From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-72036][MODERATE REGULAR] net/sched: sch_multiq: Replace direct dequeue call with peek and qdisc_dequeue_peeked Date: Sun, 16 Aug 2026 05:19:27 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-72036 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: 1 X-AL-KERNEL-Commit: 3e5fd9d14f2d228e7260251f2e4a1d41ba8f705a List-Id: CVE: CVE-2026-72036 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: net/sched: sch_multiq: Replace direct dequeue call with peek and qdisc_dequeue_peeked Commit: 3e5fd9d14f2d228e7260251f2e4a1d41ba8f705a Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3e5fd9d14f2d228e7260251f2e4a1d41ba8f705a Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72036 Analysis date: Sun, 16 Aug 2026 05:19:27 -0400 ActionableScore: 3 ActionableScore lower bound: 1 Actionable bucket: Borderline. Manual review recommended Manual review required: YES Summary: sch_multiq can desynchronize child qdisc state by directly dequeuing after peek, allowing specific traffic-control configurations to trigger a NULL pointer dereference and kernel panic during ordinary egress traffic. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-72036 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72036 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-2026-72036 MODERATE 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-476;*CWE-664;CWE-754;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference in sch_multiq can occur because multiq_dequeue peeks a child qdisc and then calls the child dequeue path directly instead of consuming the peeked skb. With a non-work-conserving child qdisc, the peeked skb can be stashed in gso_skb while the direct dequeue returns a different skb, leaving qlen and backlog state out of sync. Affected configurations can panic the kernel from softirq during ordinary egress traffic when a peeking parent and qfq style child are combined. For the CVSS the PR:L is used in the paranoid score because some deployments delegate CAP_NET_ADMIN to containers or service accounts, or an already configured vulnerable qdisc hierarchy may let a local user trigger traffic through the broken path. The issue is not directly network reachable as a standalone remote attack because the risky qdisc hierarchy normally has to be configured locally. Impact is denial of service via kernel crash, with no concrete evidence of confidentiality or integrity impact.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline. Manual review recommended (with actual score 2) YES REMOTE DANGER SIMPLEFIX NETWORK SKB LINUS QDISC PACKET DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 ActionableScoreLower=1 ## 1. ActionableScore * Conservative score: 1 * Paranoid score: 3 * Final recommended bucket: **Borderline. Manual review recommended**:: ## 2. Signal breakdown Conservative signals: * Reliable kernel crash / strong DoS: +1 The commit explicitly describes NULL dereference and kernel panic from softirq. * Common networking packet-processing path: +1 The bug is in net/sched egress qdisc dequeue handling. * Requires CAP_NET_ADMIN in typical deployments: -2 Creating the risky qdisc hierarchy normally requires traffic-control privileges. * Availability impact realistic: +1 The described outcome is a host kernel panic, not only a local process failure. Total conservative: 1 Paranoid signals: * Local unprivileged trigger: +1 If the vulnerable qdisc hierarchy is already configured, ordinary local egress traffic may trigger the crash. * Reliable kernel crash / strong DoS: +1 The failure mode is a NULL pointer dereference leading to panic. * Common networking packet-processing path: +1 The affected logic is in the packet scheduling transmit path. * Availability impact realistic: +1 Panic from softirq is system-wide availability impact. * Rare or special configuration: -1 The issue needs a specific qdisc combination involving multiq, a non-work-conserving child, and a peeking parent such as tbf/qfq. Total paranoid: 3 No memory-corruption or LPE signals are awarded. The supported primitive is qdisc state desynchronization followed by NULL pointer dereference, not UAF, OOB write, type confusion, arbitrary write, or controlled object reuse. ## 3. Reachability analysis Typical triggering requires CAP_NET_ADMIN to configure the affected qdisc hierarchy. In containerized or delegated networking environments, CAP_NET_ADMIN may be available to less trusted service accounts or container root, so the paranoid interpretation treats this as more reachable than full host-root only. The issue is not directly remote-network-triggerable as a standalone attack. Remote packets alone do not normally configure the required qdisc stack, but traffic may exercise the broken dequeue path after local configuration exists. The path is not default in the sense of ordinary qdisc setups. It depends on a specific composition of peeking and non-work-conserving qdiscs. Call-site confidence: high, because the patch directly changes the affected dequeue call and the commit explains the qdisc interaction. ## 4. Severity interpretation This behaves like a configuration-dependent local DoS, not an Important-class memory corruption issue. The realistic impact is kernel crash through NULL dereference. Theoretical escalation is not supported by the patch because there is no demonstrated stale object reuse, write-after-free, callback control, refcount takeover, or attacker-controlled corruption primitive. This should not be treated as a high-priority LPE candidate, but it is worth manual review where CAP_NET_ADMIN is delegated or complex traffic shaping is used. ## 5. One-sentence report phrase sch_multiq can desynchronize child qdisc state by directly dequeuing after peek, allowing specific traffic-control configurations to trigger a NULL pointer dereference and kernel panic during ordinary egress traffic. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Manual review is recommended for products that delegate CAP_NET_ADMIN, use containers with traffic-control permissions, or deploy complex qdisc hierarchies. For default systems without such delegation or qdisc configuration, this is closer to ordinary Moderate / local privileged DoS. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: net/sched: sch_multiq: Replace direct dequeue call with peek and qdisc_dequeue_peeked Commit: 3e5fd9d14f2d228e7260251f2e4a1d41ba8f705a Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3e5fd9d14f2d228e7260251f2e4a1d41ba8f705a Commit description: multiq_dequeue() takes a packet from a band's child with a direct ->dequeue() call after multiq_peek() peeked it. When the child is non-work-conserving the peek stashes the skb in the child's gso_skb, so the direct dequeue returns a different skb and orphans the stash, desyncing the child's qlen/backlog. With a qfq child reached through a peeking parent (e.g. tbf) this re-enters the child on an emptied list and dereferences NULL, panicking the kernel from softirq on ordinary egress. Take the packet through qdisc_dequeue_peeked(), as sch_prio already does and as sch_red and sch_sfb were just fixed to do. The helper is a no-op when the child has no stash, so a work-conserving child is unaffected. Fixes: 77be155 ("pkt_sched: Add peek emulation for non-work-conserving qdiscs.") Cc: stable@vger.kernel.org Signed-off-by: Bryam Vargas Reviewed-by: Victor Nogueira Acked-by: Jamal Hadi Salim Link: https://patch.msgid.link/20260625-b4-disp-31bcb279-v1-2-85c40b83c529@proton.me Signed-off-by: Jakub Kicinski Signed-off-by: Greg Kroah-Hartman Changed files: net/sched/sch_multiq.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=3e5fd9d14f2d228e7260251f2e4a1d41ba8f705a ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-72036 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72036 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:H/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: 1 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).