[CVE-2026-53166][MODERATE REGULAR] futex/requeue: Prevent NULL pointer dereference in remove_waiter() on self-deadlock

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

CVE: CVE-2026-53166
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: futex/requeue: Prevent NULL pointer dereference in remove_waiter() on self-deadlock
Commit: 16f8e17184b31382076f84751db5ac51fc02733e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=16f8e17184b31382076f84751db5ac51fc02733e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53166
Analysis date: Wed, 08 Jul 2026 16:55:29 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A local unprivileged process may trigger a NULL pointer dereference in the FUTEX_CMP_REQUEUE_PI path by creating a self-deadlock condition for a non-top PI waiter, causing a kernel crash and denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-53166 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53166

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-53166	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-754;*CWE-833;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference in the futex PI requeue path can occur when FUTEX_CMP_REQUEUE_PI processes a non top waiter that already owns the target PI futex. In that self deadlock case task_blocks_on_rt_mutex returns EDEADLK before waiter task is set, and the later remove_waiter path can dereference a NULL waiter task pointer and crash the kernel. For the CVSS the PR:L is used because an unprivileged local process can normally invoke futex syscalls and construct local futex wait and requeue state, although reliable triggering may require careful multi thread ordering. The issue is not network reachable and requires local code execution on the target. Impact is denial of service via kernel crash only, with no clear evidence of information disclosure or privilege escalation because the primitive is a NULL pointer dereference rather than UAF or OOB memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3)	MAYBE	DEADLOCK DANGER NULLPTR SIMPLEFIX IMPROVEONLY LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

* Local unprivileged trigger: +1
  Futex syscalls are normally reachable by unprivileged local processes. No CAP_SYS_ADMIN or CAP_NET_ADMIN is required for ordinary futex usage.

* Reliable kernel crash / strong DoS: +1
  The commit explicitly states that remove_waiter() dereferences NULL waiter->task, causing a kernel crash.

* Broad/default/common subsystem: +1 in paranoid score only
  Futex is a core, widely exposed local synchronization API used on ordinary Linux systems.

* Hard or special trigger condition: -1 in conservative score
  The crash requires a specific FUTEX_CMP_REQUEUE_PI state where a non-top waiter already owns the target PI futex. This is more specialized than a trivial syscall crash.

* No generic memory corruption: +0
  The primitive is a NULL pointer dereference. The patch does not show UAF, OOB write, double free, type confusion, refcount takeover, callback control, or attacker-controlled reclaim.

* No privilege escalation plausible signal: +0
  There is no demonstrated corruption primitive beyond NULL dereference.

* Confidentiality impact plausible: +0
  No information disclosure path is shown.

* Integrity impact plausible: +0
  No write primitive or policy bypass is shown.

## 3. Reachability analysis

A local unprivileged process can normally call futex operations and can create multi-threaded futex wait and requeue states. The affected path is not network reachable and requires local code execution on the target. Namespaces and containers do not materially reduce reachability because futex is a standard local syscall interface available inside containers. Realistic triggering likely requires careful construction of PI futex ownership and requeue ordering, but the affected subsystem is default and broadly exposed.

Call-site confidence: high. The patch and commit message show the direct path from futex_requeue() to rt_mutex_start_proxy_lock() and then remove_waiter() with waiter->task unset.

## 4. Severity interpretation

This behaves like an actionable local Moderate rather than an Important-class vulnerability. The theoretical impact is a local kernel crash through NULL pointer dereference. The realistic evidence supports denial of service only. There is no supported LPE primitive, no UAF reclaim, no stale callback, no type confusion, and no controlled write. Because futex is a broad unprivileged local API, this should not be treated as a trivial cleanup, but it also should not be escalated to Important without additional exploitation evidence.

## 5. One-sentence report phrase

A local unprivileged process may trigger a NULL pointer dereference in the FUTEX_CMP_REQUEUE_PI path by creating a self-deadlock condition for a non-top PI waiter, causing a kernel crash and denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is an unprivileged local kernel crash in the futex subsystem, which is broadly exposed, but the patch supports only a NULL pointer dereference DoS and not memory corruption or privilege escalation.

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

Patch: futex/requeue: Prevent NULL pointer dereference in remove_waiter() on self-deadlock
Commit: 16f8e17184b31382076f84751db5ac51fc02733e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=16f8e17184b31382076f84751db5ac51fc02733e

Commit description:

When FUTEX_CMP_REQUEUE_PI requeues a non-top waiter that already owns the
target PI futex, task_blocks_on_rt_mutex() returns -EDEADLK before setting
waiter->task.

The subsequent remove_waiter() in rt_mutex_start_proxy_lock() dereferences
the NULL waiter->task, causing a kernel crash.

Add a self-deadlock check for non-top waiters before calling
rt_mutex_start_proxy_lock(), analogous to the top-waiter check in
futex_lock_pi_atomic().

Fixes: 3bfdc63 ("rtmutex: Use waiter::task instead of current in remove_waiter()")
Signed-off-by: Ji'an Zhou <eilaimemedsnaimel@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/futex/requeue.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=16f8e17184b31382076f84751db5ac51fc02733e

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

The original Linux kernel CVE announcement for CVE-2026-53166 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53166

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: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: 2
  Paranoid ActionableScore: 3

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).