[CVE-2026-74657][MODERATE 7.0] ipv4: Fix fib_nlmsg_size() for RTA_VIA nexthops

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

CVE: CVE-2026-74657
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv4: Fix fib_nlmsg_size() for RTA_VIA nexthops
Commit: 0f0ca602941d0a81ae9514943ca06c55159c6385
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0f0ca602941d0a81ae9514943ca06c55159c6385
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74657
Analysis date: Sat, 22 Aug 2026 14:52:44 -0400
ActionableScore: 5
ActionableScore lower bound: 1
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
An IPv4 route notification size miscalculation for `RTA_VIA` IPv6 nexthops can cause `fib_dump_info()` to fail and hit a `WARN_ON()` in `rtmsg_fib()`, which becomes a local kernel panic DoS when `panic_on_warn` is enabled, especially if CAP_NET_ADMIN is delegated through namespaces.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74657 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74657

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: NO

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-2026-74657	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-131;CWE-617;**CWE-400;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'fib_nlmsg_size can underestimate the skb size needed for IPv4 route notifications when an IPv4 route uses an IPv6 nexthop gateway encoded as RTA_VIA instead of an IPv4 RTA_GATEWAY attribute. The undersized notification buffer can make fib_dump_info fail with EMSGSIZE and can reach a WARN_ON in rtmsg_fib. With panic_on_warn enabled, this becomes a kernel panic and local denial of service. For the CVSS the PR:L is used in the paranoid score because reliable triggering may be possible for a local actor with delegated CAP_NET_ADMIN in a network namespace or container like environment. The issue is not directly network reachable because it is triggered through route control plane updates rather than packet traffic. Impact is denial of service only and there is no evidence of UAF, OOB access, information disclosure, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 3)	YES	REMOTE DANGER NETWORK SKB IPV6  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* +1 Memory layout invariant restoration. The patch restores consistency between the skb size estimator `fib_nlmsg_size()` and the real netlink dump layout used by `fib_nexthop_info()`, including `RTA_VIA`, multipath layout, encapsulation, and optional `RTA_FLOW`.
* +1 WARN_ON / panic_on_warn DoS concern. The commit explicitly states that the undersized skb can make `fib_dump_info()` fail with `-EMSGSIZE`, reaching `WARN_ON()` in `rtmsg_fib()`, and that `panic_on_warn` turns this into a kernel panic.
* +1 Broad/common subsystem. IPv4 routing and rtnetlink route notification logic are common kernel networking control-plane paths.
* -2 Requires CAP_NET_ADMIN in typical deployments. Reliable triggering normally requires creating or modifying routes with IPv4 routes using IPv6 nexthops.

Conservative total: 1

Paranoid additions / changes:

* +1 Local unprivileged trigger in some configurations. If unprivileged user namespaces and network namespaces are enabled, a local user may obtain CAP_NET_ADMIN inside a delegated netns and exercise the route-control path.
* +1 Availability impact realistic. Under `panic_on_warn`, the WARN becomes a host kernel panic, which is system-wide and stronger than a mere failed notification.
* No CAP_NET_ADMIN penalty in the paranoid namespace interpretation, because the attacker may not have full host-root privileges.

Paranoid total: 5

Not awarded:

* No remote/network-triggerable bonus. Packet traffic alone does not create the malformed route state.
* No memory corruption bonus. The patch fixes size accounting and a WARN path, not UAF, OOB write, double-free, stale pointer reuse, or type confusion.
* No confidentiality or integrity impact. The visible impact is route notification failure and possible kernel panic.

## 3. Reachability analysis

The bug is triggered through IPv4 route control-plane updates involving nexthops where an IPv4 route uses an IPv6 gateway encoded as `RTA_VIA`. In normal host deployments this requires CAP_NET_ADMIN, so the conservative view treats it as privileged local access.

Namespaces can change the practical risk. If unprivileged users can create user and network namespaces, they may gain CAP_NET_ADMIN inside their netns and create routes that still exercise host kernel code. That makes PR effectively lower in some products, but it is configuration-dependent.

The path is not remotely reachable through ordinary packet processing. It is also not a memory corruption path based on the supplied patch. The main dangerous condition is `panic_on_warn`, where a user-triggerable WARN becomes a full kernel panic.

Call-site confidence: high. The commit explains the call chain `fib_nlmsg_size()` -> skb allocation -> `fib_dump_info()` -> `rtmsg_fib()` WARN_ON, and the patch directly updates the estimator.

## 4. Severity interpretation

This behaves like an actionable Moderate rather than Important. The issue is a local control-plane DoS condition with a strong panic outcome only when `panic_on_warn` is enabled. There is no demonstrated memory corruption, privilege escalation, information leak, or unauthorized data modification.

The conservative risk is low because typical triggering requires CAP_NET_ADMIN and non-default panic behavior for full crash impact. The paranoid risk is higher because namespace delegation can make the route update path reachable by less-privileged local users, and panic_on_warn converts the WARN into a reliable system-wide crash.

## 5. One-sentence report phrase

An IPv4 route notification size miscalculation for `RTA_VIA` IPv6 nexthops can cause `fib_dump_info()` to fail and hit a `WARN_ON()` in `rtmsg_fib()`, which becomes a local kernel panic DoS when `panic_on_warn` is enabled, especially if CAP_NET_ADMIN is delegated through namespaces.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is DoS-only with no visible memory corruption, but it should not be auto-closed because `panic_on_warn` can convert it into a full kernel panic and namespace CAP_NET_ADMIN delegation may lower the practical privilege requirement.

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

Patch: ipv4: Fix fib_nlmsg_size() for RTA_VIA nexthops
Commit: 0f0ca602941d0a81ae9514943ca06c55159c6385
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0f0ca602941d0a81ae9514943ca06c55159c6385

Commit description:

fib_nlmsg_size() still estimates nexthop space as if every gateway is
encoded as an IPv4 RTA_GATEWAY attribute. IPv4 routes can also carry an
IPv6 gateway, which fib_nexthop_info() dumps as RTA_VIA.

As a result, route notifications can allocate an skb that is too small.
fib_dump_info() then fails with -EMSGSIZE and rtmsg_fib() hits the
WARN_ON() that marks such failures as a fib_nlmsg_size() bug. With
panic_on_warn set, this becomes a kernel panic.

Mirror the actual nexthop dump layout in fib_nlmsg_size(): account for
IPv6 nexthop gateways dumped as RTA_VIA, for the no-header rtnexthop
layout used inside RTA_MULTIPATH, and for RTA_FLOW only when it is
actually present.

Fixes: d156626 ("ipv4: Allow ipv6 gateway with ipv4 routes")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/6f53fa797fcaeb26966432ed7ae9bb87c4961f37.1785411220.git.zihanx@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv4/fib_semantics.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=0f0ca602941d0a81ae9514943ca06c55159c6385

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

The original Linux kernel CVE announcement for CVE-2026-74657 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74657

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:L/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 1
  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: NO
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.
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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).