[CVE-2026-53180][MODERATE REGULAR] timers/migration: Fix livelock in tmigr_handle_remote_up()

[CVE-2026-53180][MODERATE REGULAR] timers/migration: Fix livelock in tmigr_handle_remote_up()

[AL-KERNEL]

CVE: CVE-2026-53180
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: timers/migration: Fix livelock in tmigr_handle_remote_up()
Commit: 07b3b83587fb3012619f4439389b64a955fc7836
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=07b3b83587fb3012619f4439389b64a955fc7836
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53180
Analysis date: Fri, 26 Jun 2026 04:59:23 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended / DoS-only core timer race
Manual review required: YES

Summary:
A race in timer migration can miss an expired local CPU timer after jiffies advance, leaving the timer on the wheel and causing the remote timer handling path to spin indefinitely, resulting in local denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-53180 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53180

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-2026-53180	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-835;**CWE-400;CWE-362;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 timer migration race can cause tmigr_handle_remote_cpu() to skip timer_expire_remote() for the local CPU even though jiffies advanced after the local timer wheel was processed. The affected timer can remain expired but not removed, so fetch_next_timer_interrupt_remote() keeps reporting it and the remote timer handling path can spin indefinitely. For the CVSS the PR:L is used because reliable triggering requires local code or workload control to create timer activity and CPU idle or migration timing conditions. The issue is not directly network reachable and there is no evidence of memory corruption, information disclosure, or privilege escalation. Impact is denial of service through CPU livelock or severe system responsiveness loss.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended / DoS-only core timer race (with actual score 4)  SKIP CVE-2026-53180  SKIP The Fixes patch not applied yet, so unlikely that actual: 7ee988770326fca440472200c3eb58935fe712f6 YES  NO NO unknown 	MAYBE	TIMER HARDWARE LINUS 	-	-	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended / DoS-only core timer race**::

## 2. Signal breakdown

Conservative scoring:

* Local unprivileged trigger: +1. A local workload can create timer activity and CPU idle or migration timing conditions.
* Reliable kernel crash / strong DoS: +1. The described failure is an indefinite goto-again livelock in timer migration handling.
* Availability impact realistic: +1. The livelock can persist and degrade CPU or system responsiveness, not just emit a warning.
* Broad/default/common subsystem: +1. This is core kernel timer migration code, not a rare driver path.
* Hard or unreliable race / special timing required: -1. The bug depends on jiffies advancing in a narrow window between local timer processing and remote timer evaluation.

Paranoid difference:

* The paranoid score does not subtract the timing penalty, because once the missed expired timer state is reached, the loop can be self-sustaining and repeatedly requeue the same expired event.

Not applied:

* Remote reachable: +0. No network trigger is shown.
* Generic memory corruption: +0. No UAF, OOB write, double-free, refcount reuse, or arbitrary write is indicated.
* Privilege escalation plausible: +0. No credible LPE primitive is visible from the patch.
* Security-boundary bypass: +0. No namespace, capability, credential, ptrace, or fd-theft bypass is involved.

## 3. Reachability analysis

The bug is reachable through local timer activity combined with CPU idle or timer migration behavior. A normal local user can usually create timers and workloads, but reliable triggering likely depends on scheduler timing, idle state, jiffies advancement, and the hierarchical timer migration path. No CAP_SYS_ADMIN or CAP_NET_ADMIN requirement is evident from the patch context.

Containers or namespaces may not provide a direct special interface to the bug, but unprivileged container workloads can still create timers and CPU activity. The affected code is core timer infrastructure and may be present in default kernels that include the hierarchical pull model. There is no realistic direct remote reachability unless a remote workload can indirectly cause precise local timer and CPU-idle behavior, which is not enough to score as network-triggerable.

## 4. Severity interpretation

This behaves like a DoS-only core kernel race, not like memory corruption or privilege escalation. The theoretical impact is high availability loss through CPU livelock or severe responsiveness degradation. The realistic evidence supports local denial of service, but not confidentiality or integrity impact.

Overall this is stronger than an ordinary low-risk cleanup because it affects core timer code and can spin indefinitely. It does not reach Important-class handling because there is no memory corruption, security-boundary bypass, or network-facing trigger.

## 5. One-sentence report phrase

A race in timer migration can miss an expired local CPU timer after jiffies advance, leaving the timer on the wheel and causing the remote timer handling path to spin indefinitely, resulting in local denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a core-kernel race with realistic A:H-style availability impact, but no supported memory corruption or privilege-escalation primitive.

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

Patch: timers/migration: Fix livelock in tmigr_handle_remote_up()
Commit: 07b3b83587fb3012619f4439389b64a955fc7836
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=07b3b83587fb3012619f4439389b64a955fc7836

Commit description:

tmigr_handle_remote_cpu() skips timer_expire_remote() when cpu ==
smp_processor_id(), assuming the local softirq path already handled this
CPU's timers.

This assumption is wrong because jiffies can advance after the handling of
the CPU's global timers in run_timer_base(BASE_GLOBAL) and before
tmigr_handle_remote() evaluates the expiry times.

As a consequence a timer which expires after the CPU local timer wheel
advanced and becomes expired in the remote handling is ignored and the
callback is never invoked and removed from the timer wheel.

What's worse is that fetch_next_timer_interrupt_remote() keeps reporting it
as expired, and the event is re-queued with expires == now on each
iteration.  The goto-again loop spins indefinitely.

Fix this by calling timer_expire_remote() unconditionally. That's minimal
overhead for the common case as __run_timer_base() returns immediately if
there is nothing to expire in the local wheel.

[ tglx: Amend change log and add a comment ]

Fixes: 7ee9887 ("timers: Implement the hierarchical pull model")
Reported-by: Alon Kariv <alonka@amazon.com>
Signed-off-by: Amit Matityahu <amitmat@amazon.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260603170139.33628-1-amitmat@amazon.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/time/timer_migration.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=07b3b83587fb3012619f4439389b64a955fc7836

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

The original Linux kernel CVE announcement for CVE-2026-53180 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53180

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: 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).