[CVE-2026-74686][LOW] rqspinlock: Reset tail when preserving queue on deadlock [ Upstream

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

CVE: CVE-2026-74686
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: rqspinlock: Reset tail when preserving queue on deadlock [ Upstream
Commit: f54667b0c16213285c9d9b8e3929f738b8f71826
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f54667b0c16213285c9d9b8e3929f738b8f71826
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74686
Analysis date: Sat, 22 Aug 2026 12:28:26 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A rqspinlock ABBA deadlock handling bug can leave the final waiter blocked while waiting for a next queue node, causing a local DoS-style stall in BPF-related locking paths, but the patch does not indicate memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-74686 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74686

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-2026-74686	LOW	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:N/I:N/A:H';CWE-667;*CWE-833;**CWE-400;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A stall can occur in the rqspinlock deadlock handling path when an ABBA deadlock is detected and the current waiter is the last entry in the queue. The code may wait for node next to appear instead of resetting the tail, so the task can remain blocked until another waiter arrives and may stall indefinitely if no later waiter joins. For the CVSS the PR:L is used in the paranoid score for environments where an untrusted local user can run BPF code or control a service account with limited BPF related capabilities. The issue is not network reachable and requires local execution plus a specific locking sequence. Impact is denial of service only through a stalled lock path. No evidence of memory corruption, information disclosure, or privilege escalation is supported by the patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)  SKIP CVE-2026-74686 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	LOCK DEADLOCK BPF HARDWARE LINUS 	-	-	checked

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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:

* Reliable stall / DoS concern: +1. The patch fixes a path where the final waiter may wait indefinitely for `node->next`.
* Hard or unreliable race / special timing required: -1. The described trigger requires a specific ABBA deadlock sequence and final-waiter queue state.
* Requires admin/root/CAP_* in typical deployments: -2. The affected code is under `kernel/bpf/rqspinlock.c`, and in typical evaluated product configurations BPF use is privileged or gated by CAP_BPF, CAP_SYS_ADMIN, CAP_NET_ADMIN, or root-controlled policy.

Paranoid adjustment:

* Local low-privilege or delegated-service trigger concern: +1. If an environment exposes BPF-related functionality to a reduced-privilege service account or delegated capability model, the practical privilege requirement may be closer to PR:L than PR:H.
* No memory corruption signal. The patch does not show UAF, OOB access, double-free, stale callback, refcount abuse, type confusion, or writable stale object reuse.
* No LPE signal. The impact is limited to lock-path stall behavior.

## 3. Reachability analysis

The realistic trigger is local and tied to BPF rqspinlock usage plus a specific ABBA deadlock sequence. It is not network reachable. In typical deployments, unprivileged BPF is disabled or restricted, so a normal unprivileged user should not be assumed to trigger it. Namespace or container exposure could matter only if BPF access or a BPF-using service is delegated to less trusted users, but the patch does not demonstrate a container escape or security-boundary bypass.

Call-site confidence: medium. The commit message gives a precise execution sequence and the patch is in the affected slowpath, but full higher-level BPF call sites are not included.

## 4. Severity interpretation

This behaves like a local DoS / stall bug, not an Important-class vulnerability. The theoretical worst case is an indefinite stall of a waiter in the rqspinlock path, but there is no supported memory-corruption or privilege-escalation primitive. Conservative triage can treat it as Low-like or ordinary Moderate. Paranoid triage still remains below Actionable Moderate because the issue is privileged or delegated-local, timing-dependent, and DoS-only.

## 5. One-sentence report phrase

A rqspinlock ABBA deadlock handling bug can leave the final waiter blocked while waiting for a next queue node, causing a local DoS-style stall in BPF-related locking paths, but the patch does not indicate memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a local, timing-dependent, BPF-gated stall issue with no concrete memory corruption, information disclosure, privilege escalation, or network reachability.

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

Patch: rqspinlock: Reset tail when preserving queue on deadlock [ Upstream
Commit: f54667b0c16213285c9d9b8e3929f738b8f71826
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f54667b0c16213285c9d9b8e3929f738b8f71826

Changed files:
  kernel/bpf/rqspinlock.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=f54667b0c16213285c9d9b8e3929f738b8f71826

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

The original Linux kernel CVE announcement for CVE-2026-74686 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74686

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