{"id":"CVE-2026-98720","kind":"vulnerability","aliases":["CVE-2026-98720"],"related":[],"title":"Linux KVM: nested VM entry can leave TPR writes passed through to L0","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: nVMX: Force MSR bitmap refresh if runtime eVMCS controls are modified\n\nForce a refresh of the vmcs02 MSR bitmap during nested VM-Enter if the\nruntime eVMCS controls (pin, primary, secondary, etc.) are being updated.\nIf L1 isn't intercepting TPR writes, runs L2 with TPR virtualization, and\nthen runs the same L2 with TPR virtualization disabled, KVM will fail to\nrefresh msr_bitmap02 and leave TPR in passthrough mode even though TPR\nvirtualization is disabled.  I.e. failure to refresh the bitmap lets L2 (or\nL1 by proxy) read and write L0's TPR.","summary":"The Linux kernel's KVM nested VMX code fails to refresh the vmcs02 MSR bitmap when eVMCS controls change between nested VM entries. A guest VM can then read and write the host's TPR (task priority register) directly.","severity":"unknown","cvss":null,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["kvm","linux-kernel","nested-virtualization","runtime","self-hosted"],"not_affected_if":["no-kvm","no-nested-virtualization"],"how_to_check":"- unknown","fix":"- unknown","status":"open","published_at":"2026-10-18T16:17:25+00:00","first_seen_at":"2026-10-23T21:57:16+00:00","triaged_at":"2026-10-25T05:17:24+00:00","updated_at":"2026-10-25T05:17:24+00:00","category":"runtime","confidence":0.5,"triage":{"at":"2026-10-25T05:17:24Z","model":"claude-haiku-5-5","output":{"fix":"- unknown","reach":"local","title":"Linux KVM: nested VM entry can leave TPR writes passed through to L0","impact":"medium","summary":"The Linux kernel's KVM nested VMX code fails to refresh the vmcs02 MSR bitmap when eVMCS controls change between nested VM entries. A guest VM can then read and write the host's TPR (task priority register) directly.","category":"runtime","nicknames":[],"who_cares":["linux-kernel","kvm","nested-virtualization","self-hosted"],"confidence":0.5,"needs_auth":"yes","worst_case":"Guest reads and writes host TPR, weakening virtualization isolation","analyst_note":"Find the fixed kernel versions and stable backport commits in the NVD entry or the kernel.org stable commit for this CVE.","how_to_check":"- unknown","needs_analyst":true,"not_affected_if":["no-kvm","no-nested-virtualization"],"who_is_affected":"Hosts running the Linux kernel with KVM and nested virtualization (Hyper-V enlightened VMCS, L1 hypervisors running L2 guests) are the systems where this applies. The sources do not say which kernel versions are affected or which configurations beyond nested VMX are required. Hosts without KVM nested virtualization can skip it.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"medium"},"cost_usd":0.000595,"input_tokens":3486,"output_tokens":492,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':15A,18A '-98720':16A,19A 'bitmap':45B,84C,99C,163C 'bitmap02':145C 'chang':49B 'code':38B 'control':48B,89C,109C 'cve':14A,17A 'direct':69B 'disabl':138C,157C 'enter':104C 'entri':5A,53B 'etc':113C 'even':152C 'evmc':47B,88C,108C 'fail':39B,141C 'failur':159C 'follow':75C 'forc':82C,92C 'guest':55B 'host':30B,63B 'i.e':158C 'intercept':121C 'isn':119C 'kernel':23B,33B,73C 'kvm':2A,20B,35B,80C,139C 'l0':13A,173C 'l1':118C,167C 'l2':125C,134C,165C 'leav':7A,147C 'let':164C 'linux':1A,22B,32B,72C 'linux-kernel':21B 'mode':151C 'modifi':91C 'msr':44B,83C,98C,144C 'nest':3A,25B,36B,51B,101C 'nested-virtu':24B 'nvmx':81C 'pass':10A 'passthrough':150C 'pin':110C 'primari':111C 'prioriti':67B 'proxi':169C 'read':59B,170C 'refresh':41B,85C,94C,143C,161C 'regist':68B 'resolv':79C 'run':124C,131C 'runtim':27B,87C,107C 'secondari':112C 'self':29B 'self-host':28B 'task':66B 'though':153C 'tpr':8A,65B,122C,127C,136C,148C,154C,175C 'updat':116C 'virtual':26B,128C,137C,155C 'vm':4A,52B,56B,103C 'vm-enter':102C 'vmcs02':43B,97C 'vmx':37B 'vulner':76C 'write':9A,61B,123C,172C","nicknames":[],"mitigations":[],"who_affected":"Hosts running the Linux kernel with KVM and nested virtualization (Hyper-V enlightened VMCS, L1 hypervisors running L2 guests) are the systems where this applies. The sources do not say which kernel versions are affected or which configurations beyond nested VMX are required. Hosts without KVM nested virtualization can skip it.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-18T16:17:25+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-98720","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98720","severity":null,"published_at":"2026-10-18T16:17:25+00:00","modified_at":"2026-10-19T10:19:36+00:00","first_seen_at":"2026-10-23T21:57:16+00:00","fetched_at":"2026-10-23T22:28:56+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}