[CVE-2026-99218][MODERATE 7.0] coredump: hold RCU while releasing parked threads

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

CVE: CVE-2026-99218
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: coredump: hold RCU while releasing parked threads
Commit: fd0fbe84312fda2fc187428c92c50b7d03a9d24c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fd0fbe84312fda2fc187428c92c50b7d03a9d24c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99218
Analysis date: Mon, 02 Nov 2026 22:31:28 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A local user can race multithreaded coredump completion so the kernel attempts to wake a freed task and takes `pi_lock` in reclaimed memory.

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

The original Linux kernel CVE announcement for CVE-2026-99218 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99218

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 7.0
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-2026-99218	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99218 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	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-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in coredump_finish() clears core_thread task and then calls wake_up_process() lacking an RCU read side critical section or a task reference. A parked thread can observe the cleared pointer, exit, and have its task_struct freed while the coredump client is preempted, after which try_to_wake_up() accesses pi_lock in freed memory. This is a concrete use after free access with kernel crash impact and a plausible but not proven memory corruption path, so limited confidentiality and integrity impact is retained in the base score and HHH is reserved for paranoid review. For the CVSS the PR:L value is selected because an attacker needs local user execution to create and crash a multithreaded process, while AC:H reflects the required race window. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the ActionableScore threshold rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4. The independent weaker Aegis shadow evaluated CVSS 4.7 and ActionableScore 4 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	DISK INIT LEAK UAF DEBUGFS LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3.
* Conservative score: 4. REAL UAF FLOOR 4 applies.
* Raw paranoid score: 4.
* Paranoid score: 5. The active structured post-free lock operation supports the stronger FLOOR 5 reading.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Real UAF: `curr->task` is cleared without retaining a task reference or RCU protection; the parked task can exit and free its `task_struct` before `wake_up_process(task)`.
* +1 Reliable kernel crash/strong DoS: `try_to_wake_up()` takes `pi_lock` through the freed task object, plausibly causing an Oops or host crash.
* +1 Local unprivileged trigger: the supplied evidence grounds the race in coredumping an attacker-owned multithreaded process; host administration is not required.
* -1 Hard race: exploitation requires the coredump thread to be delayed after clearing `curr->task` while the parked thread exits and is freed.
* Paranoid +1: taking `pi_lock` is a concrete write-like synchronization operation against an active structured object after its lifetime ended. This is not an arbitrary write, but it leaves post-free corruption leverage unresolved.
* No extra availability point: it would duplicate the same crash.
* No confidentiality or integrity-impact point: controlled reclaim, attacker-selected contents, disclosure, arbitrary write, and LPE are not shown.
* No authorization signal: the patch restores lifetime synchronization, not an access-control decision.
* NEW52 B5-R and NEW53 B3-PENDING do not apply.

3. Reachability analysis

A normal local user can create and crash a multithreaded process to enter coredump handling. No capability, host UID 0, namespace-scoped privilege, prior privileged object setup, or special hardware is shown as necessary. Triggering the UAF requires a timing window involving preemption or a spurious wakeup; repeated coredump attempts are plausible, but reliable race control is not established.

4. Primitive and severity reading

This is an active-structured-object UAF, not fixed stale storage: a freed `task_struct` is passed to the scheduler, which consumes and mutates its `pi_lock`. The privileged consumer is `wake_up_process()`/`try_to_wake_up()`, and the trust boundary is an unprivileged process causing kernel scheduler processing of its coredump threads.

The shown effect is crash-capable post-free access. Useful reclaimed-object control, directed corruption, disclosure, and privilege escalation remain unproven. Nevertheless, the concrete post-free lock operation across the user/kernel boundary makes this a strong unknown CLASS_B candidate, requiring review and preventing automatic downgrade.

5. One-sentence report phrase

A local user can race multithreaded coredump completion so the kernel attempts to wake a freed task and takes `pi_lock` in reclaimed memory.

6. Manual review advice

MANUAL CHECK REQUIRED

Review allocator reuse and `raw_spin_lock` behavior on the freed `task_struct`, because the patch proves a post-lifetime write-like operation but not its containment or exploitability.

7. Call-site confidence

Call-site confidence: high

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

Patch: coredump: hold RCU while releasing parked threads
Commit: fd0fbe84312fda2fc187428c92c50b7d03a9d24c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fd0fbe84312fda2fc187428c92c50b7d03a9d24c

Commit description:

coredump_finish() releases the parked threads by clearing ->task in
their core_thread entry and calling wake_up_process() on each of them.
It does that without holding a reference on the task and without being
inside rcu.

But calling wake_up_process(task) without rcu here isn't safe. A parked
thread doesn't need that wakeup to leave. A spurious wakeup or a
preemption after the store is enough for the task to go away.

So if the coredump client is preempted between the store and
wake_up_process() the thread can exit and be freed in the meantime and
try_to_wake_up() takes pi_lock in freed memory.

Hold rcu across the loop.

Fixes: a94e2d4 ("coredump: kill mm->core_done")
Cc: stable@vger.kernel.org
Acked-by: Oleg Nesterov <oleg@redhat.com>
Link: https://patch.msgid.link/20260921-work-coredump-fixes-v3-1-8e4adb1619e6@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/coredump.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=fd0fbe84312fda2fc187428c92c50b7d03a9d24c

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

The original Linux kernel CVE announcement for CVE-2026-99218 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99218

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: 4
  Paranoid ActionableScore: 5

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).