From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-99125][LOW] s390/pai: Handle multiple PMU stop callback invocations Date: Fri, 30 Oct 2026 10:38:20 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-99125 X-AL-KERNEL-Priority: LOW X-AL-KERNEL-Severity: LOW X-AL-KERNEL-Base-Severity: LOW X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 2 X-AL-KERNEL-ActionableScore-Lower: 1 X-AL-KERNEL-Commit: cf348ec613f75b9774291e47b804fd7010835a6c List-Id: 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 ====================================================================== 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: stable@vger.kernel.org # v6.19+ Signed-off-by: Thomas Richter Reviewed-by: Sumanth Korikkar Signed-off-by: Vasily Gorbik Signed-off-by: Heiko Carstens Signed-off-by: Greg Kroah-Hartman 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 alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).