From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-74743][IMPORTANT] macvlan: inherit needed_headroom and needed_tailroom from lowerdev [ Upstream Date: Wed, 26 Aug 2026 11:09:37 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-74743 X-AL-KERNEL-Priority: IMPORTANT X-AL-KERNEL-Severity: IMPORTANT X-AL-KERNEL-Base-Severity: IMPORTANT X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 7 X-AL-KERNEL-ActionableScore-Lower: 5 X-AL-KERNEL-Commit: 8cd90e850e434577bf6774778657d26d6995e53f List-Id: CVE: CVE-2026-74743 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: YES Patch: macvlan: inherit needed_headroom and needed_tailroom from lowerdev [ Upstream Commit: 8cd90e850e434577bf6774778657d26d6995e53f Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8cd90e850e434577bf6774778657d26d6995e53f Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74743 Analysis date: Wed, 26 Aug 2026 11:09:37 -0400 ActionableScore: 7 ActionableScore lower bound: 5 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: macvlan failed to inherit lowerdev needed_headroom and needed_tailroom, allowing packets on specific stacked network configurations to be built with insufficient skb space and potentially causing skb underflow, KASAN slab-use-after-free, and kernel crash. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74743 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74743 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: IMPORTANT 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-2026-74743 IMPORTANT CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;*CWE-416;CWE-119;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'macvlan devices did not inherit needed_headroom and needed_tailroom from the lower device even though hard_header_len was inherited. On lower devices that require extra packet space such as MACsec, IPsec, WireGuard, tunnels, or veth with RX headroom, upper layers may build skb buffers without enough headroom or tailroom. This can lead to skb headroom underflow or KASAN slab-use-after-free crashes when headers are prepended or trailers are appended. For the CVSS the PR:N is used for the paranoid score because a network peer may trigger packet processing on an already configured vulnerable macvlan stack without local credentials, although the configuration itself is non-default. The issue is best treated as adjacent or internal network reachable rather than public Internet reachable. Impact is at least denial of service via kernel crash and worst case may allow confidentiality or integrity impact due to memory corruption, so manual review is needed.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) YES OOB DANGER UAF NETWORK SKB KERNEL_PANIC_PLUS_UAF HARDWARE KPANIC PACKET NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=7 ActionableScoreLower=5 ## 1. ActionableScore * Conservative score: 5 * Paranoid score: 7 * Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: ## 2. Signal breakdown Conservative signals: * Remote or adjacent packet-triggerable: +2 The bug is in macvlan packet buffer sizing and can be reached through packet processing once the vulnerable macvlan over lowerdev configuration exists. This is better treated as adjacent or internal network reachable, not public Internet reachable by default. * Generic memory corruption: +2 The commit explicitly mentions skb headroom underflows and KASAN slab-use-after-free crashes. This is more than a pure WARN or resource leak. * Memory layout invariant restoration: +1 The patch restores consistency between lowerdev header or trailer space requirements and macvlan skb layout assumptions. * Reliable kernel crash / strong DoS: +1 KASAN slab-use-after-free crashes are a concrete kernel crash signal. * Common networking packet-processing path: +1 macvlan is a networking data path device and the failure occurs during skb header or tailroom handling. Negative or limiting signals: * Non-default configuration / special lowerdev requirements: -1 Triggering depends on macvlan over a lower device needing extra headroom or tailroom, such as MACsec, IPsec, WireGuard, tunnels, or veth RX headroom. * Constrained corruption / limited demonstrated exploitability: -1 The patch supports underflow or UAF crash, but does not show attacker-controlled reclaim, arbitrary write, callback control, or a demonstrated LPE primitive. Paranoid additions: * Weak LPE concern: +1 Because the affected primitive is skb memory corruption with KASAN UAF symptoms in a networking path, privilege escalation cannot be excluded without manual review, although it is not demonstrated. * Real lifetime corruption: +1 The observed failure mode includes slab-use-after-free, even if the root cause is incorrect skb space accounting. ## 3. Reachability analysis A fully unconfigured system is not exposed. The system needs a macvlan device stacked on a lower device that requires nonzero needed_headroom or needed_tailroom. Creating or changing that setup normally requires CAP_NET_ADMIN, but triggering packet processing after setup may be possible by an unprivileged local workload, a container workload using the macvlan interface, or an adjacent/internal network peer. Namespaces and containers matter because macvlan is commonly used for container or virtualization networking. A tenant or container may not be able to create the vulnerable topology, but may still be able to send traffic through it once provisioned by the host or orchestrator. Realistic exploitation is configuration-dependent. The demonstrated impact is kernel crash. The memory corruption indicators justify manual review, but the patch does not demonstrate a controlled overwrite, object replacement, or reliable privilege escalation chain. Call-site confidence: medium-high. The patch is small, but the commit message identifies concrete affected operations, dev_hard_header, macvlan_hard_header, and lowerdev tailroom append behavior. ## 4. Severity interpretation This is stronger than an ordinary Moderate because it is not merely a validation bug or resource leak. The bug affects skb layout invariants in a packet-processing path and has concrete KASAN slab-use-after-free crash evidence. Realistically, the clearest impact is DoS under specific macvlan plus lowerdev configurations. Theoretical memory corruption potential is higher than the demonstrated exploitability, so it should be treated as Actionable Moderate at minimum and a Strong Important candidate for manual triage, rather than auto-closed. ## 5. One-sentence report phrase macvlan failed to inherit lowerdev needed_headroom and needed_tailroom, allowing packets on specific stacked network configurations to be built with insufficient skb space and potentially causing skb underflow, KASAN slab-use-after-free, and kernel crash. ## 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: the patch context explicitly includes skb headroom underflow and KASAN slab-use-after-free in a networking packet path. Even though reliable LPE is not demonstrated, the memory corruption indicators and possible adjacent/internal packet trigger make this unsuitable for automatic closure. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: macvlan: inherit needed_headroom and needed_tailroom from lowerdev [ Upstream Commit: 8cd90e850e434577bf6774778657d26d6995e53f Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8cd90e850e434577bf6774778657d26d6995e53f Changed files: drivers/net/macvlan.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=8cd90e850e434577bf6774778657d26d6995e53f ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74743 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74743 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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 7.5 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: 5 Paranoid ActionableScore: 7 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: IMPORTANT KPANIC detected for this report: YES Published priority for this report (same as in Subject): IMPORTANT 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).