[CVE-2026-64572][MODERATE 7.0] ipv4: fib: free fib_alias with kfree_rcu() on insert error path [ Upstream

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

CVE: CVE-2026-64572
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipv4: fib: free fib_alias with kfree_rcu() on insert error path [ Upstream
Commit: 8150b5365f026e72250cacc527ea00be30f40105
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8150b5365f026e72250cacc527ea00be30f40105
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64572
Analysis date: Wed, 05 Aug 2026 05:29:57 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An RCU lifetime bug in IPv4 FIB insertion error handling can free a `fib_alias` before concurrent `fib_table_lookup()` readers finish, causing a local race-triggered use-after-free and potential kernel crash, with manual review recommended due to the core networking UAF context.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64572 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64572

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-2026-64572	MODERATE	CHECK WITH IMPACT FROM ORIG NN 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:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'fib_table_insert publishes a new fib_alias into an RCU protected alias list before FIB notifiers complete. If a notifier rejects the route, the error path removes the alias with hlist_del_rcu but used to free it immediately, while fib_table_lookup can still read the same entry under rcu_read_lock. This is an RCU use after free in the IPv4 FIB lookup path and can be reached by racing route insertion failure with local route lookups such as UDP connect. For the CVSS the PR:L is used for the paranoid score because reliable triggering needs local code plus CAP_NET_ADMIN or delegated network administration, but this capability is often available to container root, service accounts, or constrained network namespaces rather than only full host root. The issue is not directly network reachable because an attacker must manipulate routing state locally, although the vulnerable lookup path is exercised by normal IPv4 traffic generation. Impact is at least local denial of service via kernel crash and worst case may allow confidentiality or integrity impact due to use after free and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE READ DANGER INIT ERRORPATH UAF NETWORK HARDWARE KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak primitive: +1. This is a real RCU use-after-free, but the patch shows stale read access from `fib_table_lookup()` rather than controlled reclaim, write-after-free, callback control, or type confusion.
* Real lifetime corruption: +1. `fib_alias` is removed with `hlist_del_rcu()` but was freed immediately with `kmem_cache_free()`, violating RCU lifetime rules.
* Reliable kernel crash / strong DoS: +1. KASAN shows slab-use-after-free in the IPv4 FIB lookup path, reachable through `udp_connect()`.
* Common networking core path: +1. The stale object is consumed by IPv4 FIB lookup, a central routing path.
* Hard or unreliable race / special timing required: -1. Exploitation requires racing route insertion failure with concurrent lookup.
* Requires CAP_NET_ADMIN in typical deployments: -1. Full -2 is softened because CAP_NET_ADMIN can be delegated to containers, network namespaces, service accounts, or test environments, and is not always equivalent to full host root.

Paranoid delta:

* Paranoid manual-review uplift: +1. This is a kernel RCU UAF in a core IPv4 routing object, and the affected object is read in a hot networking path. No strong LPE primitive is demonstrated, but the class and subsystem justify not auto-closing.

## 3. Reachability analysis

The bug requires a local actor able to trigger the route insertion error path. The commit states that this needs CAP_NET_ADMIN and a registered FIB notifier that can reject a route, with netdevsim IPv4 FIB resource exhaustion being sufficient. Concurrent lookup can be triggered through normal local socket operations such as UDP `connect()`.

This is not directly network reachable. Incoming packets alone do not create the vulnerable insertion failure path. However, once the bad alias is briefly published, the lookup side is in the normal IPv4 routing path.

Namespaces and containers matter. In a default host setup this is privileged because route management requires CAP_NET_ADMIN. In delegated network namespaces or containerized environments, CAP_NET_ADMIN may be available to a reduced-privilege actor, so PR:L is a defensible paranoid interpretation.

Call-site confidence: high. The commit provides the failing call trace and the patch directly changes the free path after `fib_remove_alias()`.

## 4. Severity interpretation

This behaves like an actionable Moderate issue, not an ordinary low-risk crash. The demonstrated primitive is an RCU use-after-free with a KASAN report in a core IPv4 lookup path.

Realistic exploitation evidence supports local DoS. Theoretical memory-corruption concern exists because this is a freed slab object read under RCU, but the patch does not show attacker-controlled reclaim, write-after-free, type confusion, callback dispatch, or arbitrary read/write. Therefore the conservative score should not treat it as a strong LPE primitive.

Manual review is still warranted because kernel UAFs in central networking objects are historically easy to underestimate, especially when capability delegation lowers the practical privilege barrier.

## 5. One-sentence report phrase

An RCU lifetime bug in IPv4 FIB insertion error handling can free a `fib_alias` before concurrent `fib_table_lookup()` readers finish, causing a local race-triggered use-after-free and potential kernel crash, with manual review recommended due to the core networking UAF context.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a confirmed RCU use-after-free in a core IPv4 routing lookup object, not a pure NULL dereference or resource leak. The likely practical impact is local DoS, but CAP_NET_ADMIN delegation and the kernel UAF class justify manual triage rather than auto-closure.

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UPSTREAM PATCH SUMMARY
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Patch: ipv4: fib: free fib_alias with kfree_rcu() on insert error path [ Upstream
Commit: 8150b5365f026e72250cacc527ea00be30f40105
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8150b5365f026e72250cacc527ea00be30f40105

Changed files:
  net/ipv4/fib_trie.c
  net/ipv4/route.c
  net/ipv4/datagram.c
  net/ipv4/udp.c
  net/socket.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=8150b5365f026e72250cacc527ea00be30f40105

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

The original Linux kernel CVE announcement for CVE-2026-64572 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64572

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: 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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).