From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-74544][MODERATE REGULAR] net/sched: cls_u32: validate offshift to prevent shift-out-of-bounds [ Upstream Date: Sat, 15 Aug 2026 10:54:19 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-74544 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: 2 X-AL-KERNEL-Commit: 313cb9ffc4101a885d791facf4b5ea3d5e06144d List-Id: CVE: CVE-2026-74544 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: net/sched: cls_u32: validate offshift to prevent shift-out-of-bounds [ Upstream Commit: 313cb9ffc4101a885d791facf4b5ea3d5e06144d Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=313cb9ffc4101a885d791facf4b5ea3d5e06144d Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74544 Analysis date: Sat, 15 Aug 2026 10:54:19 -0400 ActionableScore: 3 ActionableScore lower bound: 2 Actionable bucket: Ordinary Moderate / Low-like, paranoid borderline only Manual review required: NO Summary: A local user who can create cls_u32 filters, including via user/network namespaces, can set an invalid offshift value that later triggers shift-out-of-bounds behavior during packet classification, causing an UBSAN report or possible kernel DoS on panic-configured systems. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74544 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74544 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: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-74544 MODERATE CHECK WITH IMPACT FROM ORIG NN IMPORTANT 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-1335;*CWE-20;CWE-682;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A shift-out-of-bounds bug in cls_u32 can occur because the user controlled tc_u32 offshift value was accepted without a bounds check and later used by u32_classify() when TC_U32_VAROFFSET is set. A local user who can create tc u32 filters can choose an invalid offshift and then send traffic through the classifier to trigger undefined shift behavior and a UBSAN report. For the CVSS the PR:L is used because reliable triggering requires local code or a process with CAP_NET_ADMIN in a network namespace, which may be reachable by unprivileged users when user/network namespaces are enabled. The issue is not directly network reachable because remote traffic alone cannot install the malicious classifier. Impact is primarily denial of service via warning or kernel crash on systems configured to panic on UBSAN or warnings. No concrete UAF, OOB write, information leak, or privilege escalation primitive is evident from the patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like, paranoid borderline only (with actual score 3) YES REMOTE WRITE KASAN OOB SIMPLEFIX NETWORK PACKET LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 ActionableScoreLower=2 ## 1. ActionableScore * Conservative score: 2 * Paranoid score: 3 * Final recommended bucket: **Ordinary Moderate / Low-like, paranoid borderline only**:: ## 2. Signal breakdown Conservative signals: * Local unprivileged trigger: +1. The commit explicitly says this can be triggered by an unprivileged user via user/network namespaces. * Availability impact realistic: +1. The demonstrated effect is UBSAN shift-out-of-bounds and possible warning/crash behavior, mainly DoS-like. Paranoid additional signal: * Reliable kernel crash / strong DoS: +1. Only in the paranoid interpretation, systems configured with UBSAN trap, panic_on_warn, or similar hardening/debug policies may convert the warning into a kernel panic. Signals not applied: * Remote reachable: +0. Remote packets alone cannot install the malicious cls_u32 filter. * Generic memory corruption: +0. The patch shows shift-out-of-bounds undefined behavior, but no UAF, OOB write, double free, stale pointer, type confusion, or arbitrary write. * Privilege escalation plausible: +0. No supported LPE primitive is visible. * Confidentiality / Integrity: +0. No leak or write/corruption primitive is demonstrated. * Requires admin/root penalty: not applied conservatively because user/network namespaces may allow an unprivileged local user to create the needed network context. ## 3. Reachability analysis The bug is triggered through tc cls_u32 filter creation with an invalid offshift value, followed by packet classification through u32_classify() with TC_U32_VAROFFSET set. In normal host networking this usually requires CAP_NET_ADMIN, but with unprivileged user and network namespaces enabled, a local unprivileged user may gain CAP_NET_ADMIN inside a new netns and reach the vulnerable path. The issue is not directly network-triggerable because the attacker must first configure the classifier locally. The packet-processing step is only the second stage that exercises the invalid shift. The path is not default-active unless such a tc filter is installed. Call-site confidence: high, because the commit directly identifies u32_change() as the input path and u32_classify() as the later trigger path. ## 4. Severity interpretation This behaves like an ordinary Moderate / Low-like local DoS issue, not an Important-class kernel vulnerability. The theoretical concern is C undefined behavior from an excessive shift exponent, but the concrete evidence is an UBSAN report and possible crash under specific kernel policies. There is no demonstrated memory corruption, no attacker-controlled overwrite, no information disclosure, and no privilege-escalation path. The paranoid score is only borderline because local namespace reachability plus panic-on-warning style configurations can make the DoS more actionable. ## 5. One-sentence report phrase A local user who can create cls_u32 filters, including via user/network namespaces, can set an invalid offshift value that later triggers shift-out-of-bounds behavior during packet classification, causing an UBSAN report or possible kernel DoS on panic-configured systems. ## 6. Manual review recommendation NO MANUAL CHECK NEEDED Reason: this is a validation-only fix for a local shift-out-of-bounds condition with no supported memory corruption or privilege-escalation primitive. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: net/sched: cls_u32: validate offshift to prevent shift-out-of-bounds [ Upstream Commit: 313cb9ffc4101a885d791facf4b5ea3d5e06144d Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=313cb9ffc4101a885d791facf4b5ea3d5e06144d Changed files: net/sched/cls_u32.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=313cb9ffc4101a885d791facf4b5ea3d5e06144d ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74544 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74544 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: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: 2 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).