From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-99085][LOW] sched_ext: Fix CPU hotplug hang when a dying CPU's tasks sit in the BPF scheduler [ Upstream Date: Mon, 26 Oct 2026 22:10:37 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-99085 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: 0 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: f79cd7a2df3768323fbfb9b0f96adff08fb7eb4c List-Id: CVE: CVE-2026-99085 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: sched_ext: Fix CPU hotplug hang when a dying CPU's tasks sit in the BPF scheduler [ Upstream Commit: f79cd7a2df3768323fbfb9b0f96adff08fb7eb4c Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f79cd7a2df3768323fbfb9b0f96adff08fb7eb4c Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99085 Analysis date: Mon, 26 Oct 2026 22:10:37 -0400 ActionableScore: 0 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: A host administrator controlling sched_ext and CPU hotplug can strand runnable tasks during CPU teardown, hanging CPU offlining and holding the hotplug lock. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99085 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99085 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-2026-99085 LOW https://www.kernelcve.org/list/?q=CVE-2026-99085 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=1 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-362;CWE-667;*CWE-833;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'A sched_ext lifecycle and synchronization flaw can leave runnable tasks associated with a dying CPU while those tasks remain on a user DSQ or are held by the BPF scheduler. The CPU hotplug thread can then wait indefinitely in sched_cpu_wait_empty because the runqueue may become empty without the required wakeup or an affine task may remain unable to run until the watchdog ejects the scheduler. The RCU grace period before runqueue offline can also be blocked when dispatch stops too early and a BPF-held task must run to leave an RCU read-side section. For the CVSS the base PR:H reflects that reliable triggering normally requires control of a sched_ext BPF scheduler and CPU hotplug. For the CVSS the paranoid PR:L covers deployments where these operations are delegated to a reduced-capability orchestration or service account. The issue is local and timing dependent with no supported memory corruption or information disclosure primitive. Impact is availability loss through a hung cpu_down operation and a held cpu_hotplug_lock. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 0 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 2.3 and ActionableScore 1 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 0) MAYBE BPF HARDWARE LINUS TEST - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=0 PrimitiveClass=NONE RealUAF=NO HostAdminRequired=YES NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=NO AutoDowngradeAllowed=YES RequiresHostUID0=UNKNOWN AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: -1 (+1 strong availability hang, -2 effective host-administrator privilege). * Conservative score: 0 after clamping the negative raw score. * Raw paranoid score: -1; no technically supported additional primitive or below-host-admin trigger. * Paranoid score: 0 after clamping. * Final recommended bucket: **Ordinary Moderate**:: 2. Signal breakdown * +1 Reliable strong DoS: runnable tasks stranded on a user DSQ or held by the BPF scheduler can indefinitely block `sched_cpu_wait_empty()` or the preceding RCU grace period while `cpu_hotplug_lock` remains held. * -2 Host-admin-equivalent trigger: the complete demonstrated setup requires control of a sched_ext BPF scheduler and CPU hotplug. The evidence does not establish that one below-host-admin actor can perform both operations. * No memory corruption, UAF, OOB access, authorization bypass, protected-state modification, disclosure, or integrity primitive is shown. * No extra availability point: the blocked hotplug operation and held lock are the same DoS effect. * NEW52/B5-R and NEW53/B3-PENDING do not apply: there is no write primitive. B4-PENDING does not apply because no DENY-to-ALLOW authorization change exists. 3. Reachability analysis The relevant actor must configure or control a sched_ext BPF scheduler, arrange affected tasks, and initiate CPU offlining. These are host control-plane operations; no complete user-namespace or delegated-capability path is evidenced. The legacy PR:L possibility is deployment-dependent and does not prove delegation of the full sequence. No special hardware or narrow race is required, but particular task affinity, DSQ/held-task state, and hotplug sequencing are needed. 4. Primitive and severity reading The defect is scheduler lifecycle/synchronization state that prevents runqueue draining and may block an RCU grace period. It causes an operational hang, not stale-object use or corrupted-state consumption. PrimitiveClass is NONE because the shown effect is availability-only. ImportantCandidate is NO and auto-downgrade is allowed because the complete demonstrated trigger is host-admin-equivalent and no security-boundary or corruption primitive remains unresolved. 5. One-sentence report phrase A host administrator controlling sched_ext and CPU hotplug can strand runnable tasks during CPU teardown, hanging CPU offlining and holding the hotplug lock. 6. Manual review advice NO MANUAL CHECK NEEDED The patch supports a privileged availability flaw without memory corruption, authorization bypass, or a credible lower-privilege boundary crossing. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: sched_ext: Fix CPU hotplug hang when a dying CPU's tasks sit in the BPF scheduler [ Upstream Commit: f79cd7a2df3768323fbfb9b0f96adff08fb7eb4c Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f79cd7a2df3768323fbfb9b0f96adff08fb7eb4c Changed files: kernel/sched/ext.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=f79cd7a2df3768323fbfb9b0f96adff08fb7eb4c ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99085 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99085 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:H/UI:N/S:U/C:N/I:N/A:H 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: 0 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).