{"api_version":"1","generated_at":"2026-10-29T23:21:12+00:00","cve":"CVE-2026-99178","urls":{"html":"https://cve.report/CVE-2026-99178","api":"https://cve.report/api/cve/CVE-2026-99178.json","docs":"https://cve.report/api","cve_org":"https://www.cve.org/CVERecord?id=CVE-2026-99178","nvd":"https://nvd.nist.gov/vuln/detail/CVE-2026-99178"},"summary":{"title":"x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nx86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context\n\nAn async #PF \"page not present\" event arrives through the #PF vector,\nand its reason sits in the per-CPU apf_reason.flags until\n__kvm_handle_async_pf() reads and clears it. If an NMI arrives after\nthe event is injected and before the #PF handler reads the flags, and\nthe NMI handler itself takes a #PF, that fault reads and clears the\nflags belonging to the #PF it interrupted. It then finds interrupts\ndisabled in the NMI context it interrupted, and panics:\n\n  Kernel panic - not syncing: Host injected async #PF in interrupt\ndisabled region\n  Call Trace:\n   <NMI>\n   panic+0x52/0x60\n   __kvm_handle_async_pf+0xb0/0xb0\n   exc_page_fault+0x71/0xb0\n   asm_exc_page_fault+0x27/0x30\n  RIP: 0010:__get_user_nocheck_8+0x6/0x20\n   perf_callchain_user+0x147/0x1a0\n   get_perf_callchain+0x12d/0x1c0\n   ...\n   perf_event_nmi_handler+0x26/0x50\n   exc_nmi+0xe8/0x110\n  RIP: 0010:asm_exc_page_fault+0x0/0x30\n   </NMI>\n\nThe last RIP line shows the NMI landing on the first instruction of\nthe #PF entry. perf with user call chains takes such a fault: the\nsampling NMI walks the user stack with __get_user(), which faults when\na saved frame pointer is not a valid user address or the walk runs off\nthe top of the stack. The same window exists when KVM, running as an\nL1 hypervisor with DELIVERY_AS_PF_VMEXIT, reads the flags itself after\na #PF VM-exit.\n\nThe NMI entry code saves and restores CR2 for this nesting, but not the\nasync #PF reason. Andy Lutomirski described the race, with perf as the\nexample, when async #PF delivery was reworked in 2020 [1]. Saving and\nclearing the flags across the NMI, next to CR2, was discussed there but\nnot merged.\n\nSince that rework, Linux has stopped setting KVM_ASYNC_PF_SEND_ALWAYS,\nso the host delivers \"page not present\" only for user mode faults and\na kernel-mode fault in NMI context (including kernel-mode #MC and #DB)\ncannot be an async page fault.  In that case, return without touching\nthe flags so that the interrupted handler finds them intact.\n\nTested on v7.3-rc5 in a 4-vCPU, 2 GiB guest restored from a snapshot\nwhose memory is served on demand through userfaultfd, so that reading\nit back takes async #PFs, while every thread samples hardware cycles\nwith user call chains and points its frame pointer at an unmapped page.\nWithout this patch the guest panicked as above in 3 of 3 runs. With it,\nthe guest survived 3 of 3 runs of 60 seconds, and the host counted\n148,441 to 153,334 kvm_async_pf_not_present events in the first 20\nseconds of each. On 6.18.50 the results were the same: the guest\npanicked without the patch and survived with it.","state":"PUBLISHED","assigner":"Linux","published_at":"2026-10-28 11:17:38","updated_at":"2026-10-28 11:17:38"},"problem_types":[],"metrics":[],"references":[{"url":"https://git.kernel.org/stable/c/b77e9622bbade9812e2090c66d25c372f515e921","name":"https://git.kernel.org/stable/c/b77e9622bbade9812e2090c66d25c372f515e921","refsource":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://git.kernel.org/stable/c/f45ee1ff6358a22d7e22a4c53cb3148ea97b10d9","name":"https://git.kernel.org/stable/c/f45ee1ff6358a22d7e22a4c53cb3148ea97b10d9","refsource":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://git.kernel.org/stable/c/81d99abe2bd81379abfb595e403bbbee6fbfc624","name":"https://git.kernel.org/stable/c/81d99abe2bd81379abfb595e403bbbee6fbfc624","refsource":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-99178","name":"CVE Program record","refsource":"CVE.ORG","tags":["canonical"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-99178","name":"NVD vulnerability detail","refsource":"NVD","tags":["canonical","analysis"]}],"affected":[{"source":"CNA","vendor":"Linux","product":"Linux","version":"affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 b77e9622bbade9812e2090c66d25c372f515e921 git","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 f45ee1ff6358a22d7e22a4c53cb3148ea97b10d9 git","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 81d99abe2bd81379abfb595e403bbbee6fbfc624 git","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"affected 6.18.59 semver","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"affected 7.2.13 semver","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"unaffected 6.18.59 6.18.