From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-99258][LOW] x86/hyperv: Avoid using per-cpu output page in hv_apicid_to_vp_index() [ Upstream Date: Tue, 03 Nov 2026 22:22:19 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-99258 X-AL-KERNEL-Priority: LOW X-AL-KERNEL-Severity: LOW X-AL-KERNEL-Base-Severity: LOW X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 1 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: bdba5f1b8bd0d18b1df61f769b559ed272902489 List-Id: CVE: CVE-2026-99258 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: x86/hyperv: Avoid using per-cpu output page in hv_apicid_to_vp_index() [ Upstream Commit: bdba5f1b8bd0d18b1df61f769b559ed272902489 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bdba5f1b8bd0d18b1df61f769b559ed272902489 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99258 Analysis date: Tue, 03 Nov 2026 22:22:19 -0400 ActionableScore: 1 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: A host-controlled Hyper-V environment can make an affected SEV-SNP guest store a VP index through an unallocated per-CPU output location, risking constrained guest-memory corruption or failure. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99258 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99258 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: LOW 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-2026-99258 LOW https://www.kernelcve.org/list/?q=CVE-2026-99258 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=LOW RESULT=DIFF TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: LOW, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:H/A:H';CWE-824;CWE-119;*CWE-664;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:H';BEST CVSS score: '6.3';DESCR 'In an SEV SNP CoCo VM lacking a paravisor, hv_apicid_to_vp_index can pass an unallocated per CPU output location to HVCALL_GET_VP_INDEX_FROM_APIC_ID. The hypervisor then stores the returned VP index in memory that the guest did not allocate as a hypercall output buffer, creating a constrained invalid write with possible guest memory corruption or guest failure. For the CVSS the PR:H value reflects that reliable attacker controlled triggering requires control of the host hypervisor or equivalent VM management authority rather than ordinary code execution inside the guest. The issue is not network reachable and applies only to affected Hyper V SEV SNP guest configurations. Availability impact is strongest, while confidentiality and broader integrity impact remain dependent on what guest memory is reached by the invalid output location. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 6.3 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic and the cross model evidence reconciliation 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 KPANIC_UNSUPPORTED_REMOVED and V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 3.4 and ActionableScore 1 with primitive class CLASS_B 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 LOW.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1) MAYBE SIMPLEFIX LEAK HARDWARE LINUS KPANIC_UNSUPPORTED_REMOVED V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=1 PrimitiveClass=CLASS_B RealUAF=NO HostAdminRequired=YES NamespaceScopedPrivilege=NO 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 * Raw paranoid score: 1 * Paranoid score: 1 * No floor or cap applied. * Final recommended bucket: **Ordinary Moderate**:: 2. Signal breakdown * +1 constrained invalid write: the hypervisor writes the VP-index result through a per-CPU output address that was never allocated in the affected SEV-SNP configuration. * +1 firmware/hypervisor-mediated influence: reliable adversarial triggering requires control of the host hypervisor or equivalent VM-management authority. * +1 plausible guest crash/DoS: writing into unintended guest memory can corrupt the guest or cause guest failure. * -2 proven host-administrator-equivalent prerequisite: supplied evidence identifies host hypervisor/VM-management control as necessary for reliable attacker-controlled triggering. * No retry accumulation, heap/slab retry OOB, or later corrupted-state consumer is shown; B5-R does not apply. * No arbitrary target, controlled reclaim, UAF, authorization bypass, disclosure, or protected-state overwrite is established. No additional availability point is awarded for the same possible guest failure. 3. Reachability analysis The affected path exists only in a Hyper-V SEV-SNP guest without a paravisor. Ordinary guest code is not shown to control the output address or complete trigger. The supported attacker model is a host hypervisor or equivalent VM-management actor, not a namespace-scoped guest capability. Numeric host UID 0 is not specifically established. No race or retry loop is involved. 4. Primitive and severity reading The primitive is a fixed-destination, small hypercall-result write into guest memory not allocated as an output buffer. It is a constrained CLASS_B invalid-write primitive, not a UAF or demonstrated arbitrary write. Guest corruption or failure is plausible, but the identity and later use of the overwritten memory are not shown. Consequently, no active structured-state consumer or unresolved delegated trust-boundary chain supports an Important candidate. Auto-downgrade remains allowed because the complete adversarial trigger requires host-hypervisor authority and no stronger exploitation primitive is evidenced. 5. One-sentence report phrase A host-controlled Hyper-V environment can make an affected SEV-SNP guest store a VP index through an unallocated per-CPU output location, risking constrained guest-memory corruption or failure. 6. Manual review advice MANUAL CHECK RECOMMENDED Confirm the actual output address and overwrite width in affected builds, but current evidence does not establish a stronger victim object or post-corruption consumer. 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: x86/hyperv: Avoid using per-cpu output page in hv_apicid_to_vp_index() [ Upstream Commit: bdba5f1b8bd0d18b1df61f769b559ed272902489 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bdba5f1b8bd0d18b1df61f769b559ed272902489 Changed files: arch/x86/hyperv/hv_init.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=bdba5f1b8bd0d18b1df61f769b559ed272902489 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99258 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99258 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:H/UI:N/S:U/C:N/I:L/A:H The Best / paranoid CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:H/A:H The Best / paranoid CVSS score: 6.3 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: LOW KPANIC detected for this report: NO Published priority for this report (same as in Subject): LOW 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).