From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98061][LOW] bpf: Reject tail calls directly from callback frames [ Upstream Date: Fri, 25 Sep 2026 08:41:06 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98061 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: 617c8266e49f45747f71e74177646a63f72b7025 List-Id: CVE: CVE-2026-98061 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: bpf: Reject tail calls directly from callback frames [ Upstream Commit: 617c8266e49f45747f71e74177646a63f72b7025 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=617c8266e49f45747f71e74177646a63f72b7025 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98061 Analysis date: Fri, 25 Sep 2026 08:41:06 -0400 ActionableScore: 0 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: No one-sentence report phrase was found. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98061 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98061 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-2026-98061 LOW https://www.kernelcve.org/list/?q=CVE-2026-98061 CHECK WITH IMPACT FROM ORIG NN LOW 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-617;CWE-703;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A logic error in the BPF verifier can be reached when a BPF program performs a tail call directly from a synchronous callback frame. The verifier models the taken tail call as a function return, synthesizes an unknown R0 value, and then applies the callback return contract during prepare_func_exit. Precision backtracking can consequently reach the callback calling instruction with an unexpected R0 request and trigger the callback unexpected regs WARN followed by EFAULT from BPF_PROG_LOAD. For the CVSS the PR:L in the paranoid score reflects that CAP_BPF may be delegated to a restricted service account, container root, or another limited local context without granting full host administration. The base score uses PR:H because CAP_BPF is normally a privileged capability required to reach this verifier path. The issue is not network reachable and requires local access to BPF_PROG_LOAD. Impact is normally limited to a verifier warning and failed program load, but systems configured with panic_on_warn may convert the warning into a kernel panic and local denial of service. No UAF, out of bounds access, information disclosure, or write primitive is visible in this path, so confidentiality and integrity remain unaffected.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0) SKIP CVE-2026-98061 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE INIT WARNONLY BPF SIMPLEFIX LINUS INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=0 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=UNKNOWN NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=NO ManualReview=NO AutoDowngradeAllowed=YES 1. ActionableScore * Raw conservative score: -1 * Conservative score: 0 * Raw paranoid score: 0 * Paranoid score: 0 * Conservative score was clamped to the minimum score of 0. The BPF caps and UAF floors do not apply. * Final recommended bucket: **Ordinary Moderate**:: 2. Signal breakdown * -1 Reduced privilege requirement. The trigger explicitly requires CAP_BPF, but the evidence does not establish full host-admin-equivalent privilege. * +1 Paranoid strong DoS. A reachable WARN may become a kernel panic where panic_on_warn is enabled. * +0 Conservative availability impact. The demonstrated default result is a WARN and EFAULT from BPF_PROG_LOAD, not a kernel crash. * +0 Logic and verifier-state inconsistency. No independently useful corruption primitive is established. * No UAF, stale lifetime use, OOB access, write primitive, controlled reclaim, disclosure, or authorization bypass is demonstrated. * No remote reachability or local ordinary-unprivileged trigger is established. * No extra Availability point is awarded because the conditional panic is the only possible fatal consequence. * No Confidentiality, Integrity, LPE, or structured-object leverage points are awarded. * The validation-only and BPF caps do not alter either score. 3. Reachability analysis A task with CAP_BPF can construct the callback and tail-call verifier state through BPF_PROG_LOAD. Program attachment, netdevice control, special hardware, and a race are not required for the documented trigger. The evidence does not establish whether CAP_BPF is held in a host-admin-equivalent context or delegated to a restricted service account. It also does not establish that the complete operation is available through an attacker-created user namespace. Therefore HostAdminRequired and NamespaceScopedPrivilege remain UNKNOWN. Ordinary unprivileged BPF reachability is not established. 4. Primitive and severity interpretation The exact defect is an inconsistent verifier state. A callback-frame tail call is modeled as a return, R0 is synthesized as unknown, and callback return-contract processing causes precision backtracking to reach an unexpected register state. This is CLASS_C invalid-state behavior. It is not a UAF, memory corruption, or lifetime violation. The demonstrated consequence is a verifier WARN and failed BPF program load. Kernel panic is conditional on panic_on_warn. No useful primitive remains after excluding that possible crash. ImportantCandidate is NO because there is no unresolved corruption or security-boundary primitive. AutoDowngradeAllowed is YES because the issue is privilege-gated, validation-related, and limited to WARN or conditional DoS. 5. One-sentence report phrase A CAP_BPF task can trigger an inconsistent callback tail-call verifier state that produces a WARN and failed BPF_PROG_LOAD, with kernel panic possible only when panic_on_warn is enabled. 6. Manual review recommendation NO MANUAL CHECK NEEDED The patch establishes a verifier logic error without UAF, OOB access, controlled corruption, disclosure, authorization bypass, or privilege-escalation leverage. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: bpf: Reject tail calls directly from callback frames [ Upstream Commit: 617c8266e49f45747f71e74177646a63f72b7025 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=617c8266e49f45747f71e74177646a63f72b7025 Changed files: kernel/bpf/verifier.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=617c8266e49f45747f71e74177646a63f72b7025 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98061 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98061 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:L/PR:H/UI:N/S:U/C:N/I:N/A:L 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: 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).