[CVE-2026-98256][MODERATE 7.0] signal: Prevent exec() race [ Upstream

[CVE-2026-98256][MODERATE 7.0] signal: Prevent exec() race [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98256
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: signal: Prevent exec() race [ Upstream
Commit: 621c365739828900f32a0c1cdfe5387528a62176
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=621c365739828900f32a0c1cdfe5387528a62176
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98256
Analysis date: Sat, 10 Oct 2026 14:11:14 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98256 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98256

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.

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

CVE-2026-98256	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98256 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; 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-667;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 between POSIX timer signal delivery and signal queue cleanup during a non-leader exec can corrupt the sigqueue list and leave an entry reachable after its timer is freed through RCU. A later signal operation can follow the stale list link and perform an 8 byte write into freed slab memory, as demonstrated by the KASAN use-after-free report in __send_signal_locked(). For the CVSS the PR:L value is used because an unprivileged local user can create POSIX timers, execute a multithreaded process, and send signals to owned tasks without administrative capabilities. AC:H is used because successful triggering depends on a timing window between exec, task exit, timer expiry, and queue flushing. The issue is not network reachable. Impact is at least local denial of service through memory corruption and a kernel crash. In the worst defensible case the UAF write and corrupted list links may support privilege escalation, so the paranoid score uses C:H I:H A:H and requires manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	MAYBE	LOCK INIT RACE LEAK UAF TIMER HARDWARE LINUS  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=6
ActionableScoreLower=5
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 conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* No floor or cap changed either score.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF and lifetime violation. A corrupted `sigqueue` list entry remains reachable after its containing POSIX timer is freed through RCU.
* +1 Local unprivileged trigger. An ordinary user can create POSIX timers and threads, block signals, perform a non-leader `execve()`, and use `tgkill()` against owned tasks.
* +1 Reliable kernel crash or strong DoS. KASAN demonstrates an 8-byte slab use-after-free write in `__send_signal_locked()`. A non-instrumented kernel can suffer list corruption and an Oops or panic.
* +1 Attacker-influenced repeatable lifetime manipulation. The same local actor controls timer creation, signal state, process execution, timer expiry conditions, and later signal delivery. These operations can be retried without administrative privilege.

Paranoid-only signal:

* +1 Structured stale-object exploitation leverage. The stale object is an active list node rather than passive storage. A later `list_add_tail()` follows the corrupted `list_head::prev` and writes a kernel pointer into freed slab storage. This is meaningful structured post-lifetime processing, although useful reclaim and target control remain unproven.

No negative signals:

* No host-admin penalty because below-host-admin reachability is positively established through ordinary process, timer, exec, and signal syscalls.
* No race penalty. The interleaving is timing-dependent, but the attacker controls both process-side operation sequences and can make repeated attempts.
* No niche subsystem or hardware penalty. POSIX timers, process execution, and signal queues are generic kernel facilities.

Not awarded:

* Controlled reclaim, arbitrary write, attacker-selected destination, callback control, disclosure, and demonstrated privilege escalation are not established.
* Strong exploitation leverage +2 is not awarded. The demonstrated write has list-operation-derived contents and a fixed stale-object relationship.
* No additional Availability point is awarded for the same crash.
* No Confidentiality or Integrity point is awarded merely because reclaimed storage could theoretically contain another object.
* NEW52 B5-R does not apply. This is not a retry-accumulated indexed OOB write.
* NEW53 B3-PENDING does not apply. No shared file-backed or COW storage write exists.
* NEW54 B4-PENDING does not apply. No authorization decision changes from DENY to ALLOW.

Repeated-release assessment:

* The timer reference can reach final release and the timer can be freed through RCU.
* A stale embedded `sigqueue` reference survives through the corrupted pending list.
* The later operation is a list traversal and write, not a second `put()`, `free()`, or refcount decrement.
* Concrete repeated release/refcount +1 was not awarded.
* Paranoid repeated-refcount leverage +1 was not awarded.
* FLOOR 5 was not applied because the raw conservative arithmetic already reaches 5. The case independently has below-host-admin active structured UAF processing beyond an immediate cleanup fault.

3. Reachability analysis

The supported attacker is an ordinary local user operating on processes, threads, POSIX timers, and signals under that users credentials. No host UID 0, capability, namespace privilege, prior administrator setup, special device, boot option, or unusual hardware is required.

The attacker can create the affected timer and task state, trigger non-leader `execve()`, and send permitted signals to owned tasks. The complete trigger therefore exists below host-admin privilege and does not depend on capability delegation.

The race requires an interleaving between timer signal delivery and pending-queue flushing during leader replacement and task exit. The timing is nontrivial, but the involved operations are repeatable and attacker-invocable. The evidence includes an actual reproducer-generated KASAN report, so reachability is stronger than a theoretical race inferred only from locking changes.

There is no direct network trigger.

4. Primitive and severity interpretation

The exact primitive is an 8-byte write into freed slab memory through a stale `sigqueue` list link. The timer is freed through RCU while its embedded queue entry remains linked due to concurrent non-atomic list deletion and insertion. A later signal operation follows the stale `prev` pointer and modifies the freed list node.

This is an active structured-object UAF, not fixed stale storage. The stale use is not limited to a destructor that immediately dereferences a poisoned pointer. It reaches ordinary signal queue insertion and consumes linkage state to perform another list write.

This is not a concrete repeated-release case. The post-free operation mutates list state rather than performing a second lifetime or refcount decrement on the timer.

PrimitiveClass is CLASS_B because real post-lifetime structured memory corruption is demonstrated, but controlled reclaim, directed target selection, arbitrary read or write, and privilege escalation are not. The demonstrated consequence is kernel memory corruption with realistic crash or DoS. Local privilege escalation remains a technically plausible but unproven consequence.

ImportantCandidate is YES because an unprivileged actor can reach a concrete active-object UAF write, and the exploitation value of reclaiming the freed timer storage before the later list write remains unresolved. AutoDowngradeAllowed is NO because automatically reducing this to an ordinary crash-only race would discard the established post-free list-write primitive.

Operationally, this is Actionable Moderate at minimum and requires Important-vs-Moderate review. Current evidence does not independently justify CLASS_A or a score of 7 or higher.

5. One-sentence report phrase

An unprivileged local user can race POSIX timer signal delivery with non-leader exec cleanup, leaving a freed timer sigqueue linked and causing a later signal insertion to perform an 8-byte use-after-free write into kernel slab memory.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch establishes an unprivileged active-object UAF write through corrupted list linkage, while controlled reclaim and practical privilege-escalation leverage remain unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: signal: Prevent exec() race [ Upstream
Commit: 621c365739828900f32a0c1cdfe5387528a62176
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=621c365739828900f32a0c1cdfe5387528a62176

Changed files:
  kernel/exit.c
  kernel/signal.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=621c365739828900f32a0c1cdfe5387528a62176

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

The original Linux kernel CVE announcement for CVE-2026-98256 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98256

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: 5
  Paranoid ActionableScore: 6

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).