From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-46295][MODERATE REGULAR] KVM: x86: Do IRR scan in __kvm_apic_update_irr even if PIR is empty Date: Mon, 08 Jun 2026 15:12:08 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-46295 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: 2 X-AL-KERNEL-Commit: bb1703949dcaa9a49c338dee075f659f4634214d List-Id: CVE: CVE-2026-46295 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: KVM: x86: Do IRR scan in __kvm_apic_update_irr even if PIR is empty Commit: bb1703949dcaa9a49c338dee075f659f4634214d Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bb1703949dcaa9a49c338dee075f659f4634214d Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46295 Analysis date: Mon, 08 Jun 2026 15:12:08 -0400 ActionableScore: 3 ActionableScore lower bound: 2 Actionable bucket: Borderline, manual review recommended but not Actionable Moderate Manual review required: YES Summary: A race in KVM x86 posted interrupt synchronization can cause incorrect `max_irr` reporting and trigger a host WARNING during nested VM execution, leading mainly to DoS risk in configurations where warnings are fatal. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-46295 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46295 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-2026-46295 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:N/I:N/A:H';CWE-362;CWE-703;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'KVM x86 posted interrupt handling can report an incorrect max_irr when a race occurs between PIR harvesting on the target vCPU and setting PID.ON on a sender vCPU. The interrupt is not lost because it already resides in IRR, but the second sync path can see PID.ON with an empty PIR and return -1 without scanning existing IRR. This can trigger a WARNING in vmx_check_nested_events and waste an L2 VM Enter Exit cycle. For the CVSS the PR:L is used because an attacker needs the ability to run or control local KVM guest execution, likely with nested virtualization or similar stress conditions. The issue is not network reachable and does not show a UAF, OOB access, arbitrary write, or information leak primitive. Impact is mainly denial of service through warning spam or degraded host responsiveness, with A:H only in the paranoid score if panic_on_warn or similar policy turns the warning into a host crash.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended but not Actionable Moderate (with actual score 3) MAYBE VIRT WARNONLY RACE HARDWARE TEST INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 ActionableScoreLower=2 ## 1. ActionableScore * Conservative score: 2 * Paranoid score: 3 * Final recommended bucket: **Borderline, manual review recommended but not Actionable Moderate**:: ## 2. Signal breakdown Conservative signals: * Local unprivileged or low privilege trigger: +1 A local user or process with access to `/dev/kvm` can potentially exercise the affected KVM vCPU path. This is PR:L rather than PR:H in many virtualization deployments. * Broad/common subsystem exposure: +1 KVM x86 is a common virtualization subsystem on hosts running VMs. * Hard or timing dependent race: -1 The issue requires a specific interleaving between posted interrupt delivery and PIR to IRR synchronization. * Availability impact realistic: +1 The demonstrated effect is a kernel WARNING and wasted nested VM entry/exit work. This is availability relevant, but weak in the conservative case because the interrupt is not lost and no corruption is shown. Paranoid additional signal: * Reliable kernel crash / strong DoS: +1 Only for configurations where `panic_on_warn` or similar policy turns the WARNING into a host crash. Not counted: * Generic memory corruption: +0 The patch does not show UAF, double free, OOB write, arbitrary write, SPTE/rmap lifetime corruption, or page-cache corruption. * Privilege escalation plausible: +0 No realistic LPE primitive is supported by the patch context. * Remote reachable: +0 This is not network-triggerable directly. ## 3. Reachability analysis The bug is reachable through local KVM execution paths, especially nested VM or posted interrupt stress scenarios. A normal remote network attacker cannot directly trigger it unless they already control workload execution inside a VM management environment. Privileges depend on deployment. On