[CVE-2026-90250][LOW] bpf, cgroup: Fix storage null-ptr-deref after replacing prog [ Upstream

[CVE-2026-90250][LOW] bpf, cgroup: Fix storage null-ptr-deref after replacing prog [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90250
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: bpf, cgroup: Fix storage null-ptr-deref after replacing prog [ Upstream
Commit: a033c950f6731be88a7da98604ed78f302f15b80
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a033c950f6731be88a7da98604ed78f302f15b80
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90250
Analysis date: Thu, 17 Sep 2026 23:06:04 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90250 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90250

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-2026-90250	LOW	https://www.kernelcve.org/list/?q=CVE-2026-90250 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:H';**CWE-476;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference can occur in the BPF cgroup replacement path when an attached empty program is replaced with a program that requires BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE and calls bpf_get_local_storage. The replacement path can leave the new program with uninitialized cgroup storage, so running the new program can dereference NULL and crash the kernel. For the CVSS the PR:L paranoid interpretation is used when BPF and cgroup administration are delegated to a non root service account or container root with limited but security relevant capabilities. In typical host deployments the operation requires administrative capabilities such as CAP_BPF and CAP_NET_ADMIN. The issue is not directly network reachable because the attacker must first perform a local BPF cgroup link update. Impact is denial of service via kernel panic, with no concrete evidence of information disclosure or privilege escalation from the patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	NULLPTR BPF SIMPLEFIX HARDWARE  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap changed the score. BPF privileged control-plane cap was considered, but the raw score is already below the cap. No UAF floor applies because RealUAF=NO.

2. Signal breakdown

Positive signals:

* +1: Reliable kernel crash / strong DoS. The commit explicitly says the uninitialized cgroup storage leads to a NULL pointer dereference panic when the replacement program is run.
* +1: Attacker-influenced invalid kernel control-plane state. The trigger requires replacing an attached empty cgroup BPF program with another program that expects cgroup storage, producing an invalid program/storage compatibility state.

Negative or non-awarded signals:

* +0: Real UAF not awarded. The supplied patch establishes an uninitialized or missing storage pointer and NULL dereference, not stale post-lifetime object use.
* +0: Concrete repeated release/refcount not awarded. There is no visible first release that may end object lifetime followed by a second lifetime mutation on the same dangling reference.
* +0: Strong LPE / strong exploitation leverage not awarded. No controlled reclaim, arbitrary write, callback control, refcount takeover, or credential manipulation is shown.
* +0: Structured stale-object leverage not awarded. The issue is invalid storage compatibility and NULL dereference, not stale structured object reuse.
* +0: Confidentiality and integrity not awarded. The patch context supports crash only and does not establish disclosure or unauthorized modification.
* +0: Remote reachability not awarded. The bug is reached through local BPF cgroup link update operations, not packet input.
* +0: Local unprivileged trigger not awarded. Complete below-host-admin BPF load, attach, and cgroup link update reachability is not established.
* +0: Extra availability not awarded. The same kernel panic is already counted once as reliable crash / DoS.
* +0: Race/timing penalty not applied. This is an ordered invalid replacement scenario, not a hard timing race.
* +0: Host-admin penalty not applied. Host-admin-equivalent privilege is plausible for BPF/cgroup operations in common deployments, but the supplied evidence does not fully resolve the exact privilege boundary.

Repeated release/refcount checklist:

* First release can end lifetime: no evidence.
* Exact same reference survives first release: no evidence.
* Second operation mutates lifetime/refcount: no evidence.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.

3. Reachability analysis

The direct trigger is local control-plane manipulation of cgroup BPF links. The described sequence requires attaching an empty program to a cgroup, preparing another program using BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE and bpf_get_local_storage, and using link_update to replace the first program with the second.

For BPF/XDP privilege analysis:

* Load privilege: unresolved from supplied text, but cgroup BPF program loading is normally privilege-gated by BPF-related capabilities or equivalent policy.
* Attach privilege: unresolved from supplied text, but attaching to cgroups is normally privilege-gated.
* Netdevice privilege: not central to this bug.
* Namespace privilege: not established. No evidence shows the full load, attach, link update, and trigger chain works entirely inside attacker-created namespaces.
* Root-controlled sysctls: unprivileged BPF policy may matter, but no product-specific sysctl state is supplied.

HostAdminRequired=UNKNOWN because host-admin-equivalent privilege is likely in many deployments, but not positively established for all evaluated environments from the supplied patch alone. NamespaceScopedPrivilege=UNKNOWN because the complete chain is not proven namespace-scoped. There is no direct network trigger and no special hardware requirement. No race repeatability issue is involved.

4. Primitive and severity interpretation

The exact primitive is a NULL pointer dereference caused by replacing a cgroup BPF program with another program whose cgroup storage requirements are incompatible with the original link state. The replacement path does not allocate and assign the required storage for the incoming program, so the new program may run with uninitialized storage and crash when bpf_get_local_storage is used.

This is not an active structured UAF and not fixed stale storage. It is a Class C invalid-state crash primitive. The demonstrated consequence is kernel panic / local DoS. The speculative consequence would be stronger exploitation from BPF object misuse, but the supplied code does not establish a useful corruption primitive beyond NULL dereference.

ImportantCandidate=NO because there is no real UAF, no repeated lifetime mutation, no controlled reclaim, no arbitrary read/write, and no authorization or security-boundary bypass. AutoDowngradeAllowed=YES because the established behavior is a privilege-gated local crash-only NULL dereference and no meaningful unresolved corruption leverage remains.

Likely operational severity is ordinary Moderate or Low-like in practical triage, despite CVSS possibly reaching local DoS values such as PR:L/A:H in delegated BPF environments.

5. One-sentence report phrase

A NULL pointer dereference can occur after a cgroup BPF link update replaces an attached program with another program requiring incompatible cgroup storage, causing a local kernel panic when the new program is run.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the supplied evidence supports a local privilege-gated NULL pointer dereference DoS only, with no UAF, repeated release/refcount mutation, controlled memory corruption, authorization bypass, or plausible privilege escalation primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: bpf, cgroup: Fix storage null-ptr-deref after replacing prog [ Upstream
Commit: a033c950f6731be88a7da98604ed78f302f15b80
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a033c950f6731be88a7da98604ed78f302f15b80

Changed files:
  kernel/bpf/cgroup.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=a033c950f6731be88a7da98604ed78f302f15b80

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

The original Linux kernel CVE announcement for CVE-2026-90250 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90250

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:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  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: unknown
  Paranoid ActionableScore: 2

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).