[CVE-2026-74289][MODERATE 7.0] ipv4: fib: Don't dump dying fib_info in fib_leaf_notify(). [ Upstream

[CVE-2026-74289][MODERATE 7.0] ipv4: fib: Don't dump dying fib_info in fib_leaf_notify(). [ Upstream

[AL-KERNEL]

CVE: CVE-2026-74289
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipv4: fib: Don't dump dying fib_info in fib_leaf_notify(). [ Upstream
Commit: 676482da8d938ea72c26da0fc86af2d2ec238ab2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=676482da8d938ea72c26da0fc86af2d2ec238ab2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74289
Analysis date: Sat, 15 Aug 2026 11:15:49 -0400
ActionableScore: 7
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
IPv4 FIB notification can expose a dying `fib_info` object to notifier callbacks under RCU, allowing a zero-refcount object to be referenced again and causing a UAF-style lifetime bug that can crash the kernel and warrants manual review for possible escalation impact.

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

The original Linux kernel CVE announcement for CVE-2026-74289 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74289

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
======================================================================

CVE-2026-74289	MODERATE	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-911;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'IPv4 FIB notification can expose a dying fib_info object to notifier callbacks while the table is being dumped under RCU. A notifier such as netdevsim, mlxsw, or rocker may then call fib_info_hold with a plain refcount_inc after the reference count already reached zero, which can resurrect an object that is being freed and create a use after free condition. For the CVSS the PR:L is used because reliable triggering requires local ability to exercise network control plane paths such as network namespace cleanup, devlink or notifier registration, or route lifetime manipulation, but it does not necessarily require full host root in all namespace or delegated administration setups. The issue is not directly network reachable through packet traffic. Impact is at least a local denial of service via kernel warning, crash, or corrupted lifetime state and in the paranoid case may allow confidentiality and integrity impact due to the UAF class. YES REQUIRES MANUAL CHECK.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	REMOTE DANGER UAF KERNEL_PANIC_PLUS_UAF HARDWARE IPV6 SYZBOT KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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

ActionableScore=7
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: **4**
* Paranoid score: **7**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* **Memory corruption, strong corruption primitive: +2**. The report explicitly shows `refcount_t: addition on 0` and describes a use-after-free of `fib_info`.
* **Real lifetime corruption: +1**. The bug is a refcount lifetime issue where `fib_info_hold()` can increment a dying object under RCU.
* **Reliable kernel crash / strong DoS: +1**. syzbot reached a refcount warning on the UAF path, and this can become a kernel crash especially with `panic_on_warn`.
* **Common networking core/control-plane path: +1**. IPv4 FIB notifier replay is part of core routing infrastructure, although the demonstrated path involves notifier users such as `netdevsim`, `mlxsw`, or `rocker`.
* **Hard or unreliable race / special timing required: -1**. The issue depends on an RCU/lifetime race during FIB dump and notifier registration or teardown.
* **Requires CAP_NET_ADMIN or device-management privileges in typical deployments: -2**. Reliable triggering usually needs route, netns, devlink, or simulated-device control.

Conservative total: **4**

Paranoid additions/changes:

* **Local low-privileged or delegated trigger: +1**. In namespace or delegated network-administration environments, the attacker may not be full host root while still reaching relevant network control-plane paths.
* **Weak LPE concern: +1**. The UAF/refcount resurrection affects a kernel object passed into notifier callbacks. No controlled reclaim or arbitrary write is shown, but the primitive is stronger than a pure NULL dereference.
* **Broad/default/common subsystem exposure: +1**. IPv4 FIB is broadly present, even if specific notifier consumers vary by deployment.

Paranoid total: **7**

## 3. Reachability analysis

This is not directly packet-triggerable and should not be treated as remote network exploitation. The realistic trigger is local control-plane activity involving IPv4 FIB notifier replay, network namespace cleanup, devlink reload, route lifetime changes, or notifier registration by drivers such as `netdevsim`, `mlxsw`, or `rocker`.

In typical deployments, this requires CAP_NET_ADMIN, devlink privileges, or device-management control. However, containers, user namespaces, delegated network administration, or test/dev environments can reduce the practical privilege barrier. The demonstrated syzbot path uses `netdevsim`, which is not a normal production exposure, but the fixed bug is in common IPv4 FIB notification code rather than only in the simulator.

Call-site confidence: **high**, because the trace and patch show the relevant call path through `fib_leaf_notify()`, `call_fib_entry_notifier()`, and `nsim_fib4_prepare_event()`.

## 4. Severity interpretation

This is more than an ordinary Moderate correctness bug because the patch fixes a real object lifetime violation: a dying `fib_info` can be exposed to notifier callbacks, and those callbacks can increment a zero refcount. The demonstrated impact is warning/crash/DoS, but the primitive is a UAF/refcount lifetime bug in kernel networking state.

Realistic exploitation evidence for privilege escalation is not demonstrated. There is no shown attacker-controlled reclaim, arbitrary write, type confusion, or controlled callback target. Therefore the conservative score remains borderline/manual-review level. The paranoid score is higher because kernel UAF plus refcount resurrection in a notifier path is historically worth manual review and should not be auto-closed.

## 5. One-sentence report phrase

IPv4 FIB notification can expose a dying `fib_info` object to notifier callbacks under RCU, allowing a zero-refcount object to be referenced again and causing a UAF-style lifetime bug that can crash the kernel and warrants manual review for possible escalation impact.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: the patch and syzbot trace explicitly indicate UAF/refcount lifetime corruption, not just a benign warning or resource leak. No direct LPE primitive is proven, but this class should be reviewed manually before closure.

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

Patch: ipv4: fib: Don't dump dying fib_info in fib_leaf_notify(). [ Upstream
Commit: 676482da8d938ea72c26da0fc86af2d2ec238ab2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=676482da8d938ea72c26da0fc86af2d2ec238ab2

Changed files:
  lib/refcount.c
  include/linux/refcount.h
  include/net/ip_fib.h
  drivers/net/netdevsim/fib.c
  net/core/fib_notifier.c
  net/ipv4/fib_trie.c
  drivers/net/netdevsim/dev.c
  net/devlink/dev.c
  net/devlink/core.c
  net/core/net_namespace.c
  kernel/workqueue.c
  kernel/kthread.c
  arch/x86/kernel/process.c
  arch/x86/entry/entry_64.S

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=676482da8d938ea72c26da0fc86af2d2ec238ab2

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

The original Linux kernel CVE announcement for CVE-2026-74289 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74289

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: 4
  Paranoid ActionableScore: 7

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).