many hosts, access to `/dev/kvm` is granted to users in a `kvm` group or to VM management services, so PR:L is the practical interpretation. Containers only matter if `/dev/kvm` is passed through or if a containerized workload can control KVM vCPU execution. The path is common on KVM virtualization hosts, but the demonstrated trigger depends on nested VM stress and a narrow race. The interrupt is explicitly not lost, and the main consequence is inconsistent interrupt reporting that causes a WARNING and wasted VM entry/exit work. ## 4. Severity interpretation This behaves like an ordinary Moderate or Low-like KVM correctness issue in the conservative case. It deserves manual review because it is a KVM race in interrupt handling, but the evidence supports DoS only. Theoretical risk is limited because there is no demonstrated memory lifetime bug, no KVM MMU rmap/SPTE stale pointer pattern, and no attacker-controlled overwrite or disclosure primitive. The paranoid score only raises availability for `panic_on_warn` style deployments and does not treat this as privilege escalation or memory corruption. ## 5. One-sentence report phrase A race in KVM x86 posted interrupt synchronization can cause incorrect `max_irr` reporting and trigger a host WARNING during nested VM execution, leading mainly to DoS risk in configurations where warnings are fatal. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED This is a KVM race in nested interrupt handling, so it should not be blindly auto-closed, but current evidence supports warning or DoS only and does not justify Important-class handling. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: KVM: x86: Do IRR scan in __kvm_apic_update_irr even if PIR is empty Commit: bb1703949dcaa9a49c338dee075f659f4634214d Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bb1703949dcaa9a49c338dee075f659f4634214d Commit description: Fall back to apic_find_highest_vector() when PID.ON is set but PIR turns out to be empty, to correctly report the highest pending interrupt from the existing IRR. In a nested VM stress test, the following WARNING fires in vmx_check_nested_events() when kvm_cpu_has_interrupt() reports a pending interrupt but the subsequent kvm_apic_has_interrupt() (which invokes vmx_sync_pir_to_irr() again) returns -1: WARNING: CPU: 99 PID: 57767 at arch/x86/kvm/vmx/nested.c:4449 vmx_check_nested_events+0x6bf/0x6e0 [kvm_intel] Call Trace: kvm_check_and_inject_events vcpu_enter_guest.constprop.0 vcpu_run kvm_arch_vcpu_ioctl_run kvm_vcpu_ioctl __x64_sys_ioctl do_syscall_64 entry_SYSCALL_64_after_hwframe The root cause is a race between vmx_sync_pir_to_irr() on the target vCPU and __vmx_deliver_posted_interrupt() on a sender vCPU. The sender performs two individually-atomic operations that are not a single transaction: 1. pi_test_and_set_pir(vector) -- sets the PIR bit 2. pi_test_and_set_on() -- sets PID.ON The following interleaving triggers the bug: Sender vCPU (IPI): Target vCPU (1st sync_pir_to_irr): B1: set PIR[vector] A1: pi_clear_on() A2: pi_harvest_pir() -> sees B1 bit A3: xchg() -> consumes bit, PIR=0 (1st sync returns correct max_irr) B2: set PID.ON = 1 Target vCPU (2nd sync_pir_to_irr): C1: pi_test_on() -> TRUE (from B2) C2: pi_clear_on() -> ON=0 C3: pi_harvest_pir() -> PIR empty C4: *max_irr = -1, early return IRR NOT SCANNED The interrupt is not lost (it resides in the IRR from the first sync and is recovered on the next vcpu_enter_guest() iteration), but the incorrect max_irr causes a spurious WARNING and a wasted L2 VM-Enter/VM-Exit cycle. Fixes: b41f863 ("KVM: VMX: Isolate pure loads from atomic XCHG when processing PIR") Reported-by: Farrah Chen Analyzed-by: Chenyi Qiang Cc: stable@vger.kernel.org Reviewed-by: Sean Christopherson Link: https://lore.kernel.org/kvm/20260428070349.1633238-1-chenyi.qiang@intel.com/T/ Link: https://patch.msgid.link/20260503201703.108231-2-pbonzini@redhat.com/ Signed-off-by: Paolo Bonzini Signed-off-by: Greg Kroah-Hartman Changed files: arch/x86/kvm/vmx/nested.c arch/x86/kvm/lapic.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=bb1703949dcaa9a49c338dee075f659f4634214d ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-46295 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46295 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: 2 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).