[CVE-2026-98788][MODERATE 7.0] futex: Fix private hash use-after-free on resize [ Upstream

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

CVE: CVE-2026-98788
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: futex: Fix private hash use-after-free on resize [ Upstream
Commit: 5d5dc18b3a9ac011619b7d3af4514bf9e9b6a6af
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5d5dc18b3a9ac011619b7d3af4514bf9e9b6a6af
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98788
Analysis date: Mon, 19 Oct 2026 00:53:41 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
Incorrect RCU grace period ordering during private futex hash resizing can free an active hash while an unprivileged local process still has a reader using it, creating a kernel use-after-free with crash potential and unresolved exploitation leverage.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98788 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98788

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-2026-98788	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98788 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-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in private futex hash resizing can cause a use-after-free because the recorded RCU grace period may start before the new hash pointer is published. futex_ref_drop() may then conclude that the old hash is no longer used and allow it to be freed while a concurrent reader still holds a pointer to it. A local process using private futex operations may be able to trigger the vulnerable resize and race without administrative capabilities. For the CVSS the PR:L reflects that local user code execution is required but elevated system privileges are not normally required. AC:H applies because successful triggering depends on an RCU timing window during hash replacement and reference release. Impact is at least a local denial of service through a kernel crash. In the worst defensible case the confirmed use-after-free may permit broader memory corruption and privilege escalation if the released hash memory can be reused in a controllable way. The issue is not directly network reachable. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 6 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_KPANIC to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC because KP specialist did not establish a qualifying panic. 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_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP. The independent weaker Aegis shadow evaluated CVSS 1.3 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 6)	MAYBE	DANGER UAF HARDWARE LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Applied conservative floor: 4 -> 5 under the active structured UAF floor. The stale hash remains involved in nontrivial futex lookup processing, and private futex operations are available below host-admin privilege.
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative positive signals:

* +2 Real UAF. Incorrect ordering allows the old `futex_private_hash` to be freed after an incorrectly selected RCU grace period while a reader still retains its pointer.
* +1 Local unprivileged trigger. Private futex operations operate on the calling process address space and do not normally require capabilities or UID 0.
* +1 Realistic kernel crash or strong DoS. Continued lookup through a freed hash can dereference released bucket or hash state and fault the kernel.

Paranoid additional signals:

* +1 Attacker-influenced repeatable lifetime manipulation. A process can repeatedly create private futex activity and drive hash resizing, although the exact resize threshold and repeatability are not visible in the supplied diff.
* +1 Structured stale-object leverage. The stale object is an active hash used for lookup and traversal, not passive metadata. Futex addresses and hash population are process-influenced, so post-free consumption has unresolved leverage beyond a fixed stale-buffer fault.

No negative points:

* No race penalty. The bug requires an RCU ordering window, but the supplied evidence does not establish a one-shot, externally uncontrollable, or unusually improbable race.
* No privilege penalty. Ordinary private futex use is available without host-admin-equivalent privileges.
* No niche subsystem or hardware penalty. Futexes are a common kernel synchronization interface and require no special hardware.

Important non-awarded signals:

* Controlled reclaim or attacker-shaped replacement contents are not demonstrated.
* Arbitrary or directed kernel read or write is not demonstrated.
* Confidentiality, integrity, and privilege escalation are not independently scored.
* Remote reachability is not established.
* No concrete repeated release or refcount mutation exists. `kvfree_rcu()` can end the old hash lifetime, and the reader reference survives, but the later operation is a stale hash use rather than a second `put`, `free`, or refcount mutation.
* Concrete repeated release/refcount +1 was not awarded.
* Paranoid repeated-refcount leverage +1 was not awarded.
* Weak LPE was not separately awarded.
* Extra Availability was not awarded because the same possible kernel crash is already counted once.
* B5-R does not apply because no retry-accumulated OOB write is established.
* B3-PENDING does not apply because no shared-backing write is involved.
* B4-PENDING does not apply because this is not an authorization decision defect.

3. Reachability analysis

An ordinary local process can use private futex operations on memory in its own `mm_struct`. No host UID 0 identity, host capability, namespace capability, prior privileged setup, special device, boot option, or unusual hardware is indicated.

The resize is performed internally as the private futex hash changes size. Userspace can influence the futex population and workload, but the supplied context does not show the exact resize policy or how reliably one process can force repeated pivot operations. This limits confidence in trigger reliability without changing the below-host-admin attacker model.

Namespaces are not required for the trigger, and no namespace-scoped privilege is involved. The vulnerability is not remotely reachable through ordinary network traffic. The RCU interleaving is timing-dependent, but repeated futex workload may provide multiple attempts, so a hard-race deduction is unsupported.

4. Primitive and severity interpretation

The exact primitive is premature reclamation of an active `futex_private_hash`. `mmph->batches` could identify a grace period that started before the new hash pointer was published. `futex_ref_drop()` could therefore advance reclamation while a preexisting reader still held the old hash pointer.

This is an active structured-object UAF rather than fixed stale storage. The stale use occurs in normal futex reader processing, not only in cleanup or a destructor. The first lifetime-ending operation is `kvfree_rcu()` on the old hash. A reader can retain the exact old pointer across that retirement, but no second lifetime or refcount mutation on the hash is shown.

The patch supports CLASS_B because a concrete RCU lifetime violation and post-lifetime use are established. A crash or kernel fault is a realistic demonstrated class of consequence. Controlled reclaim, arbitrary access, and local privilege escalation remain unresolved rather than proven.

ImportantCandidate is YES because an unprivileged process can reach an active structured kernel-object UAF whose stale lookup and traversal behavior requires exploitation-leverage review. AutoDowngradeAllowed is NO because reducing it automatically to an ordinary crash-only race would discard the unresolved active-object corruption surface. The evidence does not support CLASS_A or a score of 7 or higher.

5. One-sentence report phrase

Incorrect RCU grace period ordering during private futex hash resizing can free an active hash while an unprivileged local process still has a reader using it, creating a kernel use-after-free with crash potential and unresolved exploitation leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The issue is a concrete below-host-admin active structured UAF, but controlled reclaim, exact stale-field consumption, and practical privilege-escalation potential require inspection of `futex_ref_drop()` and the private hash reader and resize call paths.

7. Call-site confidence

Call-site confidence: medium

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

Patch: futex: Fix private hash use-after-free on resize [ Upstream
Commit: 5d5dc18b3a9ac011619b7d3af4514bf9e9b6a6af
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5d5dc18b3a9ac011619b7d3af4514bf9e9b6a6af

Changed files:
  kernel/futex/core.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=5d5dc18b3a9ac011619b7d3af4514bf9e9b6a6af

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

The original Linux kernel CVE announcement for CVE-2026-98788 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98788

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:L/I:L/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: 5
  Paranoid ActionableScore: 6

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).