From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-31669][IMPORTANT] mptcp: fix slab-use-after-free in __inet_lookup_established Date: Fri, 07 Aug 2026 12:00:39 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-31669 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: f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47 List-Id: CVE: CVE-2026-31669 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: YES Patch: mptcp: fix slab-use-after-free in __inet_lookup_established Commit: f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31669 Analysis date: Fri, 07 Aug 2026 12:00:39 -0400 ActionableScore: 7 ActionableScore lower bound: 5 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: A network-reachable slab-use-after-free in the MPTCP IPv6 subflow path can occur because child sockets may be allocated without `SLAB_TYPESAFE_BY_RCU`, allowing lockless established-socket lookups to access freed memory and potentially crash the kernel. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-31669 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31669 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-2026-31669 IMPORTANT CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A slab use-after-free in the MPTCP IPv6 subflow path can occur because tcpv6_prot_override copied tcpv6_prot before tcpv6_prot had a registered SLAB_TYPESAFE_BY_RCU slab cache. MPTCP v6 child sockets may then be allocated from kmalloc-4k without RCU type safe lifetime guarantees, while lockless ehash lookups in __inet_lookup_established still assume socket memory remains stable during rcu_read_lock. A remote peer can potentially trigger the race by creating and closing MPTCP IPv6 subflows while concurrent packets cause established socket lookups. For the CVSS the PR:N is used because no local account is required when an attacker can reach an exposed MPTCP IPv6 listener. The issue is network reachable but depends on MPTCP IPv6 being enabled and reachable, and the race timing makes AC:H appropriate. Impact is at least denial of service via kernel crash and in the paranoid score may include confidentiality and integrity impact due to a real UAF primitive in network socket memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) MAYBE REMOTE DANGER INIT UAF NETWORK LINUS IPV6 KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS - - 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 scoring: * Remote reachable / network-triggerable: **+2** MPTCP IPv6 subflows and established socket lookups can be influenced by a remote peer when an MPTCP IPv6 endpoint is reachable. * Memory corruption, weak/indirect corruption candidate: **+1** The bug is a real slab-use-after-free, but the patch does not show attacker-controlled reclaim, overwrite, callback control, or a reliable object replacement primitive. * Real lifetime corruption: **+1** The root cause is incorrect socket allocation lifetime semantics: MPTCP v6 child sockets are allocated from kmalloc-4k without `SLAB_TYPESAFE_BY_RCU`, while lockless ehash lookups assume RCU-safe socket lifetime. * Reliable kernel crash / strong DoS: **+1** The commit explicitly states that concurrent ehash lookup can access freed memory and trigger slab-use-after-free in `__inet_lookup_established`. * Hard or unreliable race / special timing required: **-1** Triggering depends on a race between socket free/reuse and concurrent lockless established-hash lookup. * Common networking core path or packet-processing path: **+1** The affected failure occurs in `__inet_lookup_established`, a central TCP socket lookup path. Paranoid scoring differences: * Memory corruption, strong corruption primitive: **+2 instead of +1** In the paranoid interpretation, this is treated as a stronger UAF because freed socket memory can be immediately reused while still reachable from lockless networking lookup. * Weak LPE concern: **+1** Not because privilege escalation is demonstrated, but because a network-reachable UAF in socket memory historically deserves manual review for possible object-reuse consequences. ## 3. Reachability analysis A remote peer can potentially trigger the bug by interacting with an exposed MPTCP IPv6 endpoint and causing MPTCP subflow creation and teardown while packets drive concurrent established-socket lookups. No local account is required if the vulnerable MPTCP IPv6 service is reachable, so the practical PR interpretation is **PR:N** for exposed services. The issue is configuration-dependent: MPTCP and IPv6 must be enabled and an MPTCP-capable listener must be reachable. It is not necessarily Internet-exposed in typical deployments, but it is a network-facing kernel socket lifetime bug once MPTCP IPv6 is in use. Namespaces do not materially reduce the remote-trigger concern, though containerized services using host networking or exposed MPTCP endpoints could increase practical exposure. Call-site confidence: **high**, because the commit directly identifies the affected lookup path, allocation source, missing `SLAB_TYPESAFE_BY_RCU`, and the UAF site in `__inet_lookup_established`. ## 4. Severity interpretation This should not be treated as an ordinary Moderate or auto-closed DoS-only issue. Realistic demonstrated impact is likely kernel crash or host DoS due to slab-use-after-free under race conditions. Theoretical impact is higher because the bug is a real lifetime violation in network socket memory, but there is no demonstrated arbitrary write, controlled reclaim, type confusion, or reliable privilege-escalation primitive in the patch. Conservative handling: **Actionable Moderate**. Paranoid/manual-review handling: **Strong Important candidate**. ## 5. One-sentence report phrase A network-reachable slab-use-after-free in the MPTCP IPv6 subflow path can occur because child sockets may be allocated without `SLAB_TYPESAFE_BY_RCU`, allowing lockless established-socket lookups to access freed memory and potentially crash the kernel. ## 6. Manual review recommendation **MANUAL CHECK REQUIRED** Reason: this is a network-facing race-dependent UAF in a central socket lookup path. Even if the most realistic impact is DoS, the lifetime corruption primitive is strong enough to require human review before closure. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: mptcp: fix slab-use-after-free in __inet_lookup_established Commit: f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47 Commit description: The ehash table lookups are lockless and rely on SLAB_TYPESAFE_BY_RCU to guarantee socket memory stability during RCU read-side critical sections. Both tcp_prot and tcpv6_prot have their slab caches created with this flag via proto_register(). However, MPTCP's mptcp_subflow_init() copies tcpv6_prot into tcpv6_prot_override during inet_init() (fs_initcall, level 5), before inet6_init() (module_init/device_initcall, level 6) has called proto_register(&tcpv6_prot). At that point, tcpv6_prot.slab is still NULL, so tcpv6_prot_override.slab remains NULL permanently. This causes MPTCP v6 subflow child sockets to be allocated via kmalloc (falling into kmalloc-4k) instead of the TCPv6 slab cache. The kmalloc-4k cache lacks SLAB_TYPESAFE_BY_RCU, so when these sockets are freed without SOCK_RCU_FREE (which is cleared for child sockets by design), the memory can be immediately reused. Concurrent ehash lookups under rcu_read_lock can then access freed memory, triggering a slab-use-after-free in __inet_lookup_established. Fix this by splitting the IPv6-specific initialization out of mptcp_subflow_init() into a new mptcp_subflow_v6_init(), called from mptcp_proto_v6_init() before protocol registration. This ensures tcpv6_prot_override.slab correctly inherits the SLAB_TYPESAFE_BY_RCU slab cache. Fixes: b19bc29 ("mptcp: implement delegated actions") Cc: stable@vger.kernel.org Signed-off-by: Jiayuan Chen Reviewed-by: Matthieu Baerts (NGI0) Link: https://patch.msgid.link/20260406031512.189159-1-jiayuan.chen@linux.dev Signed-off-by: Jakub Kicinski Signed-off-by: Greg Kroah-Hartman Changed files: net/mptcp/protocol.c net/mptcp/protocol.h net/mptcp/subflow.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=f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-31669 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31669 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:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 8.1 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).