From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-80840][IMPORTANT] ipv6: seg6: clear IPv4 control block on IPIP decapsulation Date: Fri, 04 Sep 2026 13:06:35 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-80840 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: 10 X-AL-KERNEL-ActionableScore-Lower: 9 X-AL-KERNEL-Commit: 10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667 List-Id: CVE: CVE-2026-80840 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: YES Patch: ipv6: seg6: clear IPv4 control block on IPIP decapsulation Commit: 10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80840 Analysis date: Fri, 04 Sep 2026 13:06:35 -0400 ActionableScore: 10 ActionableScore lower bound: 9 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: A remote packet can cause stale IPv6 skb control-block fields to be interpreted as IPv4 option state after SRv6 IPIP decapsulation, leading to an attacker-influenced slab out-of-bounds write in `__ip_options_echo()` and a kernel crash with plausible broader memory-corruption impact. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-80840 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80840 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-2026-80840 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-80840 CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-665;CWE-843;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '9.8';DESCR 'An IPv6 SRv6 End.DX4 or End.DT4 decapsulation path can pass an inner IPv4 packet to IPv4 processing without clearing the shared skb control block. Stale IP6CB fields from the outer IPv6 packet can be interpreted as IPCB IPv4 option state, causing tcp_v4_route_req and __ip_options_echo to copy attacker controlled option data past a small option buffer. A remote sender can trigger this with a crafted IPv6 packet that reaches a configured SRv6 local action and contains an IPIP inner TCP SYN. For the CVSS the PR:N because the attacker does not need a local account or authenticated access on the target, only network reachability to the affected SRv6 endpoint. Impact is kernel memory corruption with a demonstrated slab out of bounds write, so denial of service is immediate and confidentiality, integrity, or privilege escalation impact is plausible in worst case. SRv6 deployments are usually limited to provider, data center, or controlled routing domains rather than the public Internet, but exposed or reachable SRv6 endpoints should be treated as high risk.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 10) YES REMOTE WRITE KASAN OOB NETWORK SKB LINUS IPV6 KPANIC PACKET NO NO guess ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=10 ActionableScoreLower=9 ## 1. ActionableScore * Conservative score: 9 * Paranoid score: 10 * Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: ## 2. Signal breakdown * Remote reachable / network-triggerable: +2. A crafted IPv6 packet can reach the SRv6 End.DX4 or End.DT4 decapsulation path when such an endpoint is configured. * Memory corruption, strong corruption primitive: +2. The patch context describes a demonstrated KASAN slab-out-of-bounds write in `__ip_options_echo()`, with a 255-byte copy into a 40-byte option-data area. * Strong semantic reinterpretation primitive: +2. Attacker-influenced IPv6 control-block state is later consumed as IPv4 IPCB option state because `skb->cb` is shared between IP6CB and IPCB. * Paranoid semantic/lifetime impact: +1. The reinterpreted fields affect active IPv4 option processing and memory-copy length behavior. * Reliable kernel crash / strong DoS: +1. Reproducers for both End.DX4 and End.DT4 produced slab OOB writes and a kernel crash condition. * Confidentiality impact plausible: +1. Kernel heap corruption can plausibly lead to disclosure in worst-case analysis, although no direct leak is demonstrated. * Integrity impact plausible: +1. The OOB write is a concrete kernel memory corruption primitive. * Common networking packet-processing path: +1. The vulnerable path is in packet decapsulation and TCP IPv4 request processing, although gated by SRv6 configuration. * Rare AND difficult-to-reach subsystem/configuration: -1. Practical exposure requires SRv6 local actions such as End.DX4 or End.DT4. Conservative total: 9. Paranoid total: 10. ## 3. Reachability analysis The bug is triggerable by a remote sender if the target is configured as an SRv6 endpoint using End.DX4, End.DT4, or the IPv4 arm of End.DT46. No local account or authentication on the victim is required once the affected SRv6 path is reachable. This is not a default exposure for ordinary Linux hosts, and SRv6 deployments are usually limited to provider, data-center, or controlled routing domains. However, in an exposed SRv6 deployment the trigger is packet-driven and does not require a local namespace or