[CVE-2026-90317][MODERATE 7.0] bpf: Invalidate RCU pointers after final spin unlock [ Upstream

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

CVE: CVE-2026-90317
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bpf: Invalidate RCU pointers after final spin unlock [ Upstream
Commit: 7ae71629357d0a6d0bdadb929c7b296d5b7e61c2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7ae71629357d0a6d0bdadb929c7b296d5b7e61c2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90317
Analysis date: Thu, 17 Sep 2026 23:50:45 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A sleepable BPF verifier lifetime bug can leave RCU protected kptr references valid after the final spin unlock, allowing a stale `task_struct` pointer to be used in `__bpf_get_task_stack()` after another CPU frees it, causing at least local DoS and requiring manual review for possible exploitation beyond crash.

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

The original Linux kernel CVE announcement for CVE-2026-90317 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90317

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: YES

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

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

CVE-2026-90317	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90317 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:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in sleepable BPF programs because the verifier may keep RCU protected kptr references valid after the final bpf_spin_unlock() exits the only RCU protected context. Another CPU can free the referenced object before the program later uses the stale pointer, and a capability limited runtime PoC reportedly triggered a task_struct UAF in __bpf_get_task_stack(). For the CVSS the PR:L is used because triggering requires local ability to load or run the affected BPF program with delegated BPF capabilities rather than full root privileges. The issue is not network reachable and requires local BPF execution. Impact is at least local denial of service via kernel crash and in the worst case may allow privilege escalation due to UAF based memory corruption, so manual review is required.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	BPF UAF HARDWARE LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	YES	NO	checked

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

ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

## 1. ActionableScore

* Raw conservative score: 3
* Conservative score: 5
* Raw paranoid score: 3
* Paranoid score: 5
* Applied conservative floor/cap: Real UAF FLOOR 5 applied, 3 -> 5
* Applied paranoid floor/cap: Real UAF FLOOR 5 applied, 3 -> 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

## 2. Signal breakdown

Positive signals:

* +2 Real UAF / active lifetime violation. The verifier can keep an RCU protected kptr valid after the final `bpf_spin_unlock()` exits the only RCU protected context, allowing another CPU to free the referenced object before later use.
* +1 Reliable kernel crash / strong DoS. The supplied context states that a capability-limited runtime PoC triggered a `task_struct` use-after-free in `__bpf_get_task_stack()`.
* +1 Structured stale-object exploitation leverage. The stale object is an active structured kernel object, `task_struct`, and the stale use enters nontrivial helper processing rather than a shallow cleanup-only dereference.

Negative signals:

* -1 Reduced / delegated privilege. Triggering requires local ability to load or run the affected BPF program with relevant BPF/runtime capabilities, but the supplied PoC is described as capability-limited rather than requiring full host-admin-equivalent privilege.

Non-awarded signals:

* Strong LPE leverage +2 not awarded. Controlled reclaim, object replacement, arbitrary write, credential manipulation, or callback control is not established.
* Local unprivileged trigger +1 not awarded. Ordinary unprivileged BPF reachability is not established.
* Remote reachable +2 not awarded. This is a local BPF execution path, not a network-triggered bug.
* Concrete repeated release/refcount +1 not awarded. The patch does not establish a first release followed by a second lifetime-mutating put/release on the exact same dangling reference.
* Paranoid repeated-refcount +1 not awarded. No concrete second stale refcount mutation is present.
* Extra availability +1 not awarded. The same UAF/crash evidence is already counted as strong DoS.
* C/I points not awarded directly. Confidentiality or integrity impact is plausible enough for manual review, but no concrete disclosure, arbitrary write, or demonstrated LPE primitive is established.

Floor check:

* First release can end lifetime: not a repeated-release case.
* Exact same reference survives to second release: not established.
* Second operation mutates lifetime/refcount: not established.
* Concrete repeated release/refcount awarded: no.
* FLOOR 5 applied: yes, because this is a real active-object UAF involving `task_struct`, reachable below full host admin per supplied capability-limited PoC, and stale use continues into meaningful structured helper processing.

## 3. Reachability analysis

The trigger requires local BPF execution in a sleepable BPF program using `bpf_spin_lock()` / `bpf_spin_unlock()` and RCU protected kptr references. The supplied evidence says a capability-limited runtime PoC exists, so full host-admin-equivalent privilege is not positively established.

BPF loading and execution are still privilege-gated in many products through `CAP_BPF`, `CAP_SYS_ADMIN`, `CAP_PERFMON`, BPF LSM policy, or sysctl configuration such as disabled unprivileged BPF. Therefore this is not treated as ordinary unprivileged reachability.

Namespace-scoped reachability is unresolved. The input does not prove that the complete load, attach, trigger, and relevant object lifetime sequence works entirely inside attacker-created namespaces.

There is no network vector. There is no special hardware requirement. The issue is concurrency/lifetime related, but the supplied PoC and BPF-controlled sequencing make it more than a purely theoretical one-shot race.

## 4. Primitive and severity interpretation

The exact primitive is a stale RCU protected kernel pointer remaining verifier-valid after the final spin unlock exits the last RCU protected context. The stale pointer can later be used after another CPU frees the object.

This is an active structured object UAF, not fixed stale storage. The demonstrated object is `task_struct`, and the stale use occurs in `__bpf_get_task_stack()`, which performs meaningful task-related processing.

This is not merely cleanup/destructor-only behavior. The vulnerable BPF program continues to use a verifier-approved pointer after its RCU lifetime protection has ended.

PrimitiveClass is CLASS_B. A real kernel lifetime/corruption primitive exists and requires exploitation-leverage analysis, but CLASS_A is not justified because controlled reclaim, arbitrary read/write, callback control, or demonstrated privilege escalation is not established.

Demonstrated consequence is at least local DoS via kernel UAF/crash. Speculative consequence is possible privilege escalation if the stale `task_struct` use can be combined with controlled reuse or another exploitation primitive, but that is not proven by the supplied patch.

ImportantCandidate is YES because this is a real active-object BPF UAF involving `task_struct` and a capability-limited PoC, with unresolved leverage beyond crash. AutoDowngradeAllowed is NO because automatic closure would discard a meaningful unresolved kernel lifetime-corruption primitive.

## 5. One-sentence report phrase

A sleepable BPF verifier lifetime bug can leave RCU protected kptr references valid after the final spin unlock, allowing a stale `task_struct` pointer to be used in `__bpf_get_task_stack()` after another CPU frees it, causing at least local DoS and requiring manual review for possible exploitation beyond crash.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: real `task_struct` UAF, active BPF helper use after RCU protection ends, capability-limited PoC, and unresolved exploitation leverage beyond crash.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: bpf: Invalidate RCU pointers after final spin unlock [ Upstream
Commit: 7ae71629357d0a6d0bdadb929c7b296d5b7e61c2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7ae71629357d0a6d0bdadb929c7b296d5b7e61c2

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=7ae71629357d0a6d0bdadb929c7b296d5b7e61c2

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

The original Linux kernel CVE announcement for CVE-2026-90317 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90317

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