From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-64513][LOW] KVM: x86: Unconditionally recompute CR8 intercept on PPR update [ Upstream Date: Sat, 25 Jul 2026 10:39:45 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-64513 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: 1 X-AL-KERNEL-Commit: ff9c4c6428883182960cfe5c78928f0896d80ebc List-Id: CVE: CVE-2026-64513 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: KVM: x86: Unconditionally recompute CR8 intercept on PPR update [ Upstream Commit: ff9c4c6428883182960cfe5c78928f0896d80ebc Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ff9c4c6428883182960cfe5c78928f0896d80ebc Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64513 Analysis date: Sat, 25 Jul 2026 10:39:45 -0400 ActionableScore: 2 ActionableScore lower bound: 1 Actionable bucket: Ordinary Moderate / Low-like Manual review required: NO Summary: A KVM x86 logic bug can leave CR8 or TPR threshold intercept state stale after PPR updates, causing VM-entry failure and denial of service for affected guests on older or nested virtualization configurations. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-64513 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64513 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-2026-64513 LOW CHECK WITH IMPACT FROM ORIG NN MODERATE 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:N/A:H';CWE-667;CWE-703;CWE-755;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A KVM x86 logic bug can leave the VMX TPR_THRESHOLD or CR8 intercept state stale after a guest PPR update when no deliverable interrupt is pending. On affected systems this can make the next VM entry fail with a hardware error, especially on older platforms or nested virtualization setups without virtual interrupt delivery. For the CVSS the PR:L is used because reliable triggering requires the ability to run or control a guest or KVM vCPU, but it does not necessarily require full host administrator privileges in tenant or delegated virtualization environments. The issue is not network reachable by itself and is triggered through local KVM guest execution or VM control paths. Impact is denial of service for the affected VM or vCPU. There is no evidence of UAF, OOB access, stale rmap traversal, information disclosure, arbitrary write, or privilege escalation in this patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2) YES DANGER VIRT HARDWARE DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=2 ActionableScoreLower=1 ## 1. ActionableScore * Conservative score: 1 * Paranoid score: 2 * Final recommended bucket: **Ordinary Moderate / Low-like**:: ## 2. Signal breakdown Triggered signals: * Local low-privilege or tenant-triggerable VM control path: +1 A user or tenant able to run/control a KVM vCPU may influence guest APIC/TPR/PPR state. This does not require full host root in many virtualization deployments, but it does require KVM guest execution or VM control access. * Reliable VM/vCPU DoS in affected configurations: +1 The stale TPR_THRESHOLD/CR8 intercept state can cause VM entry failure with hardware error 0x7, making the affected VM or vCPU fail to run. Subtracting signals: * Legacy or non-default execution mode: -1 The issue is mainly exposed on older platforms or nested virtualization setups without virtual-interrupt delivery/APICv behavior. Modern bare-metal KVM with APICv usually avoids the failing VM-entry check. Paranoid-only addition: * Availability impact realistic: +1 In the affected nested/older-VMX configuration, repeated VM-entry failure can make the guest effectively unusable. This raises the paranoid score from 1 to 2. Not triggered: * No remote/network reachability. * No UAF, OOB, double-free, stale rmap, arbitrary write, refcount takeover, or page-cache corruption. * No plausible LPE primitive. * No confidentiality or integrity impact. ## 3. Reachability analysis The bug is reachable through KVM guest execution or VM control paths that update guest PPR/CR8/TPR-related state. A guest tenant or local user with access to KVM may be able to trigger it, but it is not reachable by network packets alone. Namespaces and containers do not automatically make this reachable unless `/dev/kvm` or equivalent VM control is delegated. The affected condition is configuration-dependent: older hardware or nested virtualization without virtual-interrupt delivery is much more relevant than modern bare-metal KVM with APICv. ## 4. Severity interpretation This behaves like an ordinary Moderate or Low-like KVM logic DoS, not an Important-class kernel vulnerability. The realistic impact is VM/vCPU denial of service due to VM-entry failure. Theoretical memory corruption potential is not supported by the patch or commit message. ## 5. One-sentence report phrase A KVM x86 logic bug can leave CR8 or TPR threshold intercept state stale after PPR updates, causing VM-entry failure and denial of service for affected guests on older or nested virtualization configurations. ## 6. Manual review recommendation NO MANUAL CHECK NEEDED Reason: this is a configuration-dependent KVM logic DoS with no demonstrated memory corruption, no host privilege escalation path, no network reachability, and no confidentiality or integrity primitive. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: KVM: x86: Unconditionally recompute CR8 intercept on PPR update [ Upstream Commit: ff9c4c6428883182960cfe5c78928f0896d80ebc Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ff9c4c6428883182960cfe5c78928f0896d80ebc Changed files: arch/x86/kvm/lapic.c arch/x86/kvm/x86.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=ff9c4c6428883182960cfe5c78928f0896d80ebc ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-64513 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64513 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:N/A:L The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 4.7 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: 1 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).