From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-46071][LOW] KVM: nSVM: Avoid clearing VMCB_LBR in vmcb12 Date: Wed, 27 May 2026 12:12:50 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-46071 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: 2 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: a3f0981a5a0e0bd51ad74cc7d9eed32294b24002 List-Id: CVE: CVE-2026-46071 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: KVM: nSVM: Avoid clearing VMCB_LBR in vmcb12 Commit: a3f0981a5a0e0bd51ad74cc7d9eed32294b24002 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a3f0981a5a0e0bd51ad74cc7d9eed32294b24002 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46071 Analysis date: Wed, 27 May 2026 12:12:50 -0400 ActionableScore: 2 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate / Low-like Manual review required: NO Summary: KVM nSVM could incorrectly mark `vmcb12` LBR state dirty during nested VM exit, causing nested LBR virtualization state correctness issues without memory corruption or host privilege impact. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-46071 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46071 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-2026-46071 LOW 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:N/I:L/A:H';CWE-670;*CWE-664;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.3';DESCR 'KVM nSVM incorrectly marked the destination VMCB LBR field dirty inside svm_copy_lbrs(), including when the destination was vmcb12 during nested VM exit. Clearing clean bits in vmcb12 is not architecturally defined and can corrupt nested LBR virtualization state or cause incorrect LBR state visibility for L1 or L2. For the CVSS the PR:L is used because triggering requires local KVM access or control of a VMM running nested SVM with LBR virtualization. AC:H is used because the issue depends on nested virtualization and LBRV configuration. The issue is not network reachable and does not involve UAF, out of bounds access, arbitrary write, information disclosure, or host privilege escalation. Impact is mainly nested guest correctness or availability and limited integrity of virtualization state.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2) YES VIRT SIMPLEFIX HARDWARE LINUS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=2 ## 1. ActionableScore * Conservative score: 2 * Paranoid score: 2 * Final recommended bucket: **Ordinary Moderate / Low-like**:: ## 2. Signal breakdown Triggered signals: * Local KVM/VMM trigger: +1 Triggering requires local KVM access or control of a VMM running nested SVM. * Integrity impact weakly plausible: +1 Incorrect dirty marking of `vmcb12` can affect nested LBR virtualization state and visibility for L1 or L2. * Availability impact plausible: +1 Broken nested LBR state may cause nested guest correctness issues or instability. Subtracting signals: * Configuration-dependent exposure: -1 Requires nested SVM and LBR virtualization. * No memory corruption or boundary escape: -1 The patch shows a state correctness bug, not UAF, OOB access, arbitrary write, or host escape. No signals apply for remote reachability, generic memory corruption, real lifetime corruption, privilege escalation, information disclosure, or sensitive-object access. ## 3. Reachability analysis The issue is reachable only through local virtualization control. An attacker would need access to KVM or control over a VMM that can run nested SVM with LBR virtualization enabled. Namespaces and containers matter only if `/dev/kvm` is delegated to an untrusted workload. The issue is not network reachable and is not triggered from ordinary guest userspace unless that user can influence nested virtualization state. ## 4. Severity interpretation This behaves like a low-risk nested virtualization state correctness issue. It may affect nested guest LBR state handling, debug/performance visibility, or guest stability. The patch does not show memory corruption, host privilege escalation, guest-to-host escape, or a meaningful security boundary bypass. Realistic impact is limited to nested guest correctness or DoS. ## 5. One-sentence report phrase KVM nSVM could incorrectly mark `vmcb12` LBR state dirty during nested VM exit, causing nested LBR virtualization state correctness issues without memory corruption or host privilege impact. ## 6. Manual review recommendation NO MANUAL CHECK NEEDED This is a nested KVM state correctness issue with no supported memory corruption, remote exposure, or host security-boundary escape. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: KVM: nSVM: Avoid clearing VMCB_LBR in vmcb12 Commit: a3f0981a5a0e0bd51ad74cc7d9eed32294b24002 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a3f0981a5a0e0bd51ad74cc7d9eed32294b24002 Commit description: svm_copy_lbrs() always marks VMCB_LBR dirty in the destination VMCB. However, nested_svm_vmexit() uses it to copy LBRs to vmcb12, and clearing clean bits in vmcb12 is not architecturally defined. Move vmcb_mark_dirty() to callers and drop it for vmcb12. This also facilitates incoming refactoring that does not pass the entire VMCB to svm_copy_lbrs(). Fixes: d20c796 ("KVM: x86: nSVM: implement nested LBR virtualization") Cc: stable@vger.kernel.org Signed-off-by: Yosry Ahmed Link: https://patch.msgid.link/20260303003421.2185681-2-yosry@kernel.org Signed-off-by: Sean Christopherson Signed-off-by: Greg Kroah-Hartman Changed files: 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=a3f0981a5a0e0bd51ad74cc7d9eed32294b24002 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-46071 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46071 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:N/I:L/A:H 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: 2 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).