From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-46076][MODERATE 7.0] KVM: nSVM: Raise #UD if unhandled VMMCALL isn't intercepted by L1 Date: Wed, 27 May 2026 11:47:14 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-46076 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: unknown X-AL-KERNEL-Commit: 924d721fae95687acedbaf624a094ed0e8b67104 List-Id: CVE: CVE-2026-46076 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: KVM: nSVM: Raise #UD if unhandled VMMCALL isn't intercepted by L1 Commit: 924d721fae95687acedbaf624a094ed0e8b67104 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=924d721fae95687acedbaf624a094ed0e8b67104 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46076 Analysis date: Wed, 27 May 2026 11:47:14 -0400 ActionableScore: 5 ActionableScore lower bound: unknown Actionable bucket: Actionable Moderate at minimum Manual review required: YES Summary: KVM nSVM could incorrectly handle an unhandled L2 VMMCALL in L0 instead of injecting `#UD`, allowing L2 to bypass the expected L1 VMMCALL interception model under specific Hyper-V nested TLB-flush conditions. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-46076 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46076 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-2026-46076 MODERATE CHECK Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:H';CWE-284;CWE-863;CWE-693;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.5';DESCR 'KVM nSVM could process an L2 VMMCALL in L0 as a normal hypercall when L1 did not request VMMCALL interception and the call was not a supported Hyper V L2 TLB flush hypercall. Architecturally that unhandled VMMCALL should raise UD, so the old behavior let L2 bypass the expected L1 interception model and act partly as if it were L1 for this hypercall path. For the CVSS the PR:L is used because the attacker needs control of code inside the nested guest, typically L2 kernel or equivalent privileged guest context, but not host privileges. AC:H is used because the condition requires nested SVM, Hyper V L2 TLB flush enablement, and a specific L1 intercept configuration. The issue is not network reachable and does not involve UAF, out of bounds access, arbitrary write, information disclosure, or direct host privilege escalation. Impact is primarily nested virtualization policy bypass and possible guest or host side availability impact depending on which hypercall path is reached.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) YES VIRT INIT HARDWARE LINUS KPANIC INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS YES NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=5 ## 1. ActionableScore * Conservative score: 5 * Paranoid score: 5 * Final recommended bucket: **Actionable Moderate at minimum**:: ## 2. Signal breakdown Triggered signals: * Local trigger from nested guest context: +1 The attacker needs code execution inside L2, typically privileged guest code, but not host privileges. * Security-boundary / policy bypass: +2 L2 can reach L0 hypercall handling even when L1 did not request VMMCALL interception, bypassing the expected nested virtualization control model. * Cross-boundary isolation/policy bypass: +1 The bug crosses the intended L2 to L1 to L0 virtualization boundary semantics. * Integrity impact plausible: +1 Incorrectly treating an L2 VMMCALL as an L1-like hypercall can affect nested virtualization state or hypercall semantics. * Availability impact plausible: +1 Mishandled hypercalls may affect guest or nested virtualization behavior, depending on the hypercall path. Subtracting signals: * Configuration-dependent / special state: -1 Requires nested SVM, Hyper-V L2 TLB flush enablement, L1 not intercepting VMMCALL, and a non-supported Hyper-V hypercall. No signals apply for memory corruption, UAF, arbitrary write, information disclosure as a demonstrated primitive, or direct host privilege escalation. ## 3. Reachability analysis The issue is reachable from L2 guest code in a nested virtualization setup. It requires an L1 configuration where VMMCALL is not intercepted, and Hyper-V L2 TLB flush support causes L0 to intercept VMMCALL for its own handling. Namespaces and containers are not directly relevant. This is not network reachable. The practical attacker is someone controlling code inside an L2 guest, usually with guest-kernel or equivalent privileges. ## 4. Severity interpretation This is an actionable Moderate nested virtualization policy bug. It is not memory corruption and does not by itself demonstrate host privilege escalation. The security concern is that L2 can bypass the expected L1 interception model and reach L0 hypercall handling when architectural behavior should be `#UD`. In environments relying on nested virtualization isolation or L1 policy enforcement, this deserves manual review. ## 5. One-sentence report phrase KVM nSVM could incorrectly handle an unhandled L2 VMMCALL in L0 instead of injecting `#UD`, allowing L2 to bypass the expected L1 VMMCALL interception model under specific Hyper-V nested TLB-flush conditions. ## 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: this is a nested virtualization boundary and policy bypass issue. It is not memory corruption, but L2 bypassing L1 interception semantics can be security-relevant in nested virtualization deployments. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: KVM: nSVM: Raise #UD if unhandled VMMCALL isn't intercepted by L1 Commit: 924d721fae95687acedbaf624a094ed0e8b67104 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=924d721fae95687acedbaf624a094ed0e8b67104 Commit description: Explicitly synthesize a #UD for VMMCALL if L2 is active, L1 does NOT want to intercept VMMCALL, nested_svm_l2_tlb_flush_enabled() is true, and the hypercall is something other than one of the supported Hyper-V hypercalls. When all of the above conditions are met, KVM will intercept VMMCALL but never forward it to L1, i.e. will let L2 make hypercalls as if it were L1. The TLFS says a whole lot of nothing about this scenario, so go with the architectural behavior, which says that VMMCALL #UDs if it's not intercepted. Opportunistically do a 2-for-1 stub trade by stub-ifying the new API instead of the helpers it uses. The last remaining "single" stub will soon be dropped as well. Suggested-by: Sean Christopherson Fixes: 3f4a812 ("KVM: nSVM: hyper-v: Enable L2 TLB flush") Cc: Vitaly Kuznetsov Cc: stable@vger.kernel.org Signed-off-by: Kevin Cheng Link: https://patch.msgid.link/20260228033328.2285047-5-chengkev@google.com [sean: rewrite changelog and comment, tag for stable, remove defunct stubs] Reviewed-by: Yosry Ahmed Reviewed-by: Vitaly Kuznetsov Link: https://patch.msgid.link/20260304002223.1105129-2-seanjc@google.com Signed-off-by: Sean Christopherson Signed-off-by: Greg Kroah-Hartman Changed files: arch/x86/kvm/hyperv.h arch/x86/kvm/svm/hyperv.h arch/x86/kvm/svm/nested.c 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=924d721fae95687acedbaf624a094ed0e8b67104 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-46076 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46076 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:L/A:L The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:H The Best / paranoid CVSS score: 6.5 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: 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).