container privilege boundary. Call-site confidence: high. The supplied input includes the relevant End.DX4 and End.DT4 call paths down to `__ip_options_echo()` and a concrete KASAN OOB write result. ## 4. Severity interpretation This behaves like an Important-class kernel networking memory-corruption issue, not an ordinary Moderate DoS. The realistic demonstrated impact is remote kernel crash through slab out-of-bounds write. The theoretical worst case includes confidentiality and integrity impact because the primitive is an attacker-influenced kernel heap overwrite in a packet-processing path. Exploitability is constrained by the need for SRv6 local action configuration, but exposed SRv6 endpoints should not be auto-closed as low-risk. ## 5. One-sentence report phrase A remote packet can cause stale IPv6 skb control-block fields to be interpreted as IPv4 option state after SRv6 IPIP decapsulation, leading to an attacker-influenced slab out-of-bounds write in `__ip_options_echo()` and a kernel crash with plausible broader memory-corruption impact. ## 6. Manual review recommendation MANUAL CHECK REQUIRED. Reason: this is a remotely reachable kernel OOB write with demonstrated KASAN evidence, attacker-influenced semantic reinterpretation of shared skb control-block data, and plausible C/I/A impact beyond simple DoS in exposed SRv6 deployments. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: ipv6: seg6: clear IPv4 control block on IPIP decapsulation Commit: 10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667 Commit description: End.DX4 and End.DT4 decapsulate an IPv4 packet through decap_and_validate() and send it directly to IPv4 routing. The inner packet therefore bypasses ip_rcv_core(), which normally clears IPCB before IPv4 interprets skb->cb. The skb instead retains IP6CB data from the outer packet. IP6CB and IPCB use the same skb->cb storage, so IP6CB(skb)->lastopt overlaps IPCB(skb)->opt.optlen and srr, while IP6CB(skb)->nhoff overlaps rr and ts. The sender can make the stale optlen byte nonzero with a valid outer extension-header chain. The reproducers put an eight-byte Destination Options header immediately after the 40-byte IPv6 header and before the Segment Routing Header. ipv6_destopt_rcv() records the sender-controlled Destination Options offset in both lastopt and nhoff, setting them to 40. On the reproduced little-endian x86-64 kernel, IPv4 therefore sees optlen = 40 and rr = 40. Both tcp_v4_save_options() and __ip_options_echo() skip option copying when optlen is zero. Here optlen is 40, so the TCP SYN path allocates room for 40 bytes of option data and calls __ip_options_echo(). The stale rr value makes that function read inner packet byte 41 as the Record Route option length. The reproducers set that sender-controlled byte to 255, so __ip_options_echo() copies 255 bytes into the 40-byte option-data area. Separate End.DX4 and End.DT4 reproducers on the unpatched v7.2-rc5 kernel both produced: BUG: KASAN: slab-out-of-bounds in __ip_options_echo() Write of size 255 The relevant End.DX4 call path is: __ip_options_echo tcp_v4_route_req tcp_conn_request tcp_v4_conn_request tcp_rcv_state_process tcp_v4_do_rcv tcp_v4_rcv ip_protocol_deliver_rcu ip_local_deliver_finish ip_local_deliver input_action_end_dx4_finish input_action_end_dx4 The relevant End.DT4 call path is: __ip_options_echo tcp_v4_route_req tcp_conn_request tcp_v4_conn_request tcp_rcv_state_process tcp_v4_do_rcv tcp_v4_rcv ip_protocol_deliver_rcu ip_local_deliver_finish ip_local_deliver input_action_end_dt4 tcp_v4_save_options() is inlined into the tcp_v4_route_req() path, so it does not appear as a separate frame. When decap_and_validate() handles IPPROTO_IPIP, save the ingress interface from IP6CB, clear IPCB, and restore the saved value. Doing this in the common decapsulation path covers End.DX4, End.DT4, and End.DT46's IPv4 arm. Use IP6CB(skb)->iif rather than skb->skb_iif. These actions run after l3mdev processing, which can replace skb_iif with the L3 master; IP6CB iif still records the receiving interface set at IPv6 ingress. Fixes: 891ef8d ("ipv6: sr: implement additional seg6local actions") Cc: stable@vger.kernel.org Suggested-by: Andrea Mayer Signed-off-by: Kyle Zeng Co-developed-by: David Lee Signed-off-by: David Lee Reviewed-by: Andrea Mayer [Commit description truncated; see the upstream URL below] Changed files: net/ipv6/seg6_local.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=10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-80840 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80840 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:H/I:H/A:H The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 9.8 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: 9 Paranoid ActionableScore: 10 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).