[CVE-2026-98787][MODERATE REGULAR] bpf: Fix missing migration protection in __rhtab_map_lookup_and_delete_batch() [ Upstream

[CVE-2026-98787][MODERATE REGULAR] bpf: Fix missing migration protection in __rhtab_map_lookup_and_delete_batch() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98787
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: bpf: Fix missing migration protection in __rhtab_map_lookup_and_delete_batch() [ Upstream
Commit: 94f41ec5fa578765b74ad5736c2f029e7aabc46d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=94f41ec5fa578765b74ad5736c2f029e7aabc46d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98787
Analysis date: Mon, 19 Oct 2026 06:18:42 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
Missing migration protection in the BPF resizable hash table batch deletion path allows this_cpu_ptr() to access per CPU memory cache state without guaranteed CPU stability, producing a kernel BUG diagnostic while actual memory corruption or privilege escalation remains unproven.

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

The original Linux kernel CVE announcement for CVE-2026-98787 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98787

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: NO

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-2026-98787	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98787 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE AEGIS_AS=0 LEGACY=MODERATE RESULT=SAME V39_AEGIS_ONE_STEP_UP=LOW->MODERATE (final reconciliation agrees; DECISION: both MODERATE)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;CWE-667;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.8';DESCR 'A race in __rhtab_map_lookup_and_delete_batch() allows the delete loop to call bpf_mem_cache_free_rcu() without disabling task migration. On CONFIG_PREEMPT_RCU systems, rcu_read_lock() does not prevent migration, so this_cpu_ptr() can select per CPU memory cache state without the required CPU stability and trigger a BUG diagnostic. Under adverse timing, the task may also access per CPU cache state while its owning CPU modifies the same state, creating a plausible but not yet proven memory corruption risk. For the CVSS the PR:L value is used because a local process must operate on a suitable BPF map or possess a usable map file descriptor, although initial map creation may require CAP_BPF or stronger privileges on hardened systems. The race requires a migration window and therefore AC:H is appropriate. The issue is not network reachable. The demonstrated impact is a kernel warning or availability degradation, while the paranoid assessment allows limited confidentiality and integrity impact and high availability impact because unsafe per CPU allocator access requires manual review. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.8 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage LOW to MODERATE because KP specialist did not establish a qualifying panic and 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 KPANIC_UNSUPPORTED_REMOVED and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 3.1 and ActionableScore 0 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	BPF SIMPLEFIX HARDWARE LINUS SYZBOT KPANIC_UNSUPPORTED_REMOVED V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=1
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: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* No floor or cap changed either score.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Weak or indirect corruption candidate. Missing migration protection permits `this_cpu_ptr()` to select per-CPU BPF memory-cache state without guaranteed CPU stability. This establishes a synchronization violation, but not actual cache corruption.
* +0 WARN or BUG diagnostic. The demonstrated result is `BUG: using smp_processor_id() in preemptible code`. This diagnostic alone does not establish a fatal kernel crash.
* +0 Reliable DoS. Panic, Oops, or host-wide failure is not demonstrated.
* +0 Real UAF. No object is shown to be used after its lifetime ends.
* +0 Strong or constrained overwrite. No OOB write, arbitrary write, stale write, or corrupted descriptor consumption is established.
* +0 Confidentiality or integrity impact. Concurrent access to the wrong per-CPU cache remains a concern, but the supplied evidence does not show corrupted state, disclosure, or unauthorized modification.
* +0 Local unprivileged trigger. A BPF map FD is required, but the complete below-host-admin creation and operation path is not established.
* +0 Privilege penalty. Host-admin-equivalent privilege is also not positively established, so no speculative penalty is applied.
* +0 Race penalty. Migration timing is required, but the evidence does not establish that the interleaving is narrow, one-shot, or difficult to repeat.
* +0 B5-R. There is no confirmed retry-accumulated heap OOB write or later consumption of corrupted structured state.
* +0 V33 active-state candidate. Active per-CPU state is involved, but attacker-influenced corruption and a concrete trust boundary are not established.
* +0 B3-PENDING. No shared-backing or file-backed write exists.
* +0 B4-PENDING. No authorization decision changes from DENY to ALLOW.

3. Reachability analysis

The vulnerable operation is the delete phase of `BPF_MAP_LOOKUP_AND_DELETE_BATCH` for a resizable hash-table map. A local process must possess a suitable BPF map FD and invoke the batch operation with deletion enabled.

The supplied material does not establish whether an ordinary user can create the required map in the evaluated product. BPF creation may be restricted by `CAP_BPF`, `CAP_SYS_ADMIN`, or unprivileged-BPF policy, while an already created map FD may potentially be delegated to a less-privileged process. Therefore `HostAdminRequired` and `NamespaceScopedPrivilege` remain `UNKNOWN`.

`CONFIG_PREEMPT_RCU` is relevant because `rcu_read_lock()` does not disable migration in that configuration. Task migration must occur during the deletion path. Syzbot demonstrates that the path is reachable, but the supplied evidence does not quantify reliability or show that migration results in more than the diagnostic.

There is no network trigger, special hardware requirement, authorization bypass, or demonstrated namespace-to-host boundary crossing.

4. Primitive and severity interpretation

The exact primitive is missing migration exclusion around a call chain that reaches `bpf_mem_cache_free_rcu()` and uses `this_cpu_ptr()`. A task can migrate while the deletion loop is active, violating the CPU-stability requirement for per-CPU cache access.

This is active cache state rather than stale storage, but the evidence establishes only unsafe CPU selection and a preemption diagnostic. It does not establish that cache metadata is actually corrupted or subsequently consumed in a dangerous state.

There is no first lifetime-ending operation, surviving dangling reference, second refcount mutation, UAF, repeated release, OOB access, controlled reclaim, or arbitrary read or write. The issue is therefore `CLASS_C`, not `CLASS_B`.

The demonstrated consequence is a kernel BUG diagnostic. A crash or per-CPU allocator-state corruption is an unresolved possibility, not a patch-supported consequence suitable for additional score. `ImportantCandidate=NO` because no independent corruption or boundary primitive is established. `AutoDowngradeAllowed=YES` because the available evidence remains diagnostic or availability-oriented without meaningful exploitation leverage.

5. One-sentence report phrase

Missing migration protection in the BPF resizable hash table batch deletion path allows this_cpu_ptr() to access per CPU memory cache state without guaranteed CPU stability, producing a kernel BUG diagnostic while actual memory corruption or privilege escalation remains unproven.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review `bpf_mem_cache_free_rcu()` and its per-CPU cache operations to determine whether migration can cause concurrent cache-state corruption rather than only the demonstrated diagnostic.

7. Call-site confidence

Call-site confidence: medium

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

Patch: bpf: Fix missing migration protection in __rhtab_map_lookup_and_delete_batch() [ Upstream
Commit: 94f41ec5fa578765b74ad5736c2f029e7aabc46d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=94f41ec5fa578765b74ad5736c2f029e7aabc46d

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=94f41ec5fa578765b74ad5736c2f029e7aabc46d

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

The original Linux kernel CVE announcement for CVE-2026-98787 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98787

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 1

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).