From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-90073][MODERATE REGULAR] net/sched: hhf: clamp quantum before hhf_change() to avoid overflow [ Upstream Date: Thu, 17 Sep 2026 17:39:40 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-90073 X-AL-KERNEL-Priority: MODERATE REGULAR X-AL-KERNEL-Severity: MODERATE REGULAR X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 3 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 2e0f0729f922c8de13801004e2131b57646c7c5f List-Id: CVE: CVE-2026-90073 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: net/sched: hhf: clamp quantum before hhf_change() to avoid overflow [ Upstream Commit: 2e0f0729f922c8de13801004e2131b57646c7c5f Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2e0f0729f922c8de13801004e2131b57646c7c5f Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90073 Analysis date: Thu, 17 Sep 2026 17:39:40 -0400 ActionableScore: 3 ActionableScore lower bound: unknown Actionable bucket: Borderline / manual review recommended Manual review required: YES Summary: HHF qdisc can accept an oversized MTU-derived quantum that overflows signed deficit accounting in hhf_dequeue and causes an infinite dequeue spin, requiring namespace scoped CAP_NET_ADMIN and resulting in local denial of service only. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-90073 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90073 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 REGULAR 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-2026-90073 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90073 CHECK WITH IMPACT FROM ORIG NN MODERATE 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-190;*CWE-835;**CWE-400;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'HHF qdisc initialization can accept an oversized quantum derived from a very large device MTU. When this value is later multiplied by the HHF weight, the signed deficit calculation in hhf_dequeue can overflow and the dequeue path may spin forever. This is a local control plane issue rather than a packet driven remote network attack. For the CVSS the PR:L is used for the paranoid score because CAP_NET_ADMIN inside a user namespace may be available to a local unprivileged user on affected systems. Impact is denial of service through high CPU usage or soft lockup. Confidentiality and integrity impact are not assigned because the patch indicates an integer overflow leading to an infinite loop, not a memory corruption primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) YES REMOTE DANGER SIMPLEFIX NETWORK HARDWARE QDISC DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=YES ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES ## 1. ActionableScore * Raw conservative score: 3 * Conservative score: 3 * Raw paranoid score: 3 * Paranoid score: 3 * Final recommended bucket: **Borderline / manual review recommended**:: No floor or cap was applied. ## 2. Signal breakdown Positive signals: * +1 Reliable strong DoS: oversized `q->quantum` derived from huge MTU can make `weight * quantum` overflow signed deficit handling in `hhf_dequeue()`, causing an infinite dequeue spin. * +1 Below-host-admin namespace-scoped reachability: supplied context states the reproducer requires `CAP_NET_ADMIN` in a user namespace, not host-admin-equivalent privilege. * +1 Attacker-controlled active qdisc configuration: attacker can configure the relevant HHF qdisc and MTU/quantum state in the affected namespace so the invalid arithmetic state reaches active scheduler processing. Non-awarded signals: * RealUAF not awarded: no stale pointer, object lifetime violation, double free, or refcount misuse is shown. * Memory corruption not awarded: the patch shows integer overflow leading to infinite loop, not OOB write, UAF, controlled overwrite, or arbitrary write. * Confidentiality and integrity not awarded: no disclosure path or unauthorized modification primitive is established. * Remote reachability not awarded: this is local control-plane configuration via `tc`/qdisc, not packet-driven remote exploitation. * Strong LPE or weak LPE not awarded: no controlled reclaim, object replacement, credential impact, callback control, or security-boundary bypass is shown. * Extra availability point not awarded: the infinite spin/soft lockup is already counted as the DoS consequence. Repeated release/refcount checks: * First release can end lifetime: no. * Exact same reference survives: no. * Second lifetime/refcount mutation: no. * Concrete repeated release/refcount +1 awarded: no. * Paranoid repeated-refcount leverage +1 awarded: no. * FLOOR 5 applied: no. ## 3. Reachability analysis The trigger is local and requires configuring a network device with an extreme MTU and attaching the HHF qdisc. The supplied context explicitly says this requires `CAP_NET_ADMIN` in a user namespace, which supports below-host-admin reachability on systems where unprivileged user namespaces and namespace-local network setup are available. The relevant operations are qdisc setup and device MTU manipulation. These are control-plane operations, not packet-processing inputs from a remote host. The complete trigger appears namespace-scoped based on the supplied statement, although real product exposure still depends on whether unprivileged user namespaces are enabled and whether the needed device/qdisc support is available. No special physical hardware is required. A dummy-like device with permissive `max_mtu` is sufficient according to the supplied context. This is not a race. The failure is deterministic once the invalid MTU-derived quantum state is installed. ## 4. Primitive and severity interpretation The primitive is an integer overflow in scheduler arithmetic: `q->quantum` can be derived from an extremely large MTU, and multiplying it by HHF weight can overflow the signed deficit logic. The demonstrated consequence is an infinite spin in `hhf_dequeue()`, causing local denial of service through CPU soft lockup or similar host responsiveness loss. This is Class C because the demonstrated behavior is DoS/invalid-state behavior without an independently useful corruption primitive. There is no UAF, no active stale object, no repeated lifetime mutation, no controlled reclaim, and no arbitrary read/write. ImportantCandidate is NO because the patch does not establish memory corruption, security-boundary bypass, filesystem ownership violation, or unresolved lifetime/refcount exploitation leverage. AutoDowngradeAllowed is YES because this remains a DoS-only arithmetic/control-plane issue, although product-specific namespace exposure should be checked before closing. Likely operational severity is Moderate-like, with higher practical relevance on systems that allow unprivileged users to create user/network namespaces and obtain namespace-scoped `CAP_NET_ADMIN`. ## 5. One-sentence report phrase HHF qdisc can accept an oversized MTU-derived quantum that overflows signed deficit accounting in hhf_dequeue and causes an infinite dequeue spin, requiring namespace scoped CAP_NET_ADMIN and resulting in local denial of service only. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Reason: no memory corruption or LPE primitive is established, but namespace-scoped `CAP_NET_ADMIN` reachability and host-impacting CPU DoS should be checked against the target product policy for unprivileged user namespaces and qdisc availability. ## 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: net/sched: hhf: clamp quantum before hhf_change() to avoid overflow [ Upstream Commit: 2e0f0729f922c8de13801004e2131b57646c7c5f Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2e0f0729f922c8de13801004e2131b57646c7c5f Changed files: net/sched/sch_hhf.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=2e0f0729f922c8de13801004e2131b57646c7c5f ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-90073 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90073 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: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: unknown Paranoid ActionableScore: 3 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: NO Published priority for this report (same as in Subject): MODERATE REGULAR 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).