From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-99030][MODERATE 7.0] net: skbuff: don't leave stale bytes in skb_copy_and_csum_bits() Date: Mon, 26 Oct 2026 23:55:38 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-99030 X-AL-KERNEL-Priority: MODERATE 7.0 X-AL-KERNEL-Severity: MODERATE 7.0 X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 5 X-AL-KERNEL-ActionableScore-Lower: 3 X-AL-KERNEL-Commit: 5f19b9cfff79508e8cb09be40d014cb0807f3854 List-Id: CVE: CVE-2026-99030 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: net: skbuff: don't leave stale bytes in skb_copy_and_csum_bits() Commit: 5f19b9cfff79508e8cb09be40d014cb0807f3854 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5f19b9cfff79508e8cb09be40d014cb0807f3854 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99030 Analysis date: Mon, 26 Oct 2026 23:55:38 -0400 ActionableScore: 5 ActionableScore lower bound: 3 Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review Manual review required: YES Summary: Unreadable skb fragments can cause stale bytes from an output buffer to be transmitted through ICMP or driver TX paths, creating a configuration-dependent kernel-memory disclosure. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99030 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99030 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 7.0 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-2026-99030 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-99030 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=3 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC) 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:N/A:N';*CWE-200;CWE-908;CWE-226;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.9';DESCR 'skb_copy_and_csum_bits can copy the linear skb data and then return when it encounters unreadable fragments while leaving the remaining destination bytes unchanged. The affected callers may transmit those stale bytes in an ICMP error or a driver TX bounce buffer, creating a concrete kernel memory disclosure path, although specialized unreadable fragment handling is required and the resulting checksum will usually be invalid. For the CVSS the PR:N value is used because a remote peer may supply traffic that reaches the affected network path without an authenticated user context, but successful disclosure depends on the enabled device memory or zero copy configuration and on observing packets that receivers normally drop. Confidentiality is L for the base score and H for the highest still defensible case because repeated packets could expose more sensitive stale memory. Integrity and availability impact are not demonstrated. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.9 and the production ActionableScore returned 4 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 3 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 4) MAYBE REMOTE SIMPLEFIX SKB LINUS KPANIC PACKET - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=5 ActionableScoreLower=3 PrimitiveClass=CLASS_B RealUAF=NO HostAdminRequired=UNKNOWN NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=YES ManualReview=REQUIRED AutoDowngradeAllowed=NO RequiresHostUID0=UNKNOWN AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: 3. * Conservative score: 3; no floor or cap applied. * Raw paranoid score: 5. * Paranoid score: 5; remote reachability is plausible from the supplied summary but not established by the patch alone. * Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**:: 2. Signal breakdown * +1: Concrete constrained disclosure primitive. Uncopied destination bytes remain stale and are subsequently transmitted. * +1: Confidentiality impact is directly supported; this is not merely an internal uninitialized read. * +1: Affected consumers are network output paths—ICMP error quotation and TX bounce-buffer handling. * Paranoid +2: A remote peer may induce relevant traffic without authentication, according to the supplied summary. The patch does not independently prove creation of unreadable fragments by remote traffic. * No rarity penalty: unreadable-fragment/device-memory configuration is specialized, but the evidence does not prove it is both rare and difficult to reach. * No integrity, availability, OOB write, UAF, arbitrary read, or authorization-bypass points. * No high-confidence C:H claim: leaked contents, repeatability, packet observability, and checksum acceptance are unresolved. 3. Reachability analysis The complete trigger requires an skb with unreadable fragments followed by one of the identified transmit consumers. Remote traffic influence is credible but configuration-dependent and not demonstrated by the diff. Required local privileges, namespace capability scope, prior device/zero-copy setup, and whether an unprivileged actor can establish the complete path remain unknown. No timing race is shown; checksum mismatch commonly limits successful observation but does not prevent transmission in every case. 4. Primitive and severity reading The primitive is disclosure of stale caller-buffer contents after a partial skb copy, not memory corruption or an out-of-bounds access. The commit explicitly states that those bytes can go onto the wire. This is CLASS_B because a real kernel-memory disclosure channel exists, while sensitive contents, reliable collection, and broad remote exploitability are unproven. ImportantCandidate is YES and automatic downgrade is disallowed because the transmitted kernel data and unresolved network exposure require human assessment; this is not proof of Important severity. 5. One-sentence report phrase Unreadable skb fragments can cause stale bytes from an output buffer to be transmitted through ICMP or driver TX paths, creating a configuration-dependent kernel-memory disclosure. 6. Manual review advice MANUAL CHECK REQUIRED Confirm remote creation of unreadable-fragment skbs, affected configurations, packet observability despite checksum failure, and the sensitivity and repeatability of leaked bytes. 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: net: skbuff: don't leave stale bytes in skb_copy_and_csum_bits() Commit: 5f19b9cfff79508e8cb09be40d014cb0807f3854 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5f19b9cfff79508e8cb09be40d014cb0807f3854 Commit description: When skb_copy_and_csum_bits() reaches unreadable frags it returns 0 after copying only the linear part, and the rest of the caller's buffer is left as it was. The callers copy into a buffer that is about to go out on the wire: an ICMP error quoting the offending packet, or a driver's TX bounce buffer in skb_copy_and_csum_dev(). Neither buffer is zeroed beforehand, so whatever was in memory there gets sent. Zero the part of the buffer we didn't fill. The checksum usually won't match the data any more, so the receiver will usually drop the packet, but either way it no longer carries anything it shouldn't. Only zero for a positive @len , a negative one from a broken caller must not turn into a huge memset(). Fixes: 65249fe ("net: add support for skbs with unreadable frags") Cc: stable@vger.kernel.org Signed-off-by: Josef Bacik Reviewed-by: Mina Almasry Link: https://patch.msgid.link/20261007-b4-skb-copy-csum-stale-bytes-v1-1-adbbde033fb3@toxicpanda.com Signed-off-by: Jakub Kicinski 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=5f19b9cfff79508e8cb09be40d014cb0807f3854 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99030 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99030 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:N/A:N The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N The Best / paranoid CVSS score: 5.9 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: 3 Paranoid ActionableScore: 5 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: YES Published priority for this report (same as in Subject): MODERATE 7.0 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).