* [CVE-2026-89988][MODERATE REGULAR] kprobes: Protect kprobe_blacklist with RCU
@ 2026-09-16 13:59 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-16 13:59 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89988
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: kprobes: Protect kprobe_blacklist with RCU
Commit: 8f406205003f741225f620f0e357ca0de71d45e3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8f406205003f741225f620f0e357ca0de71d45e3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89988
Analysis date: Wed, 16 Sep 2026 09:59:20 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89988 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89988
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-2026-89988 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89988 CHECK WITH IMPACT FROM ORIG NN IMPORTANT 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:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in kprobes can cause use after free because __within_kprobe_blacklist() traverses kprobe_blacklist without a lifetime guarantee while module unload can remove blacklist entries and free them with kfree(). A concurrent blacklist check may dereference a freed kprobe_blacklist_entry during probe validation or address checks, which can crash the kernel and may be security relevant because the stale object is part of a kernel linked list traversal. For the CVSS the PR:L is used in the paranoid score because some environments delegate tracing, perf, BPF, or reduced admin capabilities to non root service accounts, even though the typical base scenario needs higher privileges for kprobe control or module unload. The issue is local only and is not network reachable. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel use after free, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) YES OOB UAF LINUS DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6 DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::
No floor or cap changed the score.
2. Signal breakdown
Positive signals:
* +2 Real UAF / lifetime violation: `__within_kprobe_blacklist()` can traverse `kprobe_blacklist` while module unload removes entries and frees them with `kfree()`.
* +1 Reliable kernel crash / strong DoS: a concurrent reader may dereference a freed `kprobe_blacklist_entry`, plausibly causing an Oops, KASAN report, or kernel crash.
Negative signals:
* No -2 host-admin penalty was applied because the full attacker model is not completely resolved from the supplied evidence. Module unload is host-admin-like, but the reader side may be reached through kprobe validation paths and the complete one-principal trigger is not fully shown.
* No -1 race penalty was applied. The bug is a race, but the evidence does not prove a narrow, one-shot, externally uncontrollable timing window.
Non-awarded signals:
* Controlled reclaim is not demonstrated.
* Arbitrary write is not demonstrated.
* Confidentiality and integrity impact are not awarded because the stale path is shown as list traversal and address range dereference, not a concrete disclosure or write primitive.
* Structured stale-object leverage is not awarded because there is no shown attacker-controlled replacement object or useful post-reuse field consumption beyond stale traversal.
* Concrete repeated release/refcount is not awarded. There is no second `put`, `release`, `free`, or refcount mutation through the exact same dangling reference.
* Extra availability is not awarded because the same crash is already counted once.
* Remote reachability is not established.
Repeated release/refcount checklist:
* First release can end lifetime: yes, `kfree(ent)` ends the blacklist entry lifetime.
* Exact same reference survives to a second lifetime mutation: no evidence.
* Second operation is a real lifetime/refcount mutation: no.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.
3. Reachability analysis
The writer side is module unload, where `kprobe_remove_area_blacklist()` removes blacklist entries for the unloaded module. Module unload normally requires host-admin-equivalent privilege such as `CAP_SYS_MODULE`, but the supplied patch does not fully establish whether a lower-privileged actor can independently and reliably combine module unload with a concurrent blacklist check.
The reader side is `within_kprobe_blacklist()` / `__within_kprobe_blacklist()`, used during kprobe address validation. Kprobe registration and tracing control are typically privileged, but delegation to tracing or observability services can exist in real deployments. This supports manual review, but not a confirmed below-host-admin trigger.
Namespace-scoped privilege is not established. Kprobes and module unloading are not ordinary network-namespace-scoped operations. No direct network trigger is present. No special hardware requirement is present. The race appears repeatable by repeatedly loading/unloading modules and causing blacklist checks, but repeatability alone does not establish exploitation leverage beyond crash.
4. Primitive and severity interpretation
The exact primitive is a real RCU/lifetime UAF: a global list entry may be deleted and freed while an unlocked reader still traverses and dereferences it.
This is an active structured object rather than fixed stale DMA storage, but the stale object is small and the demonstrated stale use is limited to list traversal plus `start_addr` / `end_addr` comparisons. There is no demonstrated controlled reclaim, callback use, arbitrary read/write, refcount takeover, or security-boundary bypass.
This is Class B because a real kernel lifetime violation exists. It is not Class A because no strong exploitation primitive is established. It is not Class C because the patch directly establishes post-free dereference, not just an invalid state or warning.
Demonstrated consequence: local kernel crash / DoS.
Speculative consequence: stronger exploitation from heap reuse is theoretically possible for UAFs, but not supported enough here to score C/I or Important-grade leverage.
ImportantCandidate=NO because the available evidence supports a constrained UAF with crash impact and unresolved privilege reachability, but not meaningful exploitation leverage beyond stale traversal.
AutoDowngradeAllowed=YES because no controlled corruption, repeated refcount mutation, authorization bypass, COW violation, or other unresolved Important-preserving primitive is established.
Likely operational severity: borderline / Moderate-like, but worth manual review because it is a real UAF in core kprobes infrastructure.
5. One-sentence report phrase
A race in kprobes blacklist handling can allow `__within_kprobe_blacklist()` to dereference a freed blacklist entry during concurrent module unload, causing a local kernel crash with no demonstrated remote reachability or controlled privilege escalation primitive.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Reason: real kernel UAF exists, but the supplied evidence shows mainly crash-oriented stale traversal and does not establish controlled reclaim, arbitrary write, C/I impact, or a complete below-host-admin trigger.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: kprobes: Protect kprobe_blacklist with RCU
Commit: 8f406205003f741225f620f0e357ca0de71d45e3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8f406205003f741225f620f0e357ca0de71d45e3
Commit description:
__within_kprobe_blacklist() traverses kprobe_blacklist without holding
kprobe_mutex. When a module is unloaded, kprobe_remove_area_blacklist()
removes blacklist entries and immediately frees them with kfree().
A concurrent call to within_kprobe_blacklist() can therefore dereference
freed memory.
Furthermore, within_kprobe_blacklist() can be called in atomic or
non-preemptible contexts where the sleeping kprobe_mutex cannot be taken.
Protect kprobe_blacklist with RCU. Use guard(rcu)() and
list_for_each_entry_rcu() for traversal, list_add_tail_rcu() for
insertions, list_del_rcu() for deletions, and kfree_rcu() to reclaim
entries safely after a grace period.
Link: https://lore.kernel.org/all/178810004323.64882.16493230858653316962.stgit@devnote2/
Fixes: 376e242 ("kprobes: Introduce NOKPROBE_SYMBOL() macro to maintain kprobes blacklist")
Cc: [email protected]
Reported-by: Sashiko <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Assisted-by: Antigravity:gemini-3.7-flash
Signed-off-by: Masami Hiramatsu (Google) <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
include/linux/kprobes.h
kernel/kprobes.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=8f406205003f741225f620f0e357ca0de71d45e3
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89988 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89988
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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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