[CVE-2026-90392][MODERATE 7.0] bpf: Fix potential UAF when reading bpf link info [ Upstream

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

CVE: CVE-2026-90392
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bpf: Fix potential UAF when reading bpf link info [ Upstream
Commit: a5c936ac904767fc1d943d40b0308bcb2ae2509b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a5c936ac904767fc1d943d40b0308bcb2ae2509b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90392
Analysis date: Thu, 17 Sep 2026 14:45:52 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90392 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90392

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: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-90392	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90392 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:L/I:L/A:H';*CWE-416;CWE-362;CWE-367;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A use-after-free can occur in the BPF link information path because bpf_link_show_fdinfo() and bpf_link_get_info_by_fd() read the link prog pointer without RCU lifetime protection while another thread can replace it through bpf_link_update. A local process can race reading fdinfo or BPF link info against program replacement, causing stale bpf_prog fields such as tag or aux id to be read after the old program was freed. For the CVSS the PR:L is selected because triggering requires a local user context with access to BPF link file descriptors or delegated BPF capabilities, but not necessarily full administrative control in all deployments. The issue is not network reachable and is triggered through local BPF control plane operations. Impact is at least local denial of service via kernel crash and may include limited confidentiality impact through stale kernel object reads. In the paranoid interpretation, limited integrity impact is kept for manual review sensitivity due to the UAF class, but the code path does not show a direct arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5)	YES	LOCK BPF UAF  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4. No floor or cap changed the score.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap changed the score.
* Final recommended bucket: **Actionable Moderate**::

2. Signal breakdown

Conservative positive signals:

* +2 Real UAF / lifetime violation. `link->prog` may be replaced by `bpf_link_update` and the old `bpf_prog` may be freed while `bpf_link_show_fdinfo()` or `bpf_link_get_info_by_fd()` reads it without RCU protection.
* +1 Reliable kernel crash / strong DoS. The stale `bpf_prog` pointer is dereferenced through `prog->tag` and `prog->aux->id`, which can plausibly fault after the old program is freed.
* +1 Confidentiality plausible. The stale read is copied to user-visible outputs through fdinfo or `BPF_OBJ_GET_INFO_BY_FD`, so limited stale kernel-object data exposure is plausible.

Paranoid additional signal:

* +1 Attacker-influenced active-object lifetime manipulation. A caller with the required BPF link update and info-read access can repeatedly race `bpf_link_update` against fdinfo or get-info reads, directly driving the replacement that creates the stale pointer window.

Non-awarded signals:

* +0 Strong LPE / strong exploitation leverage not awarded. No controlled reclaim, attacker-shaped replacement object, arbitrary write, callback control, refcount takeover, or credential manipulation is established.
* +0 Structured stale-object exploitation leverage not separately awarded. The stale path reads limited fields, mainly `tag` and `aux->id`, and does not consume attacker-controlled replacement fields in a demonstrated security-useful way.
* +0 Concrete repeated release/refcount not awarded. The stale path does not perform a second `put`, `free`, `kref_put`, or other lifetime/refcount mutation on the same dangling object.
* +0 Integrity not awarded conservatively. The patch supports stale reads and possible crash, not unauthorized modification of kernel state.
* +0 Extra availability not awarded. The same stale dereference crash is already counted once.
* +0 Remote reachability not awarded. This is a local BPF control-plane path, not packet-triggered network input.
* No negative privilege penalty applied. Host-admin-equivalent privilege is not positively established from the supplied evidence, but below-host-admin reachability is also not fully proven.

3. Reachability analysis

The vulnerable operations are local BPF information paths: reading `/proc/.../fdinfo` for a BPF link file and calling BPF link get-info by fd. The racing writer is `bpf_link_update`, which can replace `link->prog` for an active BPF cgroup link.

The complete privilege boundary is unresolved from the supplied patch. BPF program creation, attachment, and link update are commonly privilege-gated through `CAP_BPF`, `CAP_SYS_ADMIN`, `CAP_NET_ADMIN`, sysctls, LSM policy, or prior privileged setup, depending on kernel and product configuration. Therefore `HostAdminRequired=UNKNOWN`, not YES and not NO.

Namespace-scoped privilege is also unresolved. The patch does not establish that all required stages, including program loading, link creation, link update, and info reading, are available inside attacker-created namespaces below host-admin privilege. Therefore `NamespaceScopedPrivilege=UNKNOWN`.

No prior host setup is proven as mandatory by the patch alone, but a practical exploit likely requires access to an existing or attacker-created BPF link fd plus the ability to perform `bpf_link_update`. The race is repeatable if the attacker has these operations, so no hard-race penalty is applied merely because concurrency is involved.

4. Primitive and severity interpretation

The exact primitive is a real RCU/lifetime bug: `link->prog` is read without lifetime protection while a concurrent update can replace and free the old `bpf_prog`. The stale object is an active structured kernel object, not fixed stale storage.

The demonstrated stale use is limited: the reader consumes `prog->tag` and `prog->aux->id` for reporting. This supports crash and limited stale-data disclosure, but does not establish controlled reclaim, arbitrary read/write, callback hijack, refcount manipulation, or privilege escalation.

There is no concrete repeated release/refcount operation. The first lifetime event is replacement/free of the old program. The later stale operation is a read/dereference, not a second lifetime mutation on the same object.

Primitive class is CLASS_B because a concrete active-object UAF exists. It is not CLASS_A because no strong exploitation primitive is demonstrated. It is not CLASS_C because this is more than a mere invalid state or NULL dereference: the old object can genuinely be freed while a stale reader still accesses it.

`ImportantCandidate=NO` because the established evidence indicates a constrained BPF control-plane UAF with crash and possible limited disclosure, but no demonstrated meaningful exploitation leverage sufficient to preserve Important severity. `AutoDowngradeAllowed=YES` because no controlled corruption, repeated refcount mutation, boundary bypass, or proven LPE remains unresolved. Operationally this is an actionable Moderate issue that should be reviewed, but it is not a strong Important candidate from the supplied evidence.

5. One-sentence report phrase

A local BPF link information race can read a stale `bpf_prog` pointer after `bpf_link_update` replaces and frees the old program, causing a possible kernel crash and limited stale metadata disclosure.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a real active-object UAF in BPF with user-visible stale reads, but the patch does not establish controlled reclaim, arbitrary write, repeated refcount mutation, or privilege escalation.

7. Call-site confidence

Call-site confidence: high

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

Patch: bpf: Fix potential UAF when reading bpf link info [ Upstream
Commit: a5c936ac904767fc1d943d40b0308bcb2ae2509b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a5c936ac904767fc1d943d40b0308bcb2ae2509b

Changed files:
  kernel/bpf/syscall.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=a5c936ac904767fc1d943d40b0308bcb2ae2509b

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

The original Linux kernel CVE announcement for CVE-2026-90392 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90392

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:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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).