[CVE-2026-53163][MODERATE REGULAR] locking/rtmutex: Skip remove_waiter() when waiter is not enqueued

[CVE-2026-53163][MODERATE REGULAR] locking/rtmutex: Skip remove_waiter() when waiter is not enqueued

[AL-KERNEL]

CVE: CVE-2026-53163
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: locking/rtmutex: Skip remove_waiter() when waiter is not enqueued
Commit: a388e3dfaf9538a680de5ed43a8ebb5dd45b6e53
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a388e3dfaf9538a680de5ed43a8ebb5dd45b6e53
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53163
Analysis date: Fri, 26 Jun 2026 07:03:32 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-53163 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53163

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-2026-53163	MODERATE	CHECK	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-703;BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference can occur in the rtmutex futex PI path because remove_waiter can be called for a waiter that was never enqueued and has no waiter task. A local process can trigger this through FUTEX_CMP_REQUEUE_PI when deadlock handling or a successful proxy lock path leaves the waiter without an armed task pointer. For the CVSS the PR:L is used because triggering requires local code execution through the futex syscall, but it does not normally require administrative privileges. The issue is not network reachable. Impact is denial of service via kernel crash. The patch evidence supports a NULL pointer dereference and not a UAF or write primitive, so confidentiality and integrity impact are not raised.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended (with actual score 4)  SKIP CVE-2026-53163  SKIP The Fixes patch not applied yet, so unlikely that actual: 3bfdc63936dd4773109b7b8c280c0f3b5ae7d349 YES  NO NO unknown 	MAYBE	KASAN LOCK DEADLOCK NULLPTR IMPROVEONLY LINUS SYZBOT  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=4
ActionableScoreLower=3

1. ActionableScore
* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended**::

2. Signal breakdown
* Local unprivileged trigger: +1. FUTEX_CMP_REQUEUE_PI is reachable through the futex syscall by a local process without admin privileges.
* Reliable kernel crash / strong DoS: +1. The reported path reaches a NULL pointer dereference in remove_waiter() through core rtmutex/futex PI code.
* Broad/default/common subsystem: +1. Futex and rtmutex are core kernel synchronization paths and are broadly available.
* Availability impact realistic: +1 in paranoid scoring only. If the syzbot path is reliably reproducible on affected kernels, this is a local host-wide DoS.
* Generic memory corruption: +0. The patch shows a NULL pointer dereference on waiter_task, not UAF, double-free, OOB write, or arbitrary write.
* Privilege escalation plausible: +0. No supported reclaim, overwrite, object confusion, or lifetime corruption primitive is visible from the patch.
* Hard race / special timing: 0. The trigger is a specific futex PI state path, not clearly a timing-dependent race.

3. Reachability analysis
A local unprivileged user can potentially trigger the bug through FUTEX_CMP_REQUEUE_PI and PI futex operations. Containers and sandboxes usually do not remove futex syscall reachability, so namespace isolation may not meaningfully reduce exposure unless syscall filtering blocks the relevant futex operations. The path is not network reachable. The affected code is in core locking and futex PI handling, which is default kernel functionality, although the exact trigger requires a specific requeue and deadlock or proxy-lock state.

4. Severity interpretation
This behaves like a local unprivileged kernel crash issue rather than an Important-class memory corruption vulnerability. The theoretical impact is availability only based on the patch evidence. There is no demonstrated UAF, stale pointer after free, attacker-controlled overwrite, or object reuse primitive that would justify LPE scoring. Because the path is local unprivileged and in core locking code, it should not be auto-closed without at least manual triage.

5. One-sentence report phrase
A local unprivileged process can trigger a NULL pointer dereference in the rtmutex futex PI path via FUTEX_CMP_REQUEUE_PI when remove_waiter() is called for a waiter that was never enqueued, causing a kernel crash.

6. Manual review recommendation
MANUAL CHECK RECOMMENDED. The issue is likely DoS-only, but it is reachable from an unprivileged local futex path in core kernel locking code, so it deserves manual confirmation rather than automatic closure.

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

Patch: locking/rtmutex: Skip remove_waiter() when waiter is not enqueued
Commit: a388e3dfaf9538a680de5ed43a8ebb5dd45b6e53
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a388e3dfaf9538a680de5ed43a8ebb5dd45b6e53

Commit description:

syzbot triggered the following splat in remove_waiter() via
FUTEX_CMP_REQUEUE_PI:

  KASAN: null-ptr-deref in range [0x0000000000000a88-0x0000000000000a8f]
   class_raw_spinlock_constructor
   remove_waiter+0x159/0x1200 kernel/locking/rtmutex.c:1561
   rt_mutex_start_proxy_lock+0x103/0x120
   futex_requeue+0x10e4/0x20d0
   __x64_sys_futex+0x34f/0x4d0

task_blocks_on_rt_mutex() does not arm the waiter upon deadlock detection,
leaving waiter->task nil, where 3bfdc63 ("rtmutex: Use waiter::task instead
of current in remove_waiter()") made this fatal.

Furthermore, rt_mutex_start_proxy_lock() should not be calling into remove_waiter()
upon a successfully grabbing the rtmutex. 1a1fb98 ("futex: Handle early deadlock
return correctly"), moved the remove_waiter() out of __rt_mutex_start_proxy_lock()
(where 'ret' was only ever 0 or < 0) into the wrapper. Tighten this check to
account for try_to_take_rt_mutex().

Fixes: 3bfdc63 ("rtmutex: Use waiter::task instead of current in remove_waiter()")
Reported-by: syzbot+78147abe6c524f183ee9@syzkaller.appspotmail.com
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Closes: https://lore.kernel.org/all/69f114ac.050a0220.ac8b.0003.GAE@google.com/
Link: https://patch.msgid.link/20260507112913.1019537-1-dave@stgolabs.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/locking/rtmutex.c
  kernel/locking/rtmutex_api.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=a388e3dfaf9538a680de5ed43a8ebb5dd45b6e53

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

The original Linux kernel CVE announcement for CVE-2026-53163 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53163

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: Not available
  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: 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).