[CVE-2026-90393][MODERATE 7.0] bpf: Fix potential UAF in bpf_netns_link_update_prog [ Upstream

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

CVE: CVE-2026-90393
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bpf: Fix potential UAF in bpf_netns_link_update_prog [ Upstream
Commit: d462b5481d77ce7df8a5ff89a699386f00c9214d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d462b5481d77ce7df8a5ff89a699386f00c9214d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90393
Analysis date: Thu, 17 Sep 2026 16:08:27 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90393 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90393

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-2026-90393	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90393 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:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in bpf_netns_link_update_prog can happen when two threads concurrently execute BPF_LINK_UPDATE on the same network namespace BPF link. The old_prog and program type checks were done before taking netns_bpf_mutex, so one thread can replace link prog and drop the old bpf_prog reference while another thread still evaluates the lockless type check. This creates a potential use after free in the BPF link update path. For the CVSS the PR:L is used because a local user or process must be able to update a BPF netns link, typically through BPF link update access or delegated network/BPF capabilities. The issue is not directly network reachable. Impact is at least local denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because the bug class is UAF in kernel memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	LOCK BPF RACE UAF HARDWARE  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	NO	NO	checked

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

ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Applied conservative floor: 4 -> 5 because a real active-object UAF is established in an active BPF link update path, the same attacker can plausibly drive concurrent update operations, and host-admin-equivalent privilege is not positively established.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2 Real UAF / active lifetime violation. The patch directly describes a race where one thread replaces link->prog and drops the old bpf_prog reference with bpf_prog_put(), while another thread can still dereference link->prog->type outside netns_bpf_mutex.
* +1 Reliable kernel crash / DoS plausible. A stale bpf_prog pointer dereference in kernel context can realistically fault or crash the kernel.
* +1 Attacker-influenced active-object lifetime manipulation. The same local actor able to issue concurrent BPF_LINK_UPDATE operations can drive replacement of link->prog and the stale read window.
* +1 Paranoid structured stale-object concern. The stale object is an active struct bpf_prog and the stale field is consumed in program type validation, which affects whether the update path proceeds. This is not enough for CLASS_A, but supports manual review.

Non-awarded or withheld signals:

* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Callback or function pointer influence not demonstrated.
* Confidentiality and integrity impact are not awarded as separate ActionableScore points.
* Remote reachability is not established.
* No extra Availability point is awarded because the same crash is already counted.
* Concrete repeated release/refcount signal is not awarded. The visible stale operation is a dereference of link->prog->type, not a second bpf_prog_put() on the exact same dangling old_prog reference.
* Weak LPE is not separately awarded because no additional exploitation primitive beyond the active-object UAF and stale validation read is established.
* CLASS_A is not used because controlled reclaim, arbitrary R/W, demonstrated LPE, or a concrete security-boundary bypass is not shown.

Repeated release/refcount check:

* First release can end lifetime: yes, bpf_prog_put(old_prog) can drop the final reference.
* Exact same reference survives to a second lifetime mutation: not established.
* Second operation is a real lifetime/refcount mutation on the same dangling reference: no.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: yes, but due to active-object UAF with attacker-driven concurrent update surface, not due to repeated release/refcount.

3. Reachability analysis

The trigger requires two concurrent BPF_LINK_UPDATE operations on the same network namespace BPF link. This is local control-plane reachability, not packet-driven network reachability.

The complete privilege boundary is unresolved from the supplied patch alone. BPF link update requires access to relevant BPF link and program file descriptors and may require BPF, network, or administrative capabilities depending on kernel configuration, sysctls, LSM policy, and how the link was created or delegated. Therefore HostAdminRequired is UNKNOWN rather than YES or NO.

NamespaceScopedPrivilege is UNKNOWN. The object is related to a network namespace, but the supplied evidence does not prove that all required stages, including BPF program creation, link creation, attachment, and update, are fully reachable inside attacker-created namespaces below host-admin-equivalent privilege.

Prior privileged setup may exist if a privileged service creates or delegates the BPF link or program FDs, but the patch alone does not prove the exact deployment model. No special hardware is required. The race is repeatable because the attacker can run concurrent update attempts, so no hard-race penalty is applied.

4. Primitive and severity interpretation

The exact primitive is a race-based stale dereference of an active struct bpf_prog pointer in bpf_netns_link_update_prog(). One thread can replace link->prog and drop the old program reference, while another thread performs a lockless program type check through link->prog.

This is an active structured object UAF, not fixed stale storage. It is also not merely cleanup or destructor-only behavior, because the stale read occurs in the active BPF link update validation path. The demonstrated consequence is kernel crash or DoS. The speculative consequence is that stale validation of a freed or reused bpf_prog object could have security relevance, but controlled reclaim or privilege escalation is not established by the patch.

PrimitiveClass is CLASS_B because a real kernel lifetime primitive exists and exploitation-leverage analysis is required. ImportantCandidate is YES because the UAF is in an active BPF update path with attacker-driven concurrent replacement, and automatic downgrade would risk losing a meaningful unresolved lifetime-corruption primitive. AutoDowngradeAllowed is NO for the same reason, even though CLASS_A exploitation is not demonstrated.

Likely operational severity is Actionable Moderate, with Strong Important review sensitivity.

5. One-sentence report phrase

A race in bpf_netns_link_update_prog() allows concurrent BPF_LINK_UPDATE operations on the same netns BPF link to dereference a potentially freed bpf_prog pointer, causing at least local kernel DoS and requiring manual review for possible stronger UAF impact.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real active-object UAF in the BPF link update path, with attacker-driven concurrent replacement of link->prog, but without enough evidence to prove or dismiss stronger exploitation beyond crash.

7. Call-site confidence

Call-site confidence: high

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

Patch: bpf: Fix potential UAF in bpf_netns_link_update_prog [ Upstream
Commit: d462b5481d77ce7df8a5ff89a699386f00c9214d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d462b5481d77ce7df8a5ff89a699386f00c9214d

Changed files:
  kernel/bpf/net_namespace.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=d462b5481d77ce7df8a5ff89a699386f00c9214d

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

The original Linux kernel CVE announcement for CVE-2026-90393 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90393

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: unknown
  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).