[CVE-2026-72428][MODERATE REGULAR] bpf: Fix stack slot index in nospec checks [ Upstream

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

CVE: CVE-2026-72428
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: bpf: Fix stack slot index in nospec checks [ Upstream
Commit: e62268e695075a481a6c4feeb022c19cf57422a6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e62268e695075a481a6c4feeb022c19cf57422a6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72428
Analysis date: Sat, 15 Aug 2026 08:19:17 -0400
ActionableScore: 4
ActionableScore lower bound: 0
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A BPF verifier stack-slot indexing bug can skip Spectre v4 nospec sanitization for certain fixed-offset stack writes, potentially allowing local confidentiality leakage when an attacker can load a crafted BPF program.

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

The original Linux kernel CVE announcement for CVE-2026-72428 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72428

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 REGULAR
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-2026-72428	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:N/A:N';CWE-203;*CWE-200;CWE-693;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '4.7';DESCR 'An eBPF verifier nospec check can use the wrong stack slot index when validating fixed offset stack writes in check_stack_write_fixed_off(). For a partial write such as a 4 byte write at fp-8 after another half of the slot was zeroed, the pre check may inspect bytes 0..3 while the real write updates bytes 7..4. This can leave stale stack bytes that still need speculative store bypass sanitization without nospec_result, creating a potential Spectre v4 style information leak. For the CVSS the PR:L is used because reliable triggering requires local ability to load a crafted BPF program, usually through CAP_BPF/CAP_SYS_ADMIN or an environment where unprivileged BPF is enabled. The issue is not network reachable. Impact is confidentiality leakage, not DoS or direct privilege escalation, but it should be manually reviewed because BPF verifier side channel fixes can hide serious data exposure.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2)	YES	BPF SIMPLEFIX LEAK SPECTRE LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

Conservative signals:

* Confidentiality impact plausible: +1
  The bug is a Spectre v4 / speculative store bypass mitigation gap in the BPF verifier and can plausibly expose kernel data through a side channel.

* Broad/default/common subsystem: +1
  eBPF is a broadly deployed kernel subsystem, even though actual program loading is commonly privilege gated.

* Requires admin/root/CAP_* in typical deployments: -2
  In the evaluated configuration, unprivileged BPF is disabled by default and loading the relevant BPF program requires CAP_BPF, CAP_SYS_ADMIN, CAP_NET_ADMIN, or root-controlled enablement.

Paranoid additional interpretation:

* Local unprivileged trigger: +1
  Applies only if the target configuration enables unprivileged BPF or otherwise allows a non-root local user to load the crafted program.

* Local unprivileged high-control kernel data-plane API: +1
  Applies only in the same unprivileged-BPF configuration, because BPF gives fine-grained control over verifier-visible stack state and program behavior.

* Confidentiality impact plausible: +1
  The missing nospec_result can leave bytes requiring Spectre v4 sanitization unprotected.

* Broad/default/common subsystem: +1
  BPF verifier bugs receive extra triage attention because small verifier mistakes can affect kernel data exposure.

Not awarded:

* No generic memory corruption signal. The patch fixes an incorrect slot index in a mitigation pre-check, not UAF, OOB write, double free, refcount abuse, or type confusion.
* No privilege escalation signal. The patch supports speculative information disclosure, not a demonstrated write primitive or LPE chain.
* No reliable kernel crash or strong DoS signal.
* No remote/network reachable signal.

## 3. Reachability analysis

The realistic trigger requires loading a crafted BPF program that reaches `check_stack_write_fixed_off()` with a partial stack write pattern where the pre-check inspects the wrong slot bytes. In typical hardened distributions, unprivileged BPF is disabled, so the conservative interpretation treats this as requiring privileged BPF access and applies the CAP/root penalty.

Namespaces and containers matter only if the container or delegated environment actually has the required BPF capabilities and kernel configuration permits program loading. A container root without CAP_BPF or CAP_SYS_ADMIN should not be considered able to trigger this. The path is not network reachable by packet traffic alone.

## 4. Severity interpretation

This behaves like a BPF verifier side-channel mitigation bypass, not ordinary memory corruption. The realistic impact is confidentiality-only and depends on both BPF loading permissions and microarchitectural leakage conditions.

Conservatively, this is ordinary Moderate / Low-like for products where BPF is root-only. Paranoid handling reaches borderline manual-review territory if unprivileged BPF is enabled, because verifier nospec bugs can hide meaningful kernel data exposure even when there is no direct LPE primitive.

## 5. One-sentence report phrase

A BPF verifier stack-slot indexing bug can skip Spectre v4 nospec sanitization for certain fixed-offset stack writes, potentially allowing local confidentiality leakage when an attacker can load a crafted BPF program.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this is a BPF verifier nospec mitigation bypass with plausible confidentiality impact, but it should not be escalated to Important unless the affected product enables unprivileged BPF or additional evidence shows a stronger leak or exploit chain.

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

Patch: bpf: Fix stack slot index in nospec checks [ Upstream
Commit: e62268e695075a481a6c4feeb022c19cf57422a6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e62268e695075a481a6c4feeb022c19cf57422a6

Changed files:
  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=e62268e695075a481a6c4feeb022c19cf57422a6

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

The original Linux kernel CVE announcement for CVE-2026-72428 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72428

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:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS score: 4.7

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: 0
  Paranoid ActionableScore: 4

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 REGULAR

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).