* [CVE-2026-99125][LOW] s390/pai: Handle multiple PMU stop callback invocations
@ 2026-10-30 14:38 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-30 14:38 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99125
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: s390/pai: Handle multiple PMU stop callback invocations
Commit: cf348ec613f75b9774291e47b804fd7010835a6c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cf348ec613f75b9774291e47b804fd7010835a6c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99125
Analysis date: Fri, 30 Oct 2026 10:38:20 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
A local user controlling an s390 PAI perf event can induce repeated stop callbacks and unsafe list mutation, plausibly crashing the kernel without a demonstrated UAF or exploitation primitive.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99125 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99125
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-2026-99125 LOW https://www.kernelcve.org/list/?q=CVE-2026-99125 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=LOW RESULT=DIFF TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: LOW, Aegis: MODERATE_KPANIC) 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-362;*CWE-664;*CWE-20;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 state handling flaw in the s390 PAI PMU can invoke pai_stop more than once for the same perf event. This can happen when sampling throttling stops an event and a later task schedule out stops it again. PERF_EVENT_IOC_PERIOD can also make perf_event_overflow invoke pai_stop while pai_have_samples is traversing syswide_list, creating a concrete risk of repeated list removal or invalid list traversal and a kernel crash. For the CVSS the PR:L value is selected because an attacker needs a local user context and access to create and control a supported PAI perf event. The base AC:H reflects the required callback ordering or scheduling window, while the paranoid AC:L accounts for the more controllable event limit path. No UAF, OOB access, arbitrary write, information disclosure, or privilege escalation primitive is established, so the supported impact is local denial of service. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic and the cross model evidence reconciliation rule was applied. 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. Exact triggered rule markers were KPANIC_UNSUPPORTED_REMOVED and V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 2.6 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is LOW.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE LEAK HARDWARE LINUS KPANIC_UNSUPPORTED_REMOVED V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED - - checked
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ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 1.
* Conservative score: 1; no floor or cap applied.
* Raw paranoid score: 2.
* Paranoid score: 2; no floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +1 Local unprivileged trigger: supplied evidence supports a local user controlling a supported PAI perf event and its period.
* +1 Reliable kernel crash/strong DoS: repeated `pai_stop()` and mutation during `syswide_list` traversal plausibly cause invalid list processing and a kernel fault.
* -1 base hard timing: the throttle-stop followed by scheduler-stop path requires a particular callback ordering.
* Paranoid difference: the penalty is withheld because `PERF_EVENT_IOC_PERIOD` provides a more controllable `event_limit` overflow path; the fix explicitly prohibits that ioctl.
* No extra availability point: it would duplicate the same crash.
* No UAF, heap OOB write, controlled reclaim, arbitrary write, disclosure, or privilege escalation is established.
* NEW52/B5-R does not apply: no retry-accumulated heap OOB write or later consumption of OOB-corrupted structured state is shown.
* No authorization decision changes from DENY to ALLOW; B4-PENDING is inapplicable.
3. Reachability analysis
The supported attacker is a non-UID-0 local user with access to create and control the relevant s390 PAI perf event. No namespace capability, prior host-administrator setup, or host-admin-equivalent privilege is shown as necessary. The vulnerable facility is architecture-specific, but that alone does not justify a rarity penalty. The scheduler/throttle sequence has timing sensitivity, while the period/event-limit route appears more attacker-directed.
4. Primitive and severity reading
The primitive is repeated, non-idempotent PMU stop/list-state handling: one stop may occur during throttling or overflow and another at schedule-out, while sample handling can mutate the list being traversed. This supports invalid list traversal/removal and crash-oriented state corruption, classified as CLASS_C. It is neither a lifetime UAF nor a proven useful write primitive. The shown impact is local host denial of service; non-crash exploitation is unsupported. Therefore this is not an Important candidate and may be downgraded after review.
5. One-sentence report phrase
A local user controlling an s390 PAI perf event can induce repeated stop callbacks and unsafe list mutation, plausibly crashing the kernel without a demonstrated UAF or exploitation primitive.
6. Manual review advice
MANUAL CHECK RECOMMENDED
Confirm product perf-event access policy and whether the controllable event-limit path is available to ordinary users.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: s390/pai: Handle multiple PMU stop callback invocations
Commit: cf348ec613f75b9774291e47b804fd7010835a6c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cf348ec613f75b9774291e47b804fd7010835a6c
Commit description:
Handle the following scenario:
The kernel protects itself against a very high sampling load and
throttles the sampling using:
perf_event_throttle() --> PMU->stop()
Shortly later the scheduler may terminate the task and removes it from the
CPU. It again calls
PMU->stop()
which results in two invocations of PMU->stop() called back to back.
Protect against this and check the PERF_HES_STOPPED bit on function
entry. If it is already set return.
Clear bit PERF_HES_STOPPED in PMU->start().
Prohibit ioctl(fd, PERF_EVENT_IOC_PERIOD, ...) call for this event.
It sets perf_event::event_limit to a positive value and causes
perf_event_overflow() to invoke pai_stop() call back function when
perf_event::event_limit hits zero. This is not supported because the
sample events CRYPTO_ALL and NNPA_ALL are only taken at schedule out
of a task.
Use list_for_each_entry_safe() for safe iteration over syswide_list
in pai_have_samples().
Fixes: 9f66572 ("s390/pai_crypto: Enable per-task and system-wide sampling event")
Fixes: 582cc1b ("s390/pai_ext: Enable per-task and system-wide sampling event")
Cc: [email protected] # v6.19+
Signed-off-by: Thomas Richter <[email protected]>
Reviewed-by: Sumanth Korikkar <[email protected]>
Signed-off-by: Vasily Gorbik <[email protected]>
Signed-off-by: Heiko Carstens <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
arch/s390/kernel/perf_pai.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=cf348ec613f75b9774291e47b804fd7010835a6c
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99125 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99125
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: 1
Paranoid ActionableScore: 2
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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