[CVE-2026-74656][MODERATE 7.0] ipv4: fix use-after-free in fib_nhc_update_mtu()

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

CVE: CVE-2026-74656
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipv4: fix use-after-free in fib_nhc_update_mtu()
Commit: fd39e711866498ae94fcf9acf6f422a4f045b681
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fd39e711866498ae94fcf9acf6f422a4f045b681
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74656
Analysis date: Sat, 22 Aug 2026 15:29:40 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in IPv4 nexthop exception handling can let `fib_nhc_update_mtu()` access and potentially update a `fib_nh_exception` after it was unlinked and freed via RCU, causing a local kernel crash and warranting manual review for possible stale-object corruption impact.

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

The original Linux kernel CVE announcement for CVE-2026-74656 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74656

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-2026-74656	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-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 IPv4 nexthop exception handling can make fib_nhc_update_mtu() access a fib_nh_exception entry after update_or_create_fnhe() has unlinked it and scheduled it for kfree_rcu. KASAN reports a slab use after free in fib_nhc_update_mtu(), and the old code also updated fnhe_pmtu and fnhe_mtu_locked without holding fnhe_lock during the table walk. For the CVSS the PR:L is used for the paranoid score because a local user with delegated network administration capability or a container context with CAP_NET_ADMIN may be able to trigger MTU changes and PMTU exception activity without full host root. AC:H is kept because exploitation depends on a narrow race window between MTU synchronization and PMTU exception replacement. The issue is not directly network reachable by packet traffic alone, although network or PMTU activity can help create the exception state involved in the race. Impact is at least local denial of service via kernel crash and in the worst still defensible case may include confidentiality and integrity impact because the bug is a real UAF with possible writes to freed exception objects.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE READ LOCK DANGER UAF HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Memory corruption, strong corruption primitive: +2. KASAN confirms slab use after free in `fib_nhc_update_mtu()`.
* Real lifetime corruption: +1. The stale `fib_nh_exception` object can be unlinked and freed via `kfree_rcu()` while another CPU still walks it.
* Reliable kernel crash / strong DoS: +1. The report shows a concrete KASAN UAF path during MTU update handling.
* Common networking core path: +1. This is IPv4 FIB / route exception / PMTU handling, not a rare driver-only path.
* Broad/default/common subsystem exposure: +1. IPv4 routing is widely enabled, even if this exact control path needs specific activity.
* Integrity impact plausible: +1. The old walk did not only read the stale object. It also updated `fnhe_pmtu` and `fnhe_mtu_locked`, so stale write impact is plausible.
* Hard or unreliable race / special timing required: -1. Triggering depends on racing MTU synchronization with PMTU exception replacement and RCU grace-period timing.
* Requires CAP_NET_ADMIN or delegated network administration in typical deployments: -1. Full -2 is not used because CAP_NET_ADMIN may be delegated to containers or network namespaces and is not equivalent to full host root in practical risk terms.

Paranoid delta:

* Weak LPE concern: +1. There is a real UAF with possible writes to a freed networking object, but no demonstrated reclaim, object replacement, callback control, or arbitrary write.
* Privilege penalty relaxed from -1 to 0. In environments with unprivileged user namespaces or delegated CAP_NET_ADMIN, a reduced-privilege local actor may be able to exercise the MTU and PMTU exception paths.

## 3. Reachability analysis

The most direct trigger requires local ability to change an interface MTU or otherwise invoke the `fib_sync_mtu()` path, which normally requires CAP_NET_ADMIN. Network traffic can help create or churn PMTU exception entries, but packet traffic alone does not look sufficient to trigger the vulnerable `fib_nhc_update_mtu()` walk reliably.

Namespaces and containers matter. If an untrusted user can create or control a network namespace with CAP_NET_ADMIN, practical privileges may be closer to PR:L than PR:H. The path is in common IPv4 networking code, but the race requires a specific overlap between MTU update handling and PMTU exception update/removal.

Call-site confidence: high. The provided patch includes the relevant call path and the KASAN trace: `dev_set_mtu()` -> `netif_set_mtu_ext()` -> `fib_netdev_event()` -> `fib_sync_mtu()` -> `fib_nhc_update_mtu()`.

## 4. Severity interpretation

This is stronger than an ordinary Moderate issue because it is a confirmed UAF in IPv4 routing exception handling, with both stale reads and plausible stale writes to the freed object. Realistic exploitation is still constrained by CAP_NET_ADMIN-style reachability and a hard race, so the conservative result is Actionable Moderate rather than automatically Important.

The paranoid interpretation is a Strong Important candidate because the bug is a real lifetime violation in common networking code and not merely a NULL dereference or resource leak. However, practical privilege escalation is not demonstrated. The current evidence supports manual review for exploitability, not an assumption of reliable LPE.

## 5. One-sentence report phrase

A race in IPv4 nexthop exception handling can let `fib_nhc_update_mtu()` access and potentially update a `fib_nh_exception` after it was unlinked and freed via RCU, causing a local kernel crash and warranting manual review for possible stale-object corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: confirmed UAF, RCU lifetime bug, writable stale object fields, and common IPv4 networking context. Even though the realistic trigger likely needs CAP_NET_ADMIN and a difficult race, the class and patch context are sufficient to avoid auto-closing.

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

Patch: ipv4: fix use-after-free in fib_nhc_update_mtu()
Commit: fd39e711866498ae94fcf9acf6f422a4f045b681
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fd39e711866498ae94fcf9acf6f422a4f045b681

Commit description:

fib_nhc_update_mtu() walks the nexthop exception table under RTNL, but
RTNL does not serialize this walk with PMTU exception updates. The walk
uses rcu_dereference_protected() with a constant true condition without
holding fnhe_lock.

The following interleaving can therefore occur:

  CPU 0                              CPU 1
  fib_nhc_update_mtu()               update_or_create_fnhe()
    load fnhe                          spin_lock_bh(&fnhe_lock)
                                       fnhe_remove_oldest()
                                         unlink fnhe
                                         kfree_rcu(fnhe, rcu)
    <quiescent state>
    access fnhe after grace period

KASAN reported:

  BUG: KASAN: slab-use-after-free in fib_nhc_update_mtu+0x3df/0x410
  Read of size 8 at addr ffff888107d49000 by task poc/90
  Call Trace:
   fib_nhc_update_mtu+0x3df/0x410
   fib_sync_mtu+0x7a/0xd0
   fib_netdev_event+0x229/0x3f0
   netif_set_mtu_ext+0x33a/0x570
   dev_set_mtu+0x88/0x120

The same walk updates fnhe_pmtu and fnhe_mtu_locked. These fields form a
pair and other writers serialize them with fnhe_lock. RCU alone prevents
reclamation, but would still allow concurrent writers to leave a mixed
pair.

Walk the table under RCU and acquire fnhe_lock only while updating each
exception. RCU keeps the current entry alive while the short critical
section serializes its paired PMTU fields. This avoids holding the global
lock while scanning all 2048 buckets for every nexthop.

Fixes: af7d6cc ("net: ipv4: update fnhe_pmtu when first hop's MTU changes")
Cc: stable@vger.kernel.org
Suggested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260807181710.1178747-1-nicoyip.dev@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/net/route.h
  net/ipv4/fib_semantics.c
  net/ipv4/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=fd39e711866498ae94fcf9acf6f422a4f045b681

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

The original Linux kernel CVE announcement for CVE-2026-74656 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74656

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: 5
  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.
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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
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