[CVE-2026-52910][IMPORTANT] bpf: Free reuseport cBPF prog after RCU grace period. [ Upstream

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

CVE: CVE-2026-52910
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: bpf: Free reuseport cBPF prog after RCU grace period. [ Upstream
Commit: 08264d5bba0bdd3a79bc2984fee09286aba0c4eb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=08264d5bba0bdd3a79bc2984fee09286aba0c4eb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52910
Analysis date: Mon, 22 Jun 2026 09:21:14 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local race in the BPF reuseport detach or replace path can free a classic BPF reuseport program before RCU readers finish, causing a UAF style vmalloc-out-of-bounds access in UDP reuseport packet selection and at least a kernel crash DoS, with possible higher impact requiring manual review.

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

The original Linux kernel CVE announcement for CVE-2026-52910 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52910

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: IMPORTANT
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
======================================================================

CVE-2026-52910	IMPORTANT	CHECK	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 'A race in the BPF reuseport detach or replace path can free a classic BPF reuseport program while another CPU is still using it under RCU in reuseport_select_sock(). This creates a use-after-free style condition and was observed as a KASAN vmalloc-out-of-bounds read during UDP packet processing. For the CVSS the PR:L is used because a local unprivileged process can plausibly create a UDP SO_REUSEPORT group, attach or replace a cBPF reuseport program, and race this with packet delivery. The issue is not directly network reachable by a remote attacker alone because changing the reuseport program requires local socket control. Impact is at least local denial of service via kernel crash. In the paranoid interpretation, the UAF bug class may allow confidentiality or integrity impact and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)  SKIP CVE-2026-52910  SKIP The Fixes patch not applied yet, so unlikely that actual: 538950a1b7527a0a52ccd9337e3fcd304f027f13 YES  NO NO unknown 	MAYBE	REMOTE READ OOB INIT BPF UAF KERNEL_PANIC_PLUS_UAF HARDWARE PACKET  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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

ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1. A local user can plausibly create a UDP SO_REUSEPORT group, attach or replace a cBPF reuseport program, and race it with UDP packet delivery.
* Memory corruption, strong corruption primitive: +2. The bug is an RCU lifetime UAF class issue where a classic BPF program can be freed while still reachable by reuseport_select_sock().
* Real lifetime corruption: +1. The fix explicitly defers freeing with call_rcu(), confirming an RCU grace period lifetime bug.
* Reliable kernel crash / strong DoS: +1. KASAN reports vmalloc-out-of-bounds read in the UDP receive path.
* Local unprivileged high-control kernel data-plane API: +1. BPF plus socket reuseport is a user-shaped kernel data path with controllable object lifetime and traffic timing.
* Common networking core path or packet-processing path: +1. The crash occurs in UDP lookup / reuseport packet selection.
* Weak LPE concern in paranoid score: +1. UAF in a BPF program object can be more serious than DoS, but the patch shows read-after-free rather than a demonstrated controlled overwrite or reclaim primitive.
* Confidentiality and integrity plausible in paranoid score: +1 +1. These are not proven, but are defensible for manual triage because the bug is a real UAF in a high-control networking/BPF path.

No remote-reachable +2 is applied because a remote attacker alone cannot replace or detach the reuseport program. No hard-race penalty is applied in the paranoid score because a repro exists and the race is naturally triggerable by local parallel attach/replace plus traffic.

## 3. Reachability analysis

The practical trigger is local. The attacker needs local socket control to create the reuseport group and attach or replace the cBPF reuseport program through setsockopt(). Packet delivery can be generated locally, and remote packets may help exercise the fast path, but remote traffic alone is not enough without local control over the affected socket group.

This does not normally require CAP_SYS_ADMIN or CAP_NET_ADMIN for ordinary UDP sockets owned by the process. Namespace or container environments may preserve the local attack surface if untrusted code can create UDP sockets and attach classic reuseport BPF programs. The affected path is in common networking code, but the vulnerable condition requires SO_REUSEPORT plus cBPF reuseport program replacement or detach under concurrent packet processing.

## 4. Severity interpretation

This is more than an ordinary Moderate issue because it is a real lifetime bug in a common packet-processing path, not just a NULL dereference or configuration-only DoS. The demonstrated impact is kernel crash / DoS via vmalloc-out-of-bounds read. The realistic exploitation evidence for privilege escalation is not demonstrated, so the conservative score stays in Actionable Moderate territory.

The paranoid score is Strong Important candidate because UAF in BPF/networking lifetime management historically deserves manual review, especially when local unprivileged triggering and high-control kernel objects are involved. However, it should not be treated as confirmed arbitrary code execution without additional reclaim or overwrite evidence.

## 5. One-sentence report phrase

A local race in the BPF reuseport detach or replace path can free a classic BPF reuseport program before RCU readers finish, causing a UAF style vmalloc-out-of-bounds access in UDP reuseport packet selection and at least a kernel crash DoS, with possible higher impact requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: real RCU lifetime corruption / UAF class bug, local unprivileged trigger is plausible, and the affected path combines BPF object lifetime with common UDP packet processing.

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

Patch: bpf: Free reuseport cBPF prog after RCU grace period. [ Upstream
Commit: 08264d5bba0bdd3a79bc2984fee09286aba0c4eb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=08264d5bba0bdd3a79bc2984fee09286aba0c4eb

Changed files:
  net/core/sock_reuseport.c
  lib/dump_stack.c
  mm/kasan/report.c
  net/ipv4/udp.c
  net/ipv4/ip_input.c
  include/linux/netfilter.h
  net/core/dev.c
  kernel/softirq.c
  include/linux/bottom_half.h
  include/linux/rcupdate.h
  include/net/neighbour.h
  net/ipv4/ip_output.c
  net/socket.c
  arch/x86/entry/syscall_64.c
  net/core/filter.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=08264d5bba0bdd3a79bc2984fee09286aba0c4eb

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

The original Linux kernel CVE announcement for CVE-2026-52910 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52910

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: 6
  Paranoid ActionableScore: 8

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).