[CVE-2026-43499][IMPORTANT] rtmutex: Use waiter::task instead of current in remove_waiter()

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

CVE: CVE-2026-43499
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: rtmutex: Use waiter::task instead of current in remove_waiter()
Commit: 8a1fc8d698ac5e5916e3082a0f74450d71f9611f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8a1fc8d698ac5e5916e3082a0f74450d71f9611f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43499
Analysis date: Wed, 08 Jul 2026 16:54:48 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local unprivileged user may trigger a proxy-lock rollback path in rtmutex via futex_requeue where remove_waiter uses current instead of waiter task, leaving stale PI state primed for UAF and potentially causing kernel crash or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-43499 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43499

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: IMPORTANT
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-2026-43499	IMPORTANT	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-667;CWE-362;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'rtmutex remove_waiter used current instead of waiter task during proxy-lock rollback from futex_requeue, so the waiter could be removed without holding the correct pi_lock and the waiter task pi_blocked_on pointer could remain stale. This stale pointer is explicitly described as primed for UAF and can be reached by later PI chain operations, making the impact more than a simple warning or resource leak. For the CVSS the PR:L is selected because an unprivileged local process can reach PI futex and futex_requeue paths without administrative rights, although reliable triggering requires a specific proxy-lock rollback state. The issue is not network reachable. Impact is at least local denial of service via kernel crash and in worst case may allow privilege escalation through UAF in core locking state. For the paranoid score, choose the highest still-defensible interpretation supported by the bug class and patch context, with preference for manual-review sensitivity over autoclosed false negatives.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	MAYBE	LOCK DANGER UAF LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	guess

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1
  PI futex and futex_requeue paths are reachable by local user processes without administrative privileges.

* Privilege escalation plausible: +1 conservative, +2 paranoid
  Conservative view: the patch shows a stale pi_blocked_on pointer primed for UAF, but no demonstrated reclaim or overwrite primitive. Paranoid view: this is core rtmutex priority inheritance state, and stale task or waiter state reachable by later PI-chain logic is a credible LPE concern.

* Generic memory corruption: +2
  The commit explicitly describes a dangling pointer primed for UAF, not merely a warning or bookkeeping issue.

* Real lifetime corruption: +1
  The bug leaves waiter_task->pi_blocked_on uncleared and performs dequeue under the wrong task lock, creating stale lifetime state.

* Reliable kernel crash / strong DoS: +1
  A stale PI waiter pointer in rtmutex/futex code can realistically lead to kernel crash or lock state corruption.

* Broad/default/common subsystem: +1
  Futex and rtmutex PI logic are core kernel synchronization paths and commonly available.

* Hard or unreliable state/race condition: -1
  Triggering requires a specific proxy-lock rollback path via rt_mutex_start_proxy_lock from futex_requeue, so exploitation is not trivial.

* Confidentiality impact plausible: +1 paranoid only
  Counted only in the paranoid score because UAF in core locking state may be exploitable beyond DoS, but this is not demonstrated.

* Integrity impact plausible: +1 paranoid only
  Counted only in the paranoid score because local privilege escalation is technically plausible, but no concrete write primitive is shown.

## 3. Reachability analysis

A local unprivileged process can reach the relevant futex PI and futex_requeue interfaces. The bug is not network reachable. Namespaces do not eliminate concern because futex syscalls are ordinary local process interfaces and do not generally require CAP_SYS_ADMIN or CAP_SYS_NICE. Reliable triggering likely requires constructing the proxy-lock rollback condition where waiter::task differs from current, so complexity is high, but the affected code is in a default core kernel path rather than a rare optional subsystem.

Call-site confidence: high. The commit message explicitly identifies rt_mutex_start_proxy_lock invoked from futex_requeue as the relevant non-current waiter path, and the patch changes remove_waiter locking, dequeue, pi_blocked_on clearing, and rt_mutex_adjust_prio_chain to use waiter->task.

## 4. Severity interpretation

This is stronger than an ordinary Moderate DoS bug because the patch fixes an explicit stale pointer / UAF candidate in core locking and futex priority inheritance logic. Realistic exploitation evidence supports at least local crash or kernel state corruption, while privilege escalation remains plausible but not proven. Conservative handling is Actionable Moderate. Paranoid handling is a Strong Important candidate because UAF in rtmutex PI state should not be auto-closed without manual review.

## 5. One-sentence report phrase

A local unprivileged user may trigger a proxy-lock rollback path in rtmutex via futex_requeue where remove_waiter uses current instead of waiter task, leaving stale PI state primed for UAF and potentially causing kernel crash or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a local UAF candidate in core rtmutex/futex PI logic, reachable from unprivileged local code, with plausible privilege escalation impact even though the patch does not demonstrate a concrete reclaim or write primitive.

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

Patch: rtmutex: Use waiter::task instead of current in remove_waiter()
Commit: 8a1fc8d698ac5e5916e3082a0f74450d71f9611f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8a1fc8d698ac5e5916e3082a0f74450d71f9611f

Commit description:

remove_waiter() is used by the slowlock paths, but it is also used for
proxy-lock rollback in rt_mutex_start_proxy_lock() when invoked from
futex_requeue().

In the latter case waiter::task is not current, but remove_waiter()
operates on current for the dequeue operation. That results in several
problems:

  1) the rbtree dequeue happens without waiter::task::pi_lock being held

  2) the waiter task's pi_blocked_on state is not cleared, which leaves a
     dangling pointer primed for UAF around.

  3) rt_mutex_adjust_prio_chain() operates on the wrong top priority waiter
     task

Use waiter::task instead of current in all related operations in
remove_waiter() to cure those problems.

[ tglx: Fixup rt_mutex_adjust_prio_chain(), add a comment and amend the
  	changelog ]

Fixes: 8161239 ("rtmutex: Simplify PI algorithm and make highest prio task get lock")
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Keenan Dong <keenanat2000@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/locking/rtmutex.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=8a1fc8d698ac5e5916e3082a0f74450d71f9611f

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

The original Linux kernel CVE announcement for CVE-2026-43499 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43499

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: 6
  Paranoid ActionableScore: 8

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).