[CVE-2026-74338][LOW] bpf: Reject sleepable BPF_LSM_CGROUP programs at load time [ Upstream

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

CVE: CVE-2026-74338
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: bpf: Reject sleepable BPF_LSM_CGROUP programs at load time [ Upstream
Commit: be9eaf2bb5db4ad3de61ef739fd268fd7f135737
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=be9eaf2bb5db4ad3de61ef739fd268fd7f135737
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74338
Analysis date: Sun, 16 Aug 2026 04:08:17 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A privileged local actor able to load BPF LSM cgroup programs can bypass libbpf and attach a sleepable program to a non-sleepable RCU execution path, causing a sleeping-in-invalid-context BUG warning and possible local DoS on panic-on-warn systems.

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

The original Linux kernel CVE announcement for CVE-2026-74338 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74338

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: LOW
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-2026-74338	LOW	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';*CWE-20;CWE-703;CWE-667;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '3.3';DESCR 'A sleepable BPF LSM cgroup program could be loaded even though the cgroup LSM execution path runs in a non sleepable RCU context. A privileged local actor who can load and attach BPF LSM cgroup programs could bypass libbpf and use bpf(2) directly to create a program that calls a sleepable helper such as bpf_get_dentry_xattr from file_open. This can trigger a BUG warning for sleeping function called from invalid context when the helper reaches filesystem code that may sleep. For the CVSS the PR:L is used only for the paranoid score because some environments delegate BPF or cgroup BPF control to service accounts or reduced capability administrators. The issue is not network reachable and does not show a UAF, OOB access, arbitrary write, or information leak primitive. Impact is mainly local denial of service through warning or possible panic on systems configured to panic on warnings.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)	YES	LOCK DISK BPF LINUS 	YES	NO	checked

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

ActionableScore=1
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate / Low-like**

## 2. Signal breakdown

Conservative signals:

* Requires admin/root/CAP_* in typical deployments: -2. Loading and attaching BPF LSM cgroup programs normally requires privileged BPF capability, CAP_SYS_ADMIN, CAP_BPF, or equivalent root-controlled policy.
* WARN_ON / BUG splat only: +0. The patch shows a sleeping-in-invalid-context warning path, not UAF, OOB access, arbitrary write, refcount abuse, or type confusion.
* No local unprivileged trigger: +0. The affected product configuration disables unprivileged BPF by default.
* No generic memory corruption: +0. The issue is a verifier validation gap allowing a sleepable program in a non-sleepable RCU context.
* No privilege escalation plausible signal: +0. There is no concrete reclaim, overwrite, object replacement, callback control, or security-boundary bypass.

Paranoid-only signal:

* Availability concern from warning or panic-on-warn configuration: +1. A privileged local actor can trigger a kernel BUG warning and some deployments may treat such warnings as fatal.

Effective conservative score floors at 0 after the privilege penalty. Paranoid score is 1.

## 3. Reachability analysis

The bug is reachable only by a local actor able to load and attach BPF LSM cgroup programs while setting BPF_F_SLEEPABLE. libbpf does not normally expose a sleepable lsm_cgroup section variant, but the commit shows the path can be reached by calling bpf(2) directly. In typical deployments this is not available to an unprivileged user because BPF loading and LSM cgroup attachment are capability-gated. Containers or delegated service accounts could reduce the practical privilege barrier in some environments, but this still does not become a true unprivileged trigger without explicit delegation.

The affected path is not network reachable. It is also not a default packet-processing or common remote input path. Call-site confidence is high because the commit provides a concrete call trace through bpf_get_dentry_xattr(), ext4_xattr_get(), and down_read() from the cgroup LSM hook.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like validation bug rather than an Important-class kernel vulnerability. The realistic impact is a local warning or possible local DoS on systems configured to panic on warnings. Theoretical concern is limited to executing sleepable helper paths under an RCU non-sleepable context, but the patch does not support memory corruption, privilege escalation, information disclosure, or arbitrary kernel write.

This should not be treated as Actionable Moderate unless the evaluated product delegates BPF LSM cgroup attachment to reduced-privilege users and also treats the warning as system-fatal.

## 5. One-sentence report phrase

A privileged local actor able to load BPF LSM cgroup programs can bypass libbpf and attach a sleepable program to a non-sleepable RCU execution path, causing a sleeping-in-invalid-context BUG warning and possible local DoS on panic-on-warn systems.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a privileged BPF verifier validation fix with no demonstrated memory corruption, no unprivileged trigger in the target configuration, no network reachability, and no concrete privilege-escalation primitive.

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

Patch: bpf: Reject sleepable BPF_LSM_CGROUP programs at load time [ Upstream
Commit: be9eaf2bb5db4ad3de61ef739fd268fd7f135737
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=be9eaf2bb5db4ad3de61ef739fd268fd7f135737

Changed files:
  kernel/locking/rwsem.c
  kernel/bpf/verifier.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=be9eaf2bb5db4ad3de61ef739fd268fd7f135737

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

The original Linux kernel CVE announcement for CVE-2026-74338 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74338

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:L/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS score: 3.3

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: 0
  Paranoid ActionableScore: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).