[CVE-2026-99244][LOW] pid_namespace: prevent TIF_NOTIFY_SIGNAL from interrupting the reaper

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

CVE: CVE-2026-99244
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: pid_namespace: prevent TIF_NOTIFY_SIGNAL from interrupting the reaper
Commit: 2c20ace1698d03a23467351be1d281e89b36ade3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2c20ace1698d03a23467351be1d281e89b36ade3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99244
Analysis date: Tue, 03 Nov 2026 22:59:09 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
Concurrent notification signaling can keep a PID-namespace reaper busy-looping, consuming CPU and delaying namespace teardown without a shown corruption or authorization primitive.

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

The original Linux kernel CVE announcement for CVE-2026-99244 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99244

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: LOW
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-2026-99244	LOW	https://www.kernelcve.org/list/?q=CVE-2026-99244 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=0 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both 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-835;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'The PID namespace reaper can enter a CPU busy loop because TIF_NOTIFY_SIGNAL may be raised again after it is cleared and signal_pending then prevents kernel_wait4 or an interruptible schedule from sleeping. A demonstrated trigger is concurrent live patch signaling through klp_send_signals while the namespace reaper is waiting for process cleanup. For the CVSS the PR:L value reflects that a local user or container workload may control PID namespace lifetime when namespace creation is delegated, while reliable reproduction also depends on a concurrent kernel notification source such as a privileged live patch operation. The demonstrated impact is delayed namespace teardown and CPU consumption. There is no evidence of UAF, OOB access, information disclosure, or a write primitive. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 0 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	SIMPLEFIX LEAK LINUS 	-	-	checked

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

ActionableScore=1
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* No floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 availability/resource-consumption effect: repeatedly raised `TIF_NOTIFY_SIGNAL` prevents `kernel_wait4()` and interruptible scheduling from sleeping, causing reaper busy-wait and delayed PID-namespace teardown.
* No crash point: the patch supports CPU consumption and teardown delay, not a kernel panic or host-wide fatal fault.
* No corruption point: no UAF, OOB access, invalid lifetime operation, or write primitive is present.
* No authorization point: `no_notify_signal` changes signal-pending/scheduling behavior, not a DENY-to-ALLOW security decision.
* B5-R does not apply: there is no heap OOB write, retry-accumulated offset, or later consumption of corrupted structured state.
* B3-PENDING does not apply: no shared-backing write exists.
* No extra availability point: persistent host-wide exhaustion or lasting unavailability is not shown.

3. Reachability analysis

The demonstrated sequence is compound: a PID-namespace reaper must be in teardown while another source, exemplified by privileged livepatch signaling through `klp_send_signals()`, repeatedly sets `TIF_NOTIFY_SIGNAL`. A delegated container workload may influence namespace lifetime, but the evidence does not show that the same below-host-admin actor controls the notification source or complete trigger. Therefore host-admin and namespace-scoped reachability remain unknown. No special hardware is required; timing coordination is relevant, but no separate race penalty is applied.

4. Primitive and severity reading

The exact defect is a scheduling-state busy loop: clearing `TIF_NOTIFY_SIGNAL` once is ineffective because it may be raised again. The guard makes `signal_pending()` ignore that flag while the namespace reaper waits. This is `PrimitiveClass=NONE`: the shown consequence is bounded to CPU consumption and delayed cleanup, with no memory corruption, protected-state modification, auth bypass, or demonstrated host-wide crash. Important-candidate preservation rules do not apply, and automatic downgrade is allowed.

5. One-sentence report phrase

Concurrent notification signaling can keep a PID-namespace reaper busy-looping, consuming CPU and delaying namespace teardown without a shown corruption or authorization primitive.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch and supplied evidence support an ordinary availability bug without an unresolved Important-class primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: pid_namespace: prevent TIF_NOTIFY_SIGNAL from interrupting the reaper
Commit: 2c20ace1698d03a23467351be1d281e89b36ade3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2c20ace1698d03a23467351be1d281e89b36ade3

Commit description:

Since commit 7fea700 ("zap_pid_ns_processes: clear
TIF_NOTIFY_SIGNAL along with TIF_SIGPENDING") zap_pid_ns_processes()
clears TIF_NOTIFY_SIGNAL before every kernel_wait4() call. Otherwise
kernel_wait4() doesn't sleep and returns -ERESTARTSYS if
signal_pending() is true and the reaper busy-waits.

But clearing TIF_NOTIFY_SIGNAL doesn't fix it. It can get raised again.
For example, klp_send_signals() sets it raw against any task that hasn't
transitioned yet. And it keeps sending it until the livepatch finishes.

The sleep loop waiting for pid_allocated to drop has the same problem
and it doesn't even clear anything. So a schedule() in
TASK_INTERRUPTIBLE returns immediately.

Use a no_notify_signal guard. signal_pending() will ignore
TIF_NOTIFY_SIGNAL and the pid namespace reaper sleeps until a child
exits or free_pid() wakes it.

Link: https://patch.msgid.link/20260824-work-tif_notify_signal-v2-5-6609e42b3157@kernel.org
Fixes: 7fea700 ("zap_pid_ns_processes: clear TIF_NOTIFY_SIGNAL along with TIF_SIGPENDING")
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: stable@vger.kernel.org
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/pid_namespace.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=2c20ace1698d03a23467351be1d281e89b36ade3

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

The original Linux kernel CVE announcement for CVE-2026-99244 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99244

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: unknown
  Paranoid ActionableScore: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).