From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-80590][IMPORTANT] inet: frags: strip GSO state from fragments before reassembly Date: Fri, 28 Aug 2026 02:47:28 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-80590 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: cfdbc8c2e6f9ef5d8b8e54859da03dfe682b0bee List-Id: CVE: CVE-2026-80590 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: YES Patch: inet: frags: strip GSO state from fragments before reassembly Commit: cfdbc8c2e6f9ef5d8b8e54859da03dfe682b0bee Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cfdbc8c2e6f9ef5d8b8e54859da03dfe682b0bee Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80590 Analysis date: Fri, 28 Aug 2026 02:47:28 -0400 ActionableScore: 7 ActionableScore lower bound: 5 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: Untrusted GSO metadata on IP fragments can survive reassembly and make the resulting skb reach skb_segment with a non GRO shaped frag_list, allowing a local user or guest controlled tap path to trigger a reliable kernel panic. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-80590 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80590 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-2026-80590 IMPORTANT CHECK WITH IMPACT FROM ORIG NN LOW Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';*CWE-20;CWE-617;CWE-754;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.6';DESCR 'IP fragments can incorrectly retain untrusted GSO metadata supplied through virtio_net_hdr on tun tap or AF_PACKET with PACKET_VNET_HDR. After reassembly the resulting skb may still claim to be GSO while its frag_list is not GRO shaped, and a later software segmentation path such as udp_rcv_segment, validate_xmit_skb, or ip_finish_output_gso can reach BUG_ON checks in skb_segment. For the CVSS the PR:L is used because the practical trigger requires a local user or process able to create or use the relevant tun tap, AF_PACKET, netns, or guest networking path. The issue is not public Internet reachable by normal packet traffic, but it may be reachable from a local namespace setup or from a guest when the VMM forwards virtio_net_hdr to a host tap. Impact is at least local denial of service via kernel panic. In the paranoid score, limited confidentiality and integrity impact are kept for manual review because the bug violates skb_segment layout assumptions in core networking code, although the directly demonstrated outcome is DoS.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) SKIP CVE-2026-80590 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: href YES NO NO unknown MAYBE REMOTE KASAN NETWORK SKB KERNEL_PANIC_PLUS_UAF HARDWARE IPV6 KPANIC PACKET KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 - - 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: * Local unprivileged trigger: +1. The description states that two writes to a tap by an unprivileged user in its own userns are enough. * Memory layout invariant restoration: +1. The patch restores the invariant that an IP fragment queued for reassembly must not carry GSO state that later makes the reassembled skb look like GRO shaped input. * Reliable kernel crash / strong DoS: +1. The reproducer reaches BUG_ON in skb_segment and causes kernel panic because the path runs with BH disabled. * Availability impact realistic: +1. The crash is host wide and described as reproducible across IPv4 and IPv6 fragment layouts. * Common networking core path or packet-processing path: +1. The affected path involves inet fragment reassembly, skb GSO segmentation, local delivery, forwarding, nf_defrag, and shared IPv4/IPv6 reassembly helpers. Paranoid additional signals: * Cross-boundary isolation impact: +1. The issue may be reachable from a guest when the VMM forwards virtio_net_hdr to a host tap, making guest-to-host DoS plausible. * Broad/common exposure: +1. tun/tap, AF_PACKET with PACKET_VNET_HDR, defrag, and GSO are common Linux networking facilities, even though the exact trigger is not normal Internet packet traffic. Not awarded: * Remote reachable: +0. Normal external network packets cannot directly attach virtio_net_hdr GSO metadata. * Strong memory corruption: +0. The demonstrated primitive is BUG_ON/panic from invalid skb layout assumptions, not UAF, OOB write, double free, arbitrary write, or object replacement. * Privilege escalation plausible: +0 conservative. No concrete LPE primitive is shown. * Confidentiality / Integrity impact: +0 conservative. The patch and call path show reliable crash, not disclosure or modification of protected data. ## 3. Reachability analysis The primary trigger is local. A user may reach it through tun/tap with virtio_net_hdr, AF_PACKET with PACKET_VNET_HDR, or CAP_NET_RAW in a netns where defragmentation occurs before a GSO point. Namespaces matter because userns/netns setups can reduce the practical privilege requirement compared with full host root. The path is not public Internet reachable by ordinary fragmented IP traffic because the key condition is attacker supplied GSO metadata, not merely fragmentation. A guest-to-host variant is plausible when a VMM forwards virtio_net_hdr into a host tap. Call-site confidence: high. The report includes the concrete call trace through tun_get_user, ip_local_deliver, udp_rcv_segment, GSO segmentation, and skb_segment, and the patch modifies the shared fragment queue insertion helper. ## 4. Severity interpretation This is stronger than an ordinary Moderate DoS because it is a reliable local kernel panic in core networking code