[CVE-2026-74496][MODERATE 7.0] fou: Fix use-after-free in fou_create()

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

CVE: CVE-2026-74496
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: fou: Fix use-after-free in fou_create()
Commit: a28d8903bfe76089ea4bbdbff9976965f9ef8107
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a28d8903bfe76089ea4bbdbff9976965f9ef8107
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74496
Analysis date: Sat, 15 Aug 2026 15:03:18 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in `fou_create()` can leave a freed `struct fou` reachable through `sk_user_data`, allowing a concurrent UDP tunnel receive path to dereference stale memory and potentially crash the kernel.

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

The original Linux kernel CVE announcement for CVE-2026-74496 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74496

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74496	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-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'fou_create() publishes a struct fou through sk_user_data before the object is safely linked into the per netns FOU port list. If fou_add_to_port_list() fails, the old error path freed the object while it could still be reached from the UDP tunnel socket, and a concurrent receive path could dereference the freed object in fou_from_sock(). For the CVSS the PR:L is used because reliable triggering needs local control of FOU configuration, but this may be available to an unprivileged local user through user namespace or delegated CAP_NET_ADMIN environments. The receive side can be driven by UDP traffic to the affected socket, but the vulnerability is not treated as a pure remote network issue because the dangerous error path is created through local control plane operations. Impact is at least local denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because this is an RCU protected use after free pattern with a stale sk_user_data pointer.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	REMOTE ERRORPATH UAF NETWORK KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1
  FOU configuration normally needs CAP_NET_ADMIN, but this may be reachable by a local unprivileged user through user namespace plus network namespace setups or delegated CAP_NET_ADMIN.

* Memory corruption, weak primitive: +1
  The bug is a race based UAF where `struct fou` can be freed while still reachable through `sk_user_data`. The demonstrated access is a stale dereference in receive path, not a shown write primitive or controlled object replacement.

* Real lifetime corruption: +1
  This is a real object lifetime bug involving stale `sk_user_data`, RCU readers, and freeing before socket release.

* Reliable kernel crash / strong DoS: +1
  A concurrent receive can dereference freed memory in `fou_from_sock()`, making kernel crash plausible.

* Common packet-processing relevance: +1 paranoid only
  The dereference happens from a UDP tunnel receive path. This is not a pure remote issue, but networking receive-side involvement increases manual-review relevance.

* Hard or unreliable race: -1 conservative only
  Triggering requires a timing window between the failed create path, socket release, and concurrent receive.

No LPE bonus is awarded conservatively. The patch shows UAF and stale pointer lifetime corruption, but not attacker-controlled reclaim, callback dispatch, arbitrary write, refcount takeover, or type confusion.

## 3. Reachability analysis

The dangerous state is created through the FOU control plane, specifically `fou_create()` with local port 0 and a later `fou_add_to_port_list()` failure. In normal host deployments this generally requires CAP_NET_ADMIN. In environments with unprivileged user namespaces or delegated network administration, a local unprivileged user may be able to create a net namespace and obtain enough local namespace privileges to reach the path.

The receive-side dereference can be driven by UDP traffic to the affected tunnel socket, but this is not scored as fully remote reachable because the UAF condition must first be created locally through FOU configuration. FOU is not a default common Internet-facing service, but it is a networking tunnel feature and the receive path is packet-driven once configured.

Call-site confidence: medium. The patch and commit explicitly identify `sk_user_data`, `fou_from_sock()`, concurrent receive, and `kfree_rcu()`, but full surrounding call paths are not included.

## 4. Severity interpretation

This is more than an ordinary Moderate correctness bug because it is an explicit race based UAF in networking code. Realistic impact is at least local DoS via kernel crash. Privilege escalation is not proven from the patch because there is no demonstrated reclaim or write primitive, so the conservative score stays at borderline manual-review level.

The paranoid score reaches Actionable Moderate because kernel networking UAFs involving stale socket user data and RCU lifetime rules are historically worth manual review even when the first visible symptom is only a crash.

## 5. One-sentence report phrase

A race in `fou_create()` can leave a freed `struct fou` reachable through `sk_user_data`, allowing a concurrent UDP tunnel receive path to dereference stale memory and potentially crash the kernel.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: explicit UAF plus RCU lifetime handling in networking receive path. Even though practical LPE is not demonstrated, the bug class and stale `sk_user_data` pattern should not be auto-closed without manual review.

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

Patch: fou: Fix use-after-free in fou_create()
Commit: a28d8903bfe76089ea4bbdbff9976965f9ef8107
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a28d8903bfe76089ea4bbdbff9976965f9ef8107

Commit description:

fou_create() publishes struct fou through sk_user_data before adding the
new FOU port to the per-netns list.  If fou_add_to_port_list() fails,
the error path frees fou while it is still reachable through
sk_user_data.  A concurrent receive can then dereference the freed
object in fou_from_sock().

This ordering issue was previously noted in the linked discussion.

The failure is reachable when local port 0 is requested.  Each socket
binds to a different ephemeral port, but fou_cfg_cmp() compares the
requested port 0 and reports -EALREADY once an entry already exists.

Release the tunnel socket before freeing fou so sk_user_data is cleared
first, and defer reclamation with kfree_rcu() to protect concurrent RCU
readers.  This matches the lifetime handling in fou_release().

Fixes: 2346155 ("fou: Support for foo-over-udp RX path")
Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://lore.kernel.org/netdev/20260502031401.3557229-12-kuniyu@google.com/
Cc: stable@vger.kernel.org
Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
Link: https://patch.msgid.link/20260722083858.182506-1-xuanqiang.luo@linux.dev
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv4/fou_core.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=a28d8903bfe76089ea4bbdbff9976965f9ef8107

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

The original Linux kernel CVE announcement for CVE-2026-74496 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74496

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).