From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-46300][IMPORTANT] net: skbuff: propagate shared-frag marker through frag-transfer helpers Date: Sat, 23 May 2026 08:52:27 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-46300 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: 9 X-AL-KERNEL-ActionableScore-Lower: 8 X-AL-KERNEL-Commit: fbeab9555564a1b98e8582cd106dfe46c4606991 List-Id: CVE: CVE-2026-46300 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: YES Patch: net: skbuff: propagate shared-frag marker through frag-transfer helpers Commit: fbeab9555564a1b98e8582cd106dfe46c4606991 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fbeab9555564a1b98e8582cd106dfe46c4606991 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46300 Analysis date: Sat, 23 May 2026 08:52:27 -0400 ActionableScore: 9 ActionableScore lower bound: 8 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: Missing propagation of `SKBFL_SHARED_FRAG` in SKB frag-transfer helpers can let later in-place network writers corrupt externally owned or page-cache-backed pages, including root-owned read-only file cache data, creating a plausible local privilege escalation primitive. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-46300 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46300 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-2026-46300 IMPORTANT CHECK Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-664;CWE-863;CWE-284;CWE-668;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '7.8';DESCR 'SKB frag transfer helpers failed to preserve SKBFL_SHARED_FRAG when moving page backed fragments between skbs. This can make the destination skb keep references to externally owned or page cache backed pages while skb_has_shared_frag returns false, so later in place writers such as ESP input may modify pages that should have been copied first. A local unprivileged user may be able to corrupt the page cache of a root owned read only file in affected configurations, which makes this more than a simple crash bug and creates a plausible local privilege escalation path. For the CVSS the PR:L is used because the attacker needs local user level ability to create or drive the relevant skb data path, but does not need administrative privileges on the victim. The issue is not scored as directly network reachable because remote packet delivery alone does not normally provide the page cache backed skb fragments needed for the described write primitive. Impact is high integrity impact and potentially high confidentiality and availability impact if page cache corruption is used to affect privileged code or security sensitive files.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9) YES REMOTE SKB HARDWARE KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS NO NO guess ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=9 ActionableScoreLower=8 ## 1. ActionableScore * Conservative score: 8 * Paranoid score: 9 * Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: ## 2. Signal breakdown Triggered signals: * Local unprivileged trigger: +1 The commit explicitly states that an unprivileged user can reach the bad path in affected configurations. * Strong LPE plausibility: +2 The described primitive allows writes into the page cache of a root-owned read-only file, which is a plausible local privilege escalation path. * Shared-page / page-cache / COW ownership corruption: +4 capped combined signal The same underlying primitive qualifies as strong memory corruption, Dirty-Pipe-like ownership bypass, and page-cache corruption. These are not fully stacked to avoid double counting, but the combined primitive is high risk. * Practical exploit chain strongly implied: +1 The commit gives a concrete chain involving `nft dup to local`, `nf_dup_ipv4()` / `xt_TEE`, ESP input, and authencesn-ESN stray writes. * Confidentiality impact plausible: +1 If privileged file-backed data or security-sensitive cache state can be corrupted and later consumed, confidentiality impact is plausible through follow-on privilege or policy bypass. * Integrity impact plausible: +1 Direct page-cache corruption of root-owned read-only files is a strong integrity impact. * Common networking core / packet-processing path: +1 The bug is in central SKB frag-transfer helpers and affects GRO, segmentation, skb shifting, and TCP payload cloning paths. No subtraction applied: * No hard race requirement is central to the primitive. * No CAP_SYS_ADMIN / CAP_NET_ADMIN requirement is assumed for the attacker, because the commit describes an unprivileged user corruption path when the affected system configuration exists. * No rare-subsystem penalty is applied because SKB, XFRM / ESP, netfilter duplication, and page-cache interaction are specifically called out as paths that should not be dismissed solely as niche. ## 3. Reachability analysis The realistic attacker is a local unprivileged user on a system where the relevant networking and XFRM / ESP path is configured. The bug is not scored as directly remote reachable, because remote packets alone normally do not provide the page-cache-backed SKB fragments needed for the write primitive. Namespaces or containers may matter if an unprivileged workload can generate the relevant traffic or interact with a preconfigured duplication / ESP path, but the key issue is that the attacker does not need full host-root privileges once the vulnerable path is present. The path is configuration-dependent, but not merely theoretical. The commit explicitly describes a concrete route where stripped `SKBFL_SHARED_FRAG` state lets ESP input perform in-place writes into externally owned or