[CVE-2026-74257][MODERATE 7.0] sockmap: Fix use-after-free in udp_bpf_recvmsg() [ Upstream

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

CVE: CVE-2026-74257
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: sockmap: Fix use-after-free in udp_bpf_recvmsg() [ Upstream
Commit: 81567d2b3f4dc4fc32f8b61433738bce2cafd4a1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=81567d2b3f4dc4fc32f8b61433738bce2cafd4a1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74257
Analysis date: Sat, 15 Aug 2026 07:34:56 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in the UDP sockmap receive path allows concurrent `udp_bpf_recvmsg()` callers to process the same `struct sk_msg` without socket-level serialization, causing a confirmed kernel heap use-after-free and potential local DoS, with manual review warranted for exploitability beyond crash.

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

The original Linux kernel CVE announcement for CVE-2026-74257 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74257

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-2026-74257	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free race exists in the sockmap UDP receive path because udp_bpf_recvmsg() can call sk_msg_recvmsg() without socket level serialization. sk_msg_recvmsg() peeks a struct sk_msg under a lock but then processes it locklessly, so concurrent recvmsg calls on the same UDP BPF socket can touch the same sk_msg while another thread frees it. For the CVSS the PR:L is used because reliable triggering requires local code execution or access to a process that can operate on the affected socket and sockmap context, but not necessarily full administrator control. The issue is not directly network reachable, although remote UDP packets may provide input data after the local BPF sockmap setup exists. Impact is at least a local denial of service via kernel crash and in the paranoid case may include confidentiality or integrity impact because the primitive is a confirmed kernel heap UAF race.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	REMOTE READ BPF UAF KERNEL_PANIC_PLUS_UAF HARDWARE SYZBOT  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

**Conservative scoring:**

* +1 Memory corruption, weak primitive: confirmed slab use-after-free of `struct sk_msg`, but this is a race UAF without demonstrated reclaim, object replacement, write-after-free, callback control, or arbitrary write.
* +1 Real lifetime corruption: one thread can free `sk_msg` while another thread continues lockless processing of the same object.
* +1 Reliable kernel crash / strong DoS: KASAN reports a real UAF in `sk_msg_recvmsg()`, and a non-KASAN kernel could plausibly crash.
* +1 Common networking / packet-processing related path: the bug is in UDP sockmap receive handling and `sk_msg` networking infrastructure, though exposure requires BPF sockmap setup.
* -2 Requires privileged BPF/sockmap setup in typical deployments: loading or attaching the required BPF/sockmap/XDP pieces commonly requires `CAP_BPF`, `CAP_NET_ADMIN`, `CAP_SYS_ADMIN`, or root-controlled configuration when unprivileged BPF is disabled.

**Paranoid scoring:**

* +1 Local low-privilege practical trigger: if a lower-privileged process can operate on an already configured affected socket, or if BPF/network capabilities are delegated in a container or service context, reliable triggering may not require full host-root.
* +1 Memory corruption, weak primitive: confirmed race UAF, still downgraded because no reclaim or controlled reuse is shown.
* +1 Real lifetime corruption: concurrent `recvmsg()` can access freed `sk_msg`.
* +1 Reliable kernel crash / strong DoS: demonstrated by syzbot/KASAN.
* +1 Networking data-path relevance: this is not raw remote UDP reachability, but it is still kernel networking receive-path code.

## 3. Reachability analysis

The bug is triggered by concurrent receive operations on a UDP socket using BPF sockmap/skmsg handling. It is **not directly network-triggerable** by arbitrary remote UDP traffic alone. Remote packets may supply data to the socket, but the race requires local execution against the affected socket and the sockmap/BPF context.

In default hardened product configurations where unprivileged BPF is disabled, initial setup is usually privileged and the conservative score stays low. In environments with delegated `CAP_BPF` or `CAP_NET_ADMIN`, privileged containers, service accounts managing BPF sockets, or preconfigured sockets accessible to less privileged code, the practical privilege requirement is lower, making the issue actionable.

Call-site confidence: high. The commit message and trace show the vulnerable caller `udp_bpf_recvmsg()` and the UAF site in `sk_msg_recvmsg()`.

## 4. Severity interpretation

This is more serious than an ordinary crash-only bug because the patch fixes a confirmed kernel heap UAF race in networking code. However, the demonstrated primitive is still weak: the evidence shows stale object access and crash potential, not controlled reclaim, arbitrary write, callback hijack, type confusion, or direct privilege escalation.

Conservatively, this behaves like a privileged/local DoS in BPF-gated deployments. Paranoid triage should treat it as **Actionable Moderate at minimum** because kernel networking UAF races have historically required manual review before being dismissed.

## 5. One-sentence report phrase

A race in the UDP sockmap receive path allows concurrent `udp_bpf_recvmsg()` callers to process the same `struct sk_msg` without socket-level serialization, causing a confirmed kernel heap use-after-free and potential local DoS, with manual review warranted for exploitability beyond crash.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: confirmed UAF race in kernel networking code. No strong LPE primitive is shown, but the bug class and affected lifetime path are significant enough that it should not be auto-closed solely as a low-risk BPF-gated DoS.

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

Patch: sockmap: Fix use-after-free in udp_bpf_recvmsg() [ Upstream
Commit: 81567d2b3f4dc4fc32f8b61433738bce2cafd4a1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=81567d2b3f4dc4fc32f8b61433738bce2cafd4a1

Changed files:
  net/core/skmsg.c
  lib/dump_stack.c
  mm/kasan/report.c
  net/ipv4/udp_bpf.c
  net/ipv4/af_inet.c
  net/socket.c
  arch/x86/entry/syscall_64.c
  mm/kasan/common.c
  include/linux/kasan.h
  mm/slub.c
  include/linux/slab.h
  net/ipv4/udp.c
  net/ipv4/ip_input.c
  include/linux/netfilter.h
  include/net/dst.h
  net/core/dev.c
  net/bpf/test_run.c
  kernel/bpf/syscall.c
  mm/kasan/generic.c
  include/linux/skmsg.h

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=81567d2b3f4dc4fc32f8b61433738bce2cafd4a1

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

The original Linux kernel CVE announcement for CVE-2026-74257 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74257

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: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: 2
  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.
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).