[CVE-2026-90300][MODERATE REGULAR] bpf: Clear buf on error in __bpf_get_task_stack [ Upstream

[CVE-2026-90300][MODERATE REGULAR] bpf: Clear buf on error in __bpf_get_task_stack [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90300
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: bpf: Clear buf on error in __bpf_get_task_stack [ Upstream
Commit: f523359c4789bb0396d2d642464a6e2fd4f2299d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f523359c4789bb0396d2d642464a6e2fd4f2299d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90300
Analysis date: Thu, 17 Sep 2026 18:15:09 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
`__bpf_get_task_stack()` could return an error without clearing an `ARG_PTR_TO_UNINIT_MEM` buffer, allowing a BPF program to observe stale or uninitialized kernel-derived data through the buffer, for example as a map key.

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

The original Linux kernel CVE announcement for CVE-2026-90300 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90300

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-2026-90300	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90300 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:H/I:N/A:N';CWE-908;*CWE-457;*CWE-200;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.5';DESCR '__bpf_get_task_stack can return an error without clearing the output buffer even though the helper argument is ARG_PTR_TO_UNINIT_MEM. A BPF program may then use the buffer as initialized memory, for example as a map key, and observe stale or uninitialized data. For the CVSS the PR:L is used for the paranoid score because some deployments delegate BPF access to non root users or service accounts with limited capabilities. The issue is local only and is not network reachable. Impact is primarily confidentiality loss from possible kernel memory disclosure. No direct denial of service or privilege escalation is indicated by the code path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	BPF SIMPLEFIX LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

## 1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap changed the score.

## 2. Signal breakdown

Conservative positive signals:

* +1: concrete uninitialized output-buffer defect. `__bpf_get_task_stack()` can return an error without initializing `buf`, despite the helper argument being `ARG_PTR_TO_UNINIT_MEM`.
* +1: plausible confidentiality impact. The buffer can be used directly as a BPF map key or otherwise observed by the BPF program/userspace path.
* +1: attacker-controlled helper usage once BPF execution is available. The BPF program controls the supplied buffer and can rely on the helper contract that the output memory becomes initialized.

Paranoid additional signal:

* +1: potentially sensitive kernel-memory exposure. The leaked bytes may originate from uninitialized kernel/BPF execution stack state, so the confidentiality value may be higher than a harmless deterministic-value bug.

Non-awarded signals:

* RealUAF not awarded. No freed object or stale lifetime use is shown.
* Memory corruption not awarded. The patch only adds missing `memset()` on error paths.
* Strong LPE or arbitrary read/write not awarded. No write primitive, controlled reclaim, callback control, or credential manipulation is established.
* Reliable DoS not awarded. The demonstrated issue is stale/uninitialized data exposure, not crash.
* Remote reachability not awarded. This is a local BPF helper path.
* ImportantCandidate not awarded because the issue is an information disclosure concern without a strong corruption, authorization-bypass, or demonstrated LPE primitive.

## 3. Reachability analysis

The vulnerable path is reachable by BPF programs using `bpf_get_task_stack` or `bpf_get_task_stack_sleepable`. The complete privilege boundary depends on the product configuration: loading and attaching relevant BPF programs commonly requires `CAP_BPF`, `CAP_PERFMON`, `CAP_SYS_ADMIN`, or related privileged setup, but the supplied input does not prove host-admin-equivalent privilege for all deployments.

`HostAdminRequired=UNKNOWN` is used because the exact product BPF policy is not supplied. `NamespaceScopedPrivilege=UNKNOWN` is used because no complete namespace-scoped BPF load, attach, and trigger chain is established. There is no network trigger and no special hardware prerequisite.

## 4. Primitive and severity interpretation

The exact primitive is use of an output buffer that remains uninitialized on helper error paths. This can make stale memory observable to a BPF program, for example when the buffer is used as a map key.

This is not an active structured-object UAF and not fixed stale storage after free. There is no concrete repeated release/refcount operation. The demonstrated consequence is local information disclosure or nondeterministic kernel-derived data exposure. A stronger consequence such as privilege escalation is speculative and not supported by the patch.

`PrimitiveClass=NONE` is selected because no corruption, lifetime, ownership, or authorization-bypass primitive is established. The issue is still security-relevant due to a plausible confidentiality leak. `AutoDowngradeAllowed=YES` because no unresolved Important-grade primitive is established, but manual review is recommended to determine whether the leaked bytes are sensitive enough in the evaluated product.

## 5. One-sentence report phrase

`__bpf_get_task_stack()` could return an error without clearing an `ARG_PTR_TO_UNINIT_MEM` buffer, allowing a BPF program to observe stale or uninitialized kernel-derived data through the buffer, for example as a map key.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is not memory corruption or UAF, but it may expose uninitialized kernel/BPF stack-derived data and the real impact depends on BPF availability and what data can be observed.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: bpf: Clear buf on error in __bpf_get_task_stack [ Upstream
Commit: f523359c4789bb0396d2d642464a6e2fd4f2299d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f523359c4789bb0396d2d642464a6e2fd4f2299d

Changed files:
  kernel/bpf/stackmap.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=f523359c4789bb0396d2d642464a6e2fd4f2299d

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

The original Linux kernel CVE announcement for CVE-2026-90300 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90300

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:L/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS score: 5.5

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: 3
  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).