From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98927][IMPORTANT] KVM: arm64: nv: Fix life cycle of the nested_mmus array [ Upstream Date: Mon, 26 Oct 2026 16:46:06 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98927 X-AL-KERNEL-Priority: IMPORTANT X-AL-KERNEL-Severity: IMPORTANT X-AL-KERNEL-Base-Severity: IMPORTANT X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 5 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: ea242bdc29e8a8ff0e2447a64b3841114779f3af List-Id: CVE: CVE-2026-98927 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: NO Patch: KVM: arm64: nv: Fix life cycle of the nested_mmus array [ Upstream Commit: ea242bdc29e8a8ff0e2447a64b3841114779f3af Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea242bdc29e8a8ff0e2447a64b3841114779f3af Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98927 Analysis date: Mon, 26 Oct 2026 16:46:06 -0400 ActionableScore: 5 ActionableScore lower bound: unknown Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review Manual review required: YES Summary: A KVM-capable local VMM can race nested-vCPU initialization with active L2 execution, leaving the host kernel using a freed nested Stage-2 MMU structure. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98927 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98927 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-2026-98927 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-98927 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT IMPORTANT AEGIS_AS=6 LEGACY=IMPORTANT RESULT=SAME INDEPENDENT_LEGACY=MODERATE_KPANIC INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_WINS_DISAGREEMENT PRE_MERGE_LEGACY=MODERATE_KPANIC (final reconciliation agrees; DECISION: both IMPORTANT) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in KVM arm64 nested virtualization allowed VCPU_INIT to grow and relocate nested_mmus while another vCPU was running L2 with a pointer into the old array. Freeing the relocated array left the active vCPU with a dangling pointer to a nested Stage 2 MMU object and created a concrete host kernel UAF. For the CVSS the PR:L value reflects that reliable triggering requires a local process with KVM access that can initialize vCPUs and coordinate nested execution. AC:H reflects the concurrent timing window. The issue is not directly network reachable and an isolated guest alone is not shown to control VCPU_INIT. Impact is at least host denial of service via kernel crash. Because the freed object controls shadow Stage 2 page table state, reuse could plausibly affect confidentiality and integrity, while a controlled write or proven privilege escalation chain is not established. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC because a specific evidence based protection or manual review veto was applied and FD false positive review allowed a downgrade. final reconciliation stage MODERATE_KPANIC to IMPORTANT because KP specialist did not establish a qualifying panic. 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 produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and V33_ACTIVE_STATE_BOUNDARY_CANDIDATE and IMPORTANT_CANDIDATE_MANUAL_REVIEW and KP_ONLY_DOWNGRADE_VETO and DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH and TEMP_KPANIC_MERGE_IMPORTANT_WINS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 6 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was IMPORTANT. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) MAYBE VIRT INIT NOMEMCHK UAF HARDWARE INCREASED_TO_HIGH_BASED_ON_GUESSCVSS V33_ACTIVE_STATE_BOUNDARY_CANDIDATE IMPORTANT_CANDIDATE_MANUAL_REVIEW KP_ONLY_DOWNGRADE_VETO DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH TEMP_KPANIC_MERGE_IMPORTANT_WINS KPANIC_UNSUPPORTED_REMOVED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=5 PrimitiveClass=CLASS_B RealUAF=YES HostAdminRequired=NO NamespaceScopedPrivilege=NO ImportantCandidate=YES ManualReview=REQUIRED AutoDowngradeAllowed=NO RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: 4. * Conservative score: 5; REAL UAF FLOOR 5 applied because the below-host-admin VMM can drive relocation while an active vCPU retains and uses the structured MMU pointer. * Raw paranoid score: 4. * Paranoid score: 5; no additional speculative leverage awarded. * Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**:: 2. Signal breakdown * +2: Concrete host-kernel UAF. Growing `nested_mmus` relocates and frees the old array while an L2-running vCPU retains a pointer to an element. * +1: Plausible host crash from later active use of the freed Stage-2 MMU structure. * +1: Below-host-admin local trigger is supported by the supplied evidence: a process with KVM access can initialize vCPUs and coordinate nested execution. * +1: Attacker-influenced lifetime transition. The same VMM can keep L2 active while invoking `VCPU_INIT`, causing array growth and relocation. * -1: A materially concurrent timing window is required; the supplied summary characterizes reliable triggering as AC:H. * Controlled reclaim, attacker-shaped replacement contents, arbitrary host write, confidentiality/integrity impact, and guest escape are not shown. * No separate Availability point: it would duplicate the crash. * No KVM strong-leverage bonus: an active shadow-MMU pointer is security-sensitive, but no useful post-reuse field consumption or directed host-memory operation is demonstrated. 3. Reachability analysis The supported attacker is a local VMM process with `/dev/kvm` access, not an isolated guest. It controls vCPU initialization and nested execution, while the host kernel performs relocation and later stale-pointer consumption. No host UID 0, host-admin capability, user namespace, or namespace-scoped capability is shown as necessary. No special hardware prerequisite is identified beyond arm64 nested virtualization. The trigger is a VMM-controlled concurrent sequence, not a proven guest-only path. 4. Primitive and severity reading This is an active structured-object UAF, not fixed stale storage or cleanup-only use. The old array is freed after relocation while an executing vCPU still references an embedded `kvm_s2_mmu`; the object participates in live nested Stage-2 translation processing. That is CLASS_B. Host crash is plausible; controlled reclaim, arbitrary access, LPE, and a proven guest-to-host compromise are unproven. ImportantCandidate is YES because concrete attacker-influenced active MMU-state lifetime corruption crosses the delegated KVM-interface/host-kernel trust boundary, with later privileged consumption and unresolved non-crash leverage. Auto-downgrade is therefore unsafe despite the absence of a completed exploit. 5. One-sentence report phrase A KVM-capable local VMM can race nested-vCPU initialization with active L2 execution, leaving the host kernel using a freed nested Stage-2 MMU structure. 6. Manual review advice MANUAL CHECK REQUIRED Review reclaim behavior and every stale `kvm_s2_mmu` consumer for host-memory effects beyond the supported crash path. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: KVM: arm64: nv: Fix life cycle of the nested_mmus array [ Upstream Commit: ea242bdc29e8a8ff0e2447a64b3841114779f3af Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea242bdc29e8a8ff0e2447a64b3841114779f3af Changed files: arch/arm64/include/asm/kvm_host.h arch/arm64/include/asm/kvm_nested.h arch/arm64/kvm/arm.c arch/arm64/kvm/nested.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=ea242bdc29e8a8ff0e2447a64b3841114779f3af ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98927 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98927 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:L/I:L/A:H The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 7 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: 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: IMPORTANT KPANIC detected for this report: NO 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).