* [CVE-2026-99167][MODERATE 7.0] net/sched: fix use-after-free in __tcf_action_put()
@ 2026-10-29 2:45 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-29 2:45 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99167
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net/sched: fix use-after-free in __tcf_action_put()
Commit: 2465d27ef90de9105c2bffb6c35f8ce08918e642
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2465d27ef90de9105c2bffb6c35f8ce08918e642
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99167
Analysis date: Wed, 28 Oct 2026 22:45:13 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
A race in traffic-control action release permits a confirmed four-byte slab UAF write after the action is freed, with crash impact shown and replacement-object corruption potential unresolved.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99167 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99167
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-99167 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-99167 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=3 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-667;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 __tcf_action_put can decrement tcfa_refcnt before tcfa_bindcnt lacking one lock that covers both operations, allowing another thread to remove and free the traffic control action before the delayed atomic_dec writes to tcfa_bindcnt. KASAN confirms a four byte slab use after free write, which can crash the kernel and could corrupt a replacement slab object if the freed allocation is reused. For the CVSS the PR:L value reflects that reliable triggering requires local control of traffic control action binding and concurrent release, normally through NETLINK_ROUTE with CAP_NET_ADMIN or delegated network namespace privileges. The issue is not directly reachable through ordinary packet traffic. Base impact uses CIA:LLH while the paranoid same class assessment uses CIA:HHH because a confirmed kernel heap write after free provides a plausible but unproven privilege escalation path. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were 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 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP. The independent weaker Aegis shadow evaluated CVSS 2.0 and ActionableScore 3 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 5) MAYBE REMOTE LOCK LEAK UAF NETWORK KPANIC DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 5.
* Conservative score: 5; no floor or cap changed the score.
* Raw paranoid score: 5.
* Paranoid score: 5; no extra point for theoretical slab reuse or LPE.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2 — Confirmed real UAF: one thread can free `p` after the reference decrement but before another thread executes `atomic_dec(&p->tcfa_bindcnt)`.
* +1 — Concrete post-free write leverage: KASAN confirms a four-byte write into freed slab storage. This is stronger than a read-only stale dereference, but the fixed decrement is not an arbitrary write.
* +1 — Plausible kernel crash/strong DoS from the slab UAF write.
* +1 — Attacker-influenced lifetime sequence: concurrent traffic-control action binding and release operations drive both the final-reference transition and stale write.
* No race penalty: concurrency is required, but the evidence does not establish a narrow, one-shot, or externally uncontrollable race.
* No privilege penalty: CAP_NET_ADMIN or delegated netns control is suggested, but the complete governing capability scope is not proven.
* Controlled reclaim, attacker-selected payload/target, callback control, arbitrary write, disclosure, and LPE are not shown.
* No extra availability point; it would duplicate the same crash.
* No C/I points merely because reused slab storage could theoretically be corrupted.
3. Reachability analysis
The trigger is local traffic-control action management, not ordinary packet reception. The supplied summary grounds concurrent bind/release control through `NETLINK_ROUTE`, generally involving CAP_NET_ADMIN or delegated network-management rights. It does not establish whether every required operation is authorized in an attacker-created user/net namespace or instead requires host-level control. Therefore both host-admin and complete namespace-scoped reachability remain unknown. No special hardware or prior boot configuration is indicated.
4. Primitive and severity reading
This is an active structured-object UAF with a confirmed write: the reference count may reach zero and cleanup may free the `tc_action`, after which another thread decrements `tcfa_bindcnt` through the dangling pointer. It is not a repeated release of the same reference; the second operation mutates a separate binding counter.
The primitive is CLASS_B because the patch proves an uncontrolled post-free four-byte decrement but not controlled reclaim, directed corruption, or a useful replacement object. Crash is supported; corruption of a subsequently allocated slab object is credible but not demonstrated. The concrete post-free write and unresolved delegated/namespace trust boundary require Important-versus-Moderate review, so automatic downgrade is unsafe.
5. One-sentence report phrase
A race in traffic-control action release permits a confirmed four-byte slab UAF write after the action is freed, with crash impact shown and replacement-object corruption potential unresolved.
6. Manual review advice
MANUAL CHECK REQUIRED
The confirmed post-free write is a material Class B primitive whose exploit leverage and effective CAP_NET_ADMIN boundary remain unresolved.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: net/sched: fix use-after-free in __tcf_action_put()
Commit: 2465d27ef90de9105c2bffb6c35f8ce08918e642
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2465d27ef90de9105c2bffb6c35f8ce08918e642
Commit description:
__tcf_action_put() decrements the reference count before decrementing the
binding count. Another thread can free the action between these operations,
causing the binding-count update to access freed memory. KASAN reports it
as:
BUG: KASAN: slab-use-after-free in __tcf_action_put+0x327/0x340
Write of size 4 at addr ff11000000d46420 by task tc-put-pause/64
Hold the action IDR mutex across both counter updates and IDR removal,
keeping cleanup outside the lock.
Fixes: 16af606 ("net: sched: implement reference counted action release")
Cc: [email protected]
Assisted-by: LLM
Signed-off-by: Jérémy Jean <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
net/sched/act_api.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=2465d27ef90de9105c2bffb6c35f8ce08918e642
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99167 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99167
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: unknown
Paranoid ActionableScore: 5
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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