From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98591][MODERATE 7.0] bpf: Fix bounds check for skb-backed dynptrs [ Upstream Date: Fri, 23 Oct 2026 20:21:49 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98591 X-AL-KERNEL-Priority: MODERATE 7.0 X-AL-KERNEL-Severity: MODERATE 7.0 X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 5 X-AL-KERNEL-ActionableScore-Lower: 3 X-AL-KERNEL-Commit: 1b923538598be768e9f2f258a77a083d00e0c78b List-Id: CVE: CVE-2026-98591 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: bpf: Fix bounds check for skb-backed dynptrs [ Upstream Commit: 1b923538598be768e9f2f258a77a083d00e0c78b Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1b923538598be768e9f2f258a77a083d00e0c78b Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98591 Analysis date: Fri, 23 Oct 2026 20:21:49 -0400 ActionableScore: 5 ActionableScore lower bound: 3 Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review Manual review required: YES Summary: An unsigned bounds-check underflow can back an skb BPF dynptr with an out-of-range kernel pointer, creating a potential OOB read or write that requires mandatory call-site and BPF privilege review. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98591 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98591 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-2026-98591 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98591 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (final reconciliation agrees; DECISION: both 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:H/I:H/A:H';CWE-191;CWE-823;CWE-119;*CWE-787;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'An unsigned subtraction in skb_pointer_if_linear can underflow when the requested offset exceeds skb_headlen. The bounds check may then pass and return skb data plus an out of range offset, allowing an skb backed BPF dynptr to reference invalid kernel memory. Subsequent dynptr operations may perform an OOB read or OOB write depending on the permitted operation, which can cause a kernel crash and may expose or corrupt kernel memory. For the CVSS the PR:L value reflects that a local user or process normally needs permission to execute or load a suitable BPF program, including environments where restricted BPF capabilities are delegated without full root access. The issue is not automatically network reachable. Network traffic may trigger the vulnerable path only when an already installed BPF program derives the affected dynptr offset from attacker controlled packet data. Impact is at least denial of service and may include confidentiality and integrity compromise or privilege escalation if the invalid dynptr can be used for controlled kernel memory access. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7.8 and the production ActionableScore returned 4 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the manual review requirement was applied. KP and FD specialist checks stage MODERATE_KPANIC to MODERATE because KP specialist did not establish a qualifying panic. final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK and DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 4.3 and ActionableScore 2 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 4) MAYBE OOB BPF SIMPLEFIX SKB IMPROVEONLY LINUS INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=5 ActionableScoreLower=3 PrimitiveClass=CLASS_B RealUAF=NO HostAdminRequired=UNKNOWN NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=YES ManualReview=REQUIRED AutoDowngradeAllowed=NO RequiresHostUID0=UNKNOWN AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw conservative score: 3 * Conservative score: 3 * Raw paranoid score: 5 * Paranoid score: 5 * No floor or BPF cap changed either score. * Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**:: 2. Signal breakdown Conservative signals: * +1 weak or indirect corruption candidate. Unsigned subtraction can underflow and return an attacker-influenced pointer outside the linear skb area. * +1 reliable crash concern. A dynptr backed by invalid kernel memory can fault when a later dynptr operation accesses it. * +1 active-state and trust-boundary candidate. The invalid pointer becomes active dynptr state consumed by privileged kernel BPF code, while delegated or namespace-scoped BPF reachability remains unresolved. * Total: 3. Paranoid signals: * +2 strong corruption interpretation. If the visible helper is reached by writable skb dynptr operations, the attacker-selected offset can produce an OOB kernel write rather than only an invalid read. * +1 reliable kernel crash. * +1 confidentiality plausibility. A readable invalid dynptr could expose adjacent kernel memory through a BPF-controlled output path, although that output path is not shown here. * +1 integrity plausibility. A writable dynptr could modify memory outside the skb linear region, although the exact writable call site is not supplied. * Total: 5. Not awarded: * No UAF, lifetime violation, controlled reclaim, or repeated refcount mutation exists. * No arbitrary target, callback control, credential overwrite, or demonstrated LPE is established. * No remote-reachability point. Ordinary packets cannot directly choose this helper offset without a previously installed BPF program. * No local-unprivileged point because the complete BPF load, attach, and trigger privilege chain is unresolved. * No extra availability point because it would duplicate the same potential crash. * NEW52 B5-R does not apply because there is no retry-accumulated index or retry sequence. * NEW53 B3-PENDING does not apply because shared or file-backed storage modification is not established. * NEW54 B4-PENDING does not apply because no authorization decision is bypassed. 3. Reachability analysis A suitable BPF program can influence the offset and length used for an skb-backed dynptr. The supplied patch does not establish whether the complete program load and attachment sequence is available to an ordinary user, a delegated CAP_BPF or CAP_NET_ADMIN service, a namespace-confined actor, or only a host administrator. HostAdminRequired and NamespaceScopedPrivilege therefore remain UNKNOWN. Unprivileged network traffic alone is insufficient. A network-assisted trigger is credible only after a suitable BPF program has been installed and derives the vulnerable offset from packet-controlled state. No race, special hardware, or retry sequence is required once the vulnerable BPF path is available. 4. Primitive and severity interpretation The concrete primitive is an unsigned arithmetic underflow that makes a bounds check pass and returns `skb->data + offset` outside the valid linear skb region. This invalid address then backs active BPF dynptr state. This is CLASS_B from the supplied evidence because invalid kernel-memory-backed state is established, but the exact readable or writable dynptr call site is not included. A writable dynptr path would raise the practical primitive to a directed OOB write and support CLASS_A, but that requires call-site confirmation. The demonstrated consequence is invalid active pointer state. Kernel crash, disclosure, and corruption are technically plausible downstream consequences. Controlled arbitrary read or write and privilege escalation are not demonstrated. ImportantCandidate is YES because an attacker-influenced invalid pointer is consumed by privileged BPF memory-access machinery and the delegated trust boundary remains credible but unresolved. AutoDowngradeAllowed is NO because automatically treating this as crash-only could discard a real OOB read or write primitive. 5. One-sentence report phrase An unsigned bounds-check underflow can back an skb BPF dynptr with an out-of-range kernel pointer, creating a potential OOB read or write that requires mandatory call-site and BPF privilege review. 6. Manual review recommendation MANUAL CHECK REQUIRED Confirm the callers of `skb_pointer_if_linear`, especially writable dynptr slice operations, and determine whether the complete load, attach, packet-trigger, and data-exfiltration sequence is reachable below host-admin privilege. 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: bpf: Fix bounds check for skb-backed dynptrs [ Upstream Commit: 1b923538598be768e9f2f258a77a083d00e0c78b Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1b923538598be768e9f2f258a77a083d00e0c78b Changed files: include/linux/skbuff.h 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=1b923538598be768e9f2f258a77a083d00e0c78b ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98591 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98591 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:L/UI:N/S:U/C:L/I:L/A:H The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 7.8 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: 3 Paranoid ActionableScore: 5 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).