From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98716][MODERATE REGULAR] KVM: SVM: Use the active VMCB's MSR bitmap when checking if MSR is intercepted [ Upstream Date: Sun, 18 Oct 2026 23:19:24 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98716 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: 3 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 74836ae4785b9754bb36d0b18fcc1dec4e5e98d2 List-Id: CVE: CVE-2026-98716 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: KVM: SVM: Use the active VMCB's MSR bitmap when checking if MSR is intercepted [ Upstream Commit: 74836ae4785b9754bb36d0b18fcc1dec4e5e98d2 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=74836ae4785b9754bb36d0b18fcc1dec4e5e98d2 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98716 Analysis date: Sun, 18 Oct 2026 23:19:24 -0400 ActionableScore: 3 ActionableScore lower bound: unknown Actionable bucket: Borderline / manual review recommended Manual review required: YES Summary: KVM SVM can inspect an inactive MSR permission bitmap while L2 is running, potentially causing incorrect save handling for security-sensitive MSRs such as MSR_IA32_SPEC_CTRL, but the supplied evidence does not establish host memory corruption, panic, privilege escalation, or guest escape. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98716 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98716 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: YES A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-98716 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98716 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 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:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L';CWE-693;CWE-697;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '5.3';DESCR 'KVM SVM checked a cached nested MSR permission bitmap instead of the bitmap referenced by the active VMCB when deciding whether an MSR write was intercepted. When L2 runs with a bitmap other than msrpm02, KVM can make an incorrect save or restore decision for directly accessible MSRs such as MSR_IA32_SPEC_CTRL, creating stale or cross context architectural MSR state. A malicious nested guest or an untrusted virtualization workload may be able to influence this condition through nested SVM and its MSR interception policy. For the CVSS the PR:L is used because the attacker needs execution inside a guest or access to a KVM virtualization context, but full host administrative privileges are not required. The issue is local and is not directly network reachable. Impact can include weakened isolation or incorrect speculative execution control state, with limited confidentiality and integrity impact considered plausible. Reliable host compromise and memory corruption are not established, and exploitation depends on nested virtualization, active VMCB selection, and CPU support. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.3 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class 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_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 0.0 and ActionableScore 0 with primitive class CLASS_C and manual review RECOMMENDED. 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 Borderline / manual review recommended (with actual score 3) MAYBE OOB VIRT SIMPLEFIX HARDWARE DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw conservative score: 3 * Conservative score: 3 * Raw paranoid score: 3 * Paranoid score: 3 * No floor or cap was applied. * Final recommended bucket: **Borderline / manual review recommended**:: 2. Signal breakdown * +1 below-host-admin guest reachability. An L1 guest using nested SVM controls its nested MSR interception policy and can run L2 without host-administrator powers. * +1 weak indirect active-state defect. `msr_write_intercepted()` consults a bitmap that may not belong to the active VMCB, producing an incorrect interception result. * +1 limited integrity concern. The incorrect result can affect whether directly accessible architectural MSR state, including `MSR_IA32_SPEC_CTRL`, is saved across context transitions. * No UAF, OOB access, heap corruption, arbitrary write, controlled reclaim, callback control, or refcount corruption is established. * No reliable host crash, information disclosure, privilege escalation, or guest escape is demonstrated. * No additional guest-to-host exploitation leverage was awarded merely because the defect is in host KVM code. * V33 was not activated. The patch establishes selection of the wrong active-state source, but not attacker-controlled corruption written into an active structured kernel object. * NEW52 B5-R and NEW53 B3-PENDING do not apply. There is no retry-accumulated OOB write or shared-backing write. * AuthorizationDecisionBypass is NO because the defect concerns MSR interception and state management, not an authorization DENY-to-ALLOW decision. 3. Reachability analysis The credible attacker is a malicious L1 guest that is allowed to use nested SVM. L1 can configure nested MSR interception policy and execute L2, while host KVM performs the incorrect bitmap check. This is a guest-controlled trigger below host-administrator privilege, although nested virtualization must already be exposed by the host. A local VMM process with `/dev/kvm` access can also construct the relevant nested setup. Host root is not required for the vulnerable operation after access to an existing VM is granted. The trigger is not network-reachable by itself and is unrelated to Linux user or network namespaces. This is not a compound guest plus VMM lifetime trigger. No host-side object deletion, memslot replacement, MMU notifier operation, or other VMM-controlled teardown is required. No race or narrow timing window is identified. 4. Primitive and severity interpretation The exact primitive is an incorrect active-state lookup. KVM checks `svm->nested.msrpm` whenever L2 is active even though the active VMCB may reference a different MSR permission bitmap. Consequently, KVM can misclassify whether an MSR write is intercepted and make an incorrect save-state decision. This is CLASS_C invalid-state behavior, not a memory-lifetime violation. The affected state is active architectural virtualization state, but no corruption of heap objects, pointers, descriptors, or page tables is established. There is no repeated release or refcount operation. The patch-supported practical consequence is possible cross-context mishandling of security-sensitive MSR state. Limited integrity impact is plausible because `MSR_IA32_SPEC_CTRL` controls speculative-execution behavior. Stronger consequences such as host memory corruption, host panic, confidentiality compromise, privilege escalation, or VM escape remain unproven. ImportantCandidate is NO because no Class A or meaningful Class B corruption primitive is established. AutoDowngradeAllowed is YES because the unresolved impact concerns architectural state correctness rather than a concrete kernel-memory or authorization-boundary primitive. Manual review remains recommended because the affected MSR is security-sensitive and the bug crosses nested virtualization contexts. 5. One-sentence report phrase KVM SVM can inspect an inactive MSR permission bitmap while L2 is running, potentially causing incorrect save handling for security-sensitive MSRs such as MSR_IA32_SPEC_CTRL, but the supplied evidence does not establish host memory corruption, panic, privilege escalation, or guest escape. 6. Manual review recommendation MANUAL CHECK RECOMMENDED Review the callers of `msr_write_intercepted()` and the host restore path to determine whether an L2-controlled `MSR_IA32_SPEC_CTRL` value can survive into L1 or host execution and measurably weaken speculative-execution isolation. 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: KVM: SVM: Use the active VMCB's MSR bitmap when checking if MSR is intercepted [ Upstream Commit: 74836ae4785b9754bb36d0b18fcc1dec4e5e98d2 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=74836ae4785b9754bb36d0b18fcc1dec4e5e98d2 Changed files: arch/x86/kvm/svm/svm.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=74836ae4785b9754bb36d0b18fcc1dec4e5e98d2 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98716 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98716 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:N The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L The Best / paranoid CVSS score: 5.3 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: 3 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).