page-cache-backed pages. ## 4. Severity interpretation This behaves more like an Important-class kernel vulnerability than an ordinary Moderate DoS. The issue is not just a crash, warning, or metadata leak. It is an ownership / COW boundary violation where SKB helpers lose the shared-frag marker, causing later writers to treat shared page-backed fragments as private. The realistic evidence supports at least Actionable Moderate and likely Important handling because the patch context describes unprivileged page-cache corruption of root-owned read-only file data. The paranoid score is higher because Dirty-Pipe-like primitives have historically been underestimated during early triage, but the score is still kept below a remote Critical-style bucket because direct network-only exploitation is not established. ## 5. One-sentence report phrase Missing propagation of `SKBFL_SHARED_FRAG` in SKB frag-transfer helpers can let later in-place network writers corrupt externally owned or page-cache-backed pages, including root-owned read-only file cache data, creating a plausible local privilege escalation primitive. ## 6. Manual review recommendation MANUAL CHECK REQUIRED This should not be auto-closed because the patch describes a concrete page-cache corruption primitive reachable by an unprivileged local user in affected configurations, with plausible local privilege escalation impact rather than DoS-only behavior. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: net: skbuff: propagate shared-frag marker through frag-transfer helpers Commit: fbeab9555564a1b98e8582cd106dfe46c4606991 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fbeab9555564a1b98e8582cd106dfe46c4606991 Commit description: Two frag-transfer helpers (__pskb_copy_fclone() and skb_shift()) fail to propagate the SKBFL_SHARED_FRAG bit in skb_shinfo()->flags when moving frags from source to destination. __pskb_copy_fclone() defers the rest of the shinfo metadata to skb_copy_header() after copying frag descriptors, but that helper only carries over gso_{size,segs, type} and never touches skb_shinfo()->flags; skb_shift() moves frag descriptors directly and leaves flags untouched. As a result, the destination skb keeps a reference to the same externally-owned or page-cache-backed pages while reporting skb_has_shared_frag() as false. The mismatch is harmful in any in-place writer that uses skb_has_shared_frag() to decide whether shared pages must be detoured through skb_cow_data(). ESP input is one such writer (esp4.c, esp6.c), and a single nft 'dup to ' rule -- or any other nf_dup_ipv4() / xt_TEE caller -- is enough to land a pskb_copy()'d skb in esp_input() with the marker stripped, letting an unprivileged user write into the page cache of a root-owned read-only file via authencesn-ESN stray writes. Set SKBFL_SHARED_FRAG on the destination whenever frag descriptors were actually moved from the source. skb_copy() and skb_copy_expand() share skb_copy_header() too but linearize all paged data into freshly allocated head storage and emerge with nr_frags == 0, so skb_has_shared_frag() returns false on its own; they need no change. The same omission exists in skb_gro_receive() and skb_gro_receive_list(). The former moves the incoming skb's frag descriptors into the accumulator's last sub-skb via two paths (a direct frag-move loop and the head_frag + memcpy path); the latter chains the incoming skb whole onto p's frag_list. Downstream skb_segment() reads only skb_shinfo(p)->flags, and skb_segment_list() reuses each sub-skb's shinfo as the nskb -- both p and lp must carry the marker. The same omission also exists in tcp_clone_payload(), which builds an MTU probe skb by moving frag descriptors from skbs on sk_write_queue into a freshly allocated nskb. The helper falls into the same family and warrants the same fix for consistency; no TCP TX-side in-place writer is currently known to reach a user page through this gap, but a future consumer depending on the marker would regress silently. The same omission exists in skb_segment(): the per-iteration flag merge takes only head_skb's flag, and the inner switch that rebinds frag_skb to list_skb on head_skb-frags exhaustion does not fold the new frag_skb's flag into nskb. Fold frag_skb's flag at both sites so segments drawing frags from frag_list members carry the marker. Fixes: cef401d ("net: fix possible wrong checksum generation") Fixes: f4c50a4 ("xfrm: esp: avoid in-place decrypt on shared skb frags") Suggested-by: Sabrina Dubroca Suggested-by: Sultan Alsawaf Suggested-by: Ben Hutchings Suggested-by: Lin Ma Suggested-by: Jingguo Tan Suggested-by: Aaron Esau Cc: stable@vger.kernel.org Signed-off-by: Hyunwoo Kim Tested-by: Rajat Gupta Link: https://patch.msgid.link/ageeJfJHwgzmKXbh@v4bel Signed-off-by: Paolo Abeni [bwh: Backported to 5.10: - Set the SKBTX_SHARED_FRAG flag in skb_shared_info::tx_flags, instead of SKBFL_SHARED_FRAG in skb_shared_info::flags - skb_gro_receive() and skb_gro_receive_list() are in skbuff.c here - Drop change to tcp_clone_payload(), which does not exist here - Adjust context in skb_shift() ] Signed-off-by: Ben Hutchings Signed-off-by: Greg Kroah-Hartman Changed files: net/core/skbuff.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=fbeab9555564a1b98e8582cd106dfe46c4606991 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-46300 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46300 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:L/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 7.8 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: 8 Paranoid ActionableScore: 9 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).