map may require BPF privileges, sufficient memory limits, and prior privileged provisioning, but the supplied evidence does not resolve the exact product policy.

A delegated map FD gives a credible below-host-admin trigger model, but delegation is not established for every affected deployment. The complete trigger is not shown to be achievable through attacker-created user or network namespaces. No special hardware is required, although the demonstrated severe behavior used a large ARM64 system with `CONFIG_PREEMPT_NONE`. This is deterministic long traversal rather than a timing race.

4. Primitive and severity interpretation

The exact primitive is an attacker-invoked, insufficiently preemptible traversal of a very large BPF hash table. Long empty-bucket runs remain inside an RCU read-side and instrumentation-disabled section, producing CPU starvation and delaying Tasks RCU progress.

This is `CLASS_C` DoS behavior. It is neither active-state corruption nor a lifetime violation. The demonstrated consequences are soft lockup and delayed kernel teardown operations. Kernel memory corruption, useful stale-state consumption, protected-state modification, and privilege escalation are unsupported.

`ImportantCandidate=NO` because no Important-preserving corruption or security-boundary primitive exists. `AutoDowngradeAllowed=YES` because the established impact remains availability-only, although deployment-specific BPF FD delegation warrants review.

5. One-sentence report phrase

A BPF hash map batch operation can traverse a very large bucket array without adequate rescheduling, causing prolonged CPU soft lockups and stalling Tasks RCU users until the syscall completes.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Verify whether untrusted processes can receive suitable BPF map FDs and whether affected hosts permit sufficiently large maps, since those deployment facts determine practical DoS exposure.

7. Call-site confidence

Call-site confidence: high

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

Patch: bpf: Avoid soft lockup in __htab_map_lookup_and_delete_batch() [ Upstream
Commit: 5c8b1869cb99a32a962388848cd19604fb41263e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5c8b1869cb99a32a962388848cd19604fb41263e

Changed files:
  kernel/bpf/hashtab.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=5c8b1869cb99a32a962388848cd19604fb41263e

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

The original Linux kernel CVE announcement for CVE-2026-98644 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98644

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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2
  Paranoid ActionableScore: 3

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).