From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-99055][MODERATE REGULAR] tcp: reject net_iov in zerocopy receive mapping hints Date: Mon, 26 Oct 2026 21:11:50 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-99055 X-AL-KERNEL-Priority: MODERATE REGULAR X-AL-KERNEL-Severity: MODERATE REGULAR X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 1 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 563d3564513496b9ce3b3ba7a5154427554d0f06 List-Id: CVE: CVE-2026-99055 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: tcp: reject net_iov in zerocopy receive mapping hints Commit: 563d3564513496b9ce3b3ba7a5154427554d0f06 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=563d3564513496b9ce3b3ba7a5154427554d0f06 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99055 Analysis date: Mon, 26 Oct 2026 21:11:50 -0400 ActionableScore: 1 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: TCP zerocopy receive can dereference a NULL page for a `net_iov` fragment, causing a kernel crash on a specialized and not yet hardware-validated receive path. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99055 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99055 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 REGULAR Manual review required: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-99055 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-99055 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: 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:N/I:N/A:H';**CWE-476;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.9';DESCR 'A NULL pointer dereference in can_map_frag can occur when TCP zerocopy receive processes an unreadable skb containing a net_iov fragment after copying a readable prefix. skb_frag_page returns NULL for net_iov and the old path passes that value to PageCompound. The demonstrated primitive is a kernel NULL dereference and denial of service without evidence of UAF, OOB access, information disclosure, or an arbitrary write. For the CVSS the PR:N value applies to the worst case where a network service automatically uses this receive path for data controlled by an unauthenticated remote peer. Exploitation still has high complexity because it requires CONFIG_NET_DEVMEM, a supported DMA buf bound RX queue, a suitable fragment layout, and hardware reachability that has not yet been tested end to end. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.9 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1) MAYBE REMOTE KASAN NULLPTR NETWORK SKB HARDWARE LINUS TEST MEMORY DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=1 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=UNKNOWN NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES RequiresHostUID0=UNKNOWN AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: 1. * Conservative score: 1; no floor or cap applied. * Raw paranoid score: 1. * Paranoid score: 1; remote reachability remains unproven and receives no bonus. * Final recommended bucket: **Ordinary Moderate**:: 2. Signal breakdown * +1 Reliable kernel crash/strong DoS: `skb_frag_page()` returns NULL for a `net_iov`, and `PageCompound()` dereferences it. KASAN demonstrated the fault and panic under `panic_on_warn`. * +0 weak corrupt concern: this is a NULL dereference, not a write, UAF, OOB access, or useful stale-object operation. * No remote-reachability bonus: actual hardware end-to-end reachability was not tested and requires `CONFIG_NET_DEVMEM`, a suitable NIC, and a DMA-buf-bound RX queue. * No rarity penalty: the prerequisites constrain exposure, but the supplied evidence does not prove sufficiently rare hardware or an attacker-inaccessible deployment. * No extra Availability point: it would duplicate the same crash. * No confidentiality, integrity, authorization, B3-PENDING, B4-PENDING, or B5-R primitive is shown. 3. Reachability analysis The diagnostic directly injected the `net_iov`; it does not prove a normal attacker can create the fragment. A real path requires prior host-side device and RX-queue configuration plus supported hardware. A remote peer may influence TCP data and fragment sequencing once such a service exists, but the complete remote trigger is unverified. Required host privilege, namespace capability scope, and UID requirements therefore remain unknown. No timing race is involved. 4. Primitive and severity reading The exact primitive is a deterministic NULL pointer dereference while evaluating an unreadable non-page-backed skb fragment. It is CLASS_C crash/DoS behavior, with no active-state corruption or later corrupted-state consumption. The patch-supported effect is a kernel fault; arbitrary write, disclosure, UAF, and privilege escalation are unsupported. ImportantCandidate is NO and auto-downgrade is allowed because no material non-crash primitive remains. 5. One-sentence report phrase TCP zerocopy receive can dereference a NULL page for a `net_iov` fragment, causing a kernel crash on a specialized and not yet hardware-validated receive path. 6. Manual review advice MANUAL CHECK RECOMMENDED Confirm real NIC reachability, queue provisioning requirements, and whether remotely supplied traffic can reliably produce the required fragment sequence. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: tcp: reject net_iov in zerocopy receive mapping hints Commit: 563d3564513496b9ce3b3ba7a5154427554d0f06 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=563d3564513496b9ce3b3ba7a5154427554d0f06 Commit description: After copying a readable prefix, receive_fallback_to_copy() asks tcp_zerocopy_set_hint_for_skb() where page mapping can resume. If the next skb is unreadable, find_next_mappable_frag() passes its net_iov fragment to can_map_frag(). skb_frag_page() returns NULL for a net_iov, but can_map_frag() dereferences it in PageCompound(). A v7.2 KASAN run on a connected TCP socket with 64 readable bytes followed by a 4096-byte NET_IOV_DMABUF fragment reported: BUG: KASAN: null-ptr-deref in can_map_frag tcp_zerocopy_receive -> can_map_frag Kernel panic - not syncing: KASAN: panic_on_warn set The diagnostic inserted the net_iov directly because the test host has no devmem-capable NIC. Hardware end-to-end reachability remains untested and requires CONFIG_NET_DEVMEM plus a supported DMA-buf-bound RX queue. Reject all net_iov fragments before skb_frag_page(). This covers both DMABUF and IOURING net_iov types while leaving page-backed checks unchanged. With the guard, the same queue copied the readable prefix, returned a 4096-byte skip hint, and completed without a fault. Fixes: 9f6b619 ("net: support non paged skb frags") Cc: stable@vger.kernel.org Signed-off-by: Daehyeon Ko <4ncienth@gmail.com> Reviewed-by: Mina Almasry Link: https://patch.msgid.link/20260930102524.1659847-1-4ncienth@gmail.com Signed-off-by: Jakub Kicinski Signed-off-by: Greg Kroah-Hartman Changed files: net/ipv4/tcp.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=563d3564513496b9ce3b3ba7a5154427554d0f06 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99055 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99055 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:N/I:N/A:H The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H 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: unknown Paranoid ActionableScore: 1 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: NO Published priority for this report (same as in Subject): MODERATE REGULAR 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).