[CVE-2026-74583][MODERATE 7.0] net/sched: cls_route: fix fastmap use-after-free on filter [ Upstream

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

CVE: CVE-2026-74583
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net/sched: cls_route: fix fastmap use-after-free on filter [ Upstream
Commit: 7897198b26445b4009a057bda1986b94a99e5d5f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7897198b26445b4009a057bda1986b94a99e5d5f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74583
Analysis date: Fri, 21 Aug 2026 13:08:10 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in cls_route fastmap handling can republish a route4_filter after deletion and RCU deferred free, allowing later packet classification to read a stale freed object and trigger a KASAN confirmed kernel use after free.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74583 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74583

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-2026-74583	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:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in the cls_route fastmap cache can store a stale route4_filter pointer after the writer has already reset the cache and scheduled the filter for RCU deferred free. A later packet with the same id and iif tuple can hit that stale fastmap entry and read f res from freed memory, producing a real use-after-free condition confirmed by KASAN reports. For the CVSS the PR:L is used for the paranoid score because reduced capability root, container root, or a service account with delegated network admin may be able to modify tc filters without full host administration. The issue is not directly network reachable because reliable triggering requires local control plane changes to cls_route filters, although packet traffic is used to exercise the stale cache entry. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to a real UAF primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE LOCK RACE UAF NETWORK TEST KPANIC PACKET 	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 race UAF primitive: +1. The patch describes a real stale `route4_filter` pointer surviving into fastmap and later being read after RCU free.
* Real lifetime corruption: +1. This is an RCU lifetime bug with a stale pointer after deferred free.
* Reliable kernel crash / strong DoS: +1. KASAN slab use after free reports were reproduced in the route4 fastmap paths.
* Weak LPE concern: +1. The primitive is a real UAF with stale object reuse, but no controlled reclaim, write after free, callback control, or refcount takeover is shown.
* Common packet processing path: +1. Once cls_route is configured, normal classified packets exercise the stale fastmap entry.
* Requires CAP_NET_ADMIN in typical deployments: -2. Adding, changing, or deleting tc route filters normally requires network administration privilege.
* Hard race / timing required: -1. The bug depends on concurrent classification and filter delete/change, and the reproducer used stress plus an artificial delay.

Paranoid adjustment:

* Local unprivileged or reduced privilege trigger: +1. In deployments with user namespaces, container root, delegated CAP_NET_ADMIN, or network management service accounts, the practical privilege level can be lower than full host root.
* The paranoid score remains capped at 5 because this is a race UAF without demonstrated attacker controlled reclaim, arbitrary write, callback dispatch, or type confusion.

## 3. Reachability analysis

The bug is not directly remote reachable. Packet traffic is needed to hit the stale fastmap entry, but reliable triggering also requires local control plane changes to cls_route filters through tc/netlink. In typical host deployments this requires CAP_NET_ADMIN, so the conservative interpretation treats it as privileged local. In containerized or namespace enabled environments, CAP_NET_ADMIN may be delegated to a reduced privilege root or service account, which supports the paranoid lower privilege interpretation. The affected subsystem is not always configured by default, but traffic control and net/sched are common enough that this should not be treated as a rare hardware-only issue.

Call-site confidence: high. The commit message describes the reader and writer paths, the RCU free sequence, the stale fastmap republish, and KASAN-confirmed UAF behavior.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is a confirmed kernel UAF in a packet processing path. Realistic evidence supports local DoS and manual review. Theoretical LPE is plausible enough to avoid auto-close, but the patch does not show controlled reclaim, attacker-chosen replacement object, write after free, type confusion, or callback hijack. Therefore it fits Actionable Moderate at minimum rather than a clear Strong Important candidate.

## 5. One-sentence report phrase

A race in cls_route fastmap handling can republish a route4_filter after deletion and RCU deferred free, allowing later packet classification to read a stale freed object and trigger a KASAN confirmed kernel use after free.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: confirmed UAF, RCU lifetime bug, packet processing involvement, and possible reduced privilege CAP_NET_ADMIN exposure make this unsuitable for automatic closure even though practical LPE is not demonstrated.

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

Patch: net/sched: cls_route: fix fastmap use-after-free on filter [ Upstream
Commit: 7897198b26445b4009a057bda1986b94a99e5d5f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7897198b26445b4009a057bda1986b94a99e5d5f

Changed files:
  net/sched/cls_route.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=7897198b26445b4009a057bda1986b94a99e5d5f

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

The original Linux kernel CVE announcement for CVE-2026-74583 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74583

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: 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).