and may cross a guest-to-host boundary in common virtualization setups. It does not currently justify confirmed Important on memory-corruption or privilege-escalation grounds because the demonstrated effect is a BUG_ON, not attacker-controlled memory corruption. The conservative score is Actionable Moderate. The paranoid score reaches Strong Important candidate because of local unprivileged reachability, reproducible host-wide panic, common networking paths, and plausible guest-to-host exposure. ## 5. One-sentence report phrase Untrusted GSO metadata on IP fragments can survive reassembly and make the resulting skb reach skb_segment with a non GRO shaped frag_list, allowing a local user or guest controlled tap path to trigger a reliable kernel panic. ## 6. Manual review recommendation MANUAL CHECK REQUIRED Manual review is required because the bug is locally triggerable without full host root in namespace based setups, affects central skb and fragment reassembly paths, and has plausible guest-to-host DoS exposure, even though confirmed confidentiality, integrity, or LPE impact is not demonstrated. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: inet: frags: strip GSO state from fragments before reassembly Commit: cfdbc8c2e6f9ef5d8b8e54859da03dfe682b0bee Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cfdbc8c2e6f9ef5d8b8e54859da03dfe682b0bee Commit description: A virtio_net_hdr (tun/tap, or AF_PACKET with PACKET_VNET_HDR) can mark an IPv4 or IPv6 fragment as GSO; nothing relates gso_type to frag_off. inet_frag_reasm_prepare()/inet_frag_reasm_finish() keep the first fragment's skb as the head of the reassembled datagram, including its shinfo->gso_size/gso_type/gso_segs, and chain the remaining fragments on frag_list with whatever linear/paged layout they arrived with. After ip_defrag() (ip_local_deliver(), nf_defrag_ipv4, ...) the reassembled skb therefore still claims to be GSO (SKB_GSO_DODGY), and the next software segmentation point - udp_rcv_segment() on local delivery, validate_xmit_skb(), or the ip_finish_output_gso() slow path - hands it to skb_segment(). skb_segment()'s frag_list walk assumes GRO-shaped input and hits one of its BUG_ON()s. Two writes to a tap by an unprivileged user in its own userns are enough: kernel BUG at net/core/skbuff.c:4899! Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI CPU: 0 UID: 1000 PID: 82 Comm: poc Not tainted 7.2.0-pentest+ #2 RIP: 0010:skb_segment+0x20ca/0x48b0 Call Trace: __udp_gso_segment+0x29a/0x27d0 udp4_ufo_fragment+0x458/0x6c0 inet_gso_segment+0x429/0x1340 skb_mac_gso_segment+0x233/0x4f0 __skb_gso_segment+0x308/0x660 udp_queue_rcv_skb+0x440/0xad0 udp_unicast_rcv_skb+0xc7/0x2c0 udp_rcv+0x16ce/0x2260 ip_protocol_deliver_rcu+0x197/0x2d0 ip_local_deliver+0x430/0x690 ip_rcv+0x16f/0x1f0 __netif_receive_skb_one_core+0x15e/0x1c0 __netif_receive_skb+0x1e/0x110 netif_receive_skb+0xf6/0x5c0 tun_rx_batched.isra.0+0x3ab/0x790 tun_get_user+0x17c3/0x3550 tun_chr_write_iter+0xba/0x1b0 vfs_write+0x646/0x1130 Kernel panic - not syncing: Fatal exception in interrupt This runs with BH disabled, so it is a panic rather than an oops. The same is reachable with CAP_NET_RAW in a netns where a defrag point precedes a GSO point, and from a guest whose VMM forwards virtio_net_hdr to a tap. The SKB_GSO_DODGY frag_list checks added by commit 3dcbdb1 ("net: gso: Fix skb_segment splat when splitting gso_size mangled skb having linear-headed frag_list") and by commit 9e4b7a9 ("net: gso: fix panic on frag_list with mixed head alloc types") do not cover it: page-backed heads skip them, and kmalloc heads skip them when gso_size == skb_headlen(head), which the sender controls. An skb entering a frag queue is an IP fragment by definition and cannot legitimately carry GSO state: GRO does not merge fragments and the stack segments before it fragments, so only untrusted sources are affected. This has been reachable since commit f43798c ("tun: Allow GSO using virtio_net_hdr"), the first path that let userspace attach GSO metadata to an IP fragment. Reset the GSO fields of every fragment as it is queued, in inet_frag_queue_insert(), which IPv4, IPv6, nf_conntrack_reasm and 6lowpan reassembly share; then neither the head nor the frag_list members of the reassembled skb carry them (the members matter too: the ip_do_fragment()/ip6_fragment() fast paths send them out as they are). The head may remain CHECKSUM_PARTIAL; that is already accepted on receive and resolved by skb_checksum_help() in ip_do_fragment()/ip6_fragment() on forward. Tested on top of net.git ( dc4b95b ), x86_64: the tap reproducer above, two further IPv4 frag_list geometries that reach BUG_ON(i >= nfrags) and BUG_ON(!list_skb->head_frag), and an IPv6 fragment-header variant (udp6_ufo_fragment()) each panic the unpatched kernel; with this patch all four datagrams are delivered intact and nothing is logged. Fixes: f43798c ("tun: Allow GSO using virtio_net_hdr") Cc: stable@kernel.org Suggested-by: Eric Dumazet Signed-off-by: Xinyang Ge Signed-off-by: Paolo Abeni [Commit description truncated; see the upstream URL below] Changed files: net/core/skbuff.c net/ipv4/inet_fragment.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=cfdbc8c2e6f9ef5d8b8e54859da03dfe682b0bee ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-80590 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80590 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:H The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H The Best / paranoid CVSS score: 6.6 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).