[CVE-2026-74658][MODERATE REGULAR] futex: Prevent robust futex exit race some more

[CVE-2026-74658][MODERATE REGULAR] futex: Prevent robust futex exit race some more

[AL-KERNEL]

CVE: CVE-2026-74658
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: futex: Prevent robust futex exit race some more
Commit: 83b0f71d5a313a765754acab51d2ecc5de76e0b9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=83b0f71d5a313a765754acab51d2ecc5de76e0b9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74658
Analysis date: Sat, 22 Aug 2026 18:20:04 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Ordinary Moderate / Low-like, with borderline manual-review only for critical futex users
Manual review required: NO

Summary:
A robust futex exit race can lose a one shot wakeup when a woken waiter exits before acquiring the futex or rearming FUTEX_WAITERS, allowing remaining waiters to sleep indefinitely and causing a local denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-74658 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74658

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: NO

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

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

CVE-2026-74658	MODERATE	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-362;CWE-667;*CWE-833;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A race in robust futex exit handling can lose a one shot wakeup when a woken waiter dies before it can acquire the futex or re arm FUTEX_WAITERS. A third thread may acquire the futex through the uncontended fast path and later unlock it without seeing FUTEX_WAITERS, leaving other waiters sleeping forever behind a free futex. For the CVSS the PR:L is used because reliable triggering requires local code execution in a process that can participate in or share the affected futex state. The issue is not directly network reachable and is triggered through local futex synchronization behavior rather than packet traffic. Impact is denial of service through a hang of affected threads or services. No confidentiality or integrity impact is assigned because the patch indicates a lost wakeup logic race and does not show a memory corruption primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like, with borderline manual-review only for critical futex users (with actual score 3)	YES	LOCK RACE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Low-like, with borderline manual-review only for critical futex users**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  Robust futexes are reachable by local unprivileged processes using ordinary futex synchronization.
* Availability impact realistic: +1
  The bug can leave waiters sleeping forever, causing a local application or service hang.
* Broad/default/common subsystem: +1
  Futexes are a core Linux synchronization primitive used broadly.
* Hard or unreliable race / special timing required: -1
  The scenario requires a specific interleaving: one waiter is woken, another task acquires the futex through the fast path, and the woken waiter exits before rearming FUTEX_WAITERS.

Paranoid adjustment:

* Paranoid score adds practical service-impact concern: +1
  If the affected robust futex is inside a critical multi-threaded service, the lost wakeup can produce a persistent service-level DoS. This does not imply kernel memory corruption or privilege escalation.

Not counted:

* No generic memory corruption.
  The patch fixes a lost wakeup and FUTEX_WAITERS consistency issue, not UAF, OOB, double-free, refcount misuse, or type confusion.
* No privilege escalation plausible signal.
  There is no supported primitive for controlled overwrite, stale object reuse, callback control, or credential manipulation.
* No remote reachability.
  This is a local synchronization path, not packet-triggered.
* No confidentiality or integrity impact.
  The demonstrated consequence is waiters sleeping forever.

## 3. Reachability analysis

A local unprivileged process can create robust futex users and attempt to trigger the interleaving with multiple threads and task exit. Containers and namespaces do not fundamentally remove futex reachability, but the impact is normally confined to processes or services sharing the affected futex state. The path is default-enabled because futex support is core kernel functionality. Realistic exploitation requires local code execution and timing control, but no elevated capability is required.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like local DoS rather than an Important-class kernel vulnerability. The issue is a race and lost-wakeup logic flaw that can hang waiters, but it does not show memory corruption, kernel crash, privilege escalation, or cross-boundary data access. The paranoid score is slightly higher only because hangs in critical services can be operationally significant.

## 5. One-sentence report phrase

A robust futex exit race can lose a one shot wakeup when a woken waiter exits before acquiring the futex or rearming FUTEX_WAITERS, allowing remaining waiters to sleep indefinitely and causing a local denial of service.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can usually be auto-closed for urgent security triage because the patch supports local DoS only, with no memory corruption or privilege escalation primitive. Manual review is only useful if the product relies on robust futexes inside critical host services where service deadlock has elevated operational impact.

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

Patch: futex: Prevent robust futex exit race some more
Commit: 83b0f71d5a313a765754acab51d2ecc5de76e0b9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=83b0f71d5a313a765754acab51d2ecc5de76e0b9

Commit description:

A robust futex unlock stores 0 over the whole futex value - wiping
FUTEX_WAITERS - and wakes a single waiter. That wakeup is a one-shot
notification: the protocol relies on its recipient to either acquire the
futex (and eventually unlock while aware of the remaining contention) or
re-arm FUTEX_WAITERS before sleeping again.  If the woken waiter is killed
before it can do either, the kernel must jump in and wake the next task
down the line.

This is a known complication of the futex protocol with a previous
partial fix in commit ca16d5b ("futex: Prevent robust futex exit
race"). Unfortunately, that fix is insufficient.

If a third task re-acquired the futex through the uncontended fast
path in the meantime, the notification is lost: robust exit processing
sees that it is owned by another task and does nothing, while the new
owner sees no FUTEX_WAITERS when it unlocks and wakes nobody.
The remaining waiters sleep forever behind a free futex:

  A owns the futex, B and C sleep in FUTEX_WAIT
                                        uval == A | FUTEX_WAITERS
  A robust unlock: store 0, FUTEX_WAKE(1) wakes B
                                        uval == 0
  D fast path acquire: cmpxchg(0 -> D)
                                        uval == D, no FUTEX_WAITERS
  B killed before acting on the wakeup
  B exit walk, pending op: owner D != B -> no action
  D unlock: no FUTEX_WAITERS -> no wake
                                        C sleeps forever

This is clearly a shortcoming in the implementation, which fails to keep
the FUTEX_WAITERS bit consistent.

Work around this by augmenting the robust list exit processing to also
perform the extra wakeup if the futex word is owned by another thread but
FUTEX_WAITERS is not set.

This does not fix the problem of a non-contended take over/release and free
sequence, which has been discussed for years and has been addressed by
commit 3ca9595 ("futex: Add support for unlocking robust futexes") and
subsequent changes, but failed to take the problem described above into
account.

A more complete solution which is based on the in kernel unlock of
contended robust futexes has been discussed in the context of this change
and should show up in mainline sooner than later.

[ tglx: Amend change log slightly and fixup coding style ]

Fixes: ca16d5b ("futex: Prevent robust futex exit race")
Signed-off-by: Keno Fischer <keno@juliahub.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Assisted-by: ClaudeCode:claude-fable-5 tla+
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260730194705.38981-1-keno@juliacomputing.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  kernel/futex/core.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=83b0f71d5a313a765754acab51d2ecc5de76e0b9

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

The original Linux kernel CVE announcement for CVE-2026-74658 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74658

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: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: 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).