* semver","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"unaffected 7.2.13 7.2.* semver","platforms":[]},{"source":"CNA","vendor":"Linux","product":"Linux","version":"unaffected 7.3 * original_commit_for_fix","platforms":[]}],"timeline":[],"solutions":[],"workarounds":[],"exploits":[],"credits":[],"nvd_cpes":[],"vendor_comments":[],"enrichments":{"kev":null,"epss":null,"legacy_qids":[]},"source_records":{"cve_program":{"containers":{"cna":{"affected":[{"defaultStatus":"unaffected","product":"Linux","programFiles":["arch/x86/kernel/kvm.c"],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","vendor":"Linux","versions":[{"lessThan":"b77e9622bbade9812e2090c66d25c372f515e921","status":"affected","version":"1da177e4c3f41524e886b7f1b8a0c1fc7321cac2","versionType":"git"},{"lessThan":"f45ee1ff6358a22d7e22a4c53cb3148ea97b10d9","status":"affected","version":"1da177e4c3f41524e886b7f1b8a0c1fc7321cac2","versionType":"git"},{"lessThan":"81d99abe2bd81379abfb595e403bbbee6fbfc624","status":"affected","version":"1da177e4c3f41524e886b7f1b8a0c1fc7321cac2","versionType":"git"},{"lessThan":"6.18.59","status":"affected","version":"0","versionType":"semver"},{"lessThan":"7.2.13","status":"affected","version":"0","versionType":"semver"}]},{"defaultStatus":"affected","product":"Linux","programFiles":["arch/x86/kernel/kvm.c"],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","vendor":"Linux","versions":[{"lessThanOrEqual":"6.18.*","status":"unaffected","version":"6.18.59","versionType":"semver"},{"lessThanOrEqual":"7.2.*","status":"unaffected","version":"7.2.13","versionType":"semver"},{"lessThanOrEqual":"*","status":"unaffected","version":"7.3","versionType":"original_commit_for_fix"}]}],"cpeApplicability":[{"nodes":[{"cpeMatch":[{"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionEndExcluding":"6.18.59","vulnerable":true},{"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionEndExcluding":"7.2.13","vulnerable":true},{"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionEndExcluding":"7.3","vulnerable":true}],"negate":false,"operator":"OR"}]}],"descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\nx86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context\n\nAn async #PF \"page not present\" event arrives through the #PF vector,\nand its reason sits in the per-CPU apf_reason.flags until\n__kvm_handle_async_pf() reads and clears it. If an NMI arrives after\nthe event is injected and before the #PF handler reads the flags, and\nthe NMI handler itself takes a #PF, that fault reads and clears the\nflags belonging to the #PF it interrupted. It then finds interrupts\ndisabled in the NMI context it interrupted, and panics:\n\n  Kernel panic - not syncing: Host injected async #PF in interrupt\ndisabled region\n  Call Trace:\n   <NMI>\n   panic+0x52/0x60\n   __kvm_handle_async_pf+0xb0/0xb0\n   exc_page_fault+0x71/0xb0\n   asm_exc_page_fault+0x27/0x30\n  RIP: 0010:__get_user_nocheck_8+0x6/0x20\n   perf_callchain_user+0x147/0x1a0\n   get_perf_callchain+0x12d/0x1c0\n   ...\n   perf_event_nmi_handler+0x26/0x50\n   exc_nmi+0xe8/0x110\n  RIP: 0010:asm_exc_page_fault+0x0/0x30\n   </NMI>\n\nThe last RIP line shows the NMI landing on the first instruction of\nthe #PF entry. perf with user call chains takes such a fault: the\nsampling NMI walks the user stack with __get_user(), which faults when\na saved frame pointer is not a valid user address or the walk runs off\nthe top of the stack. The same window exists when KVM, running as an\nL1 hypervisor with DELIVERY_AS_PF_VMEXIT, reads the flags itself after\na #PF VM-exit.\n\nThe NMI entry code saves and restores CR2 for this nesting, but not the\nasync #PF reason. Andy Lutomirski described the race, with perf as the\nexample, when async #PF delivery was reworked in 2020 [1]. Saving and\nclearing the flags across the NMI, next to CR2, was discussed there but\nnot merged.\n\nSince that rework, Linux has stopped setting KVM_ASYNC_PF_SEND_ALWAYS,\nso the host delivers \"page not present\" only for user mode faults and\na kernel-mode fault in NMI context (including kernel-mode #MC and #DB)\ncannot be an async page fault.  In that case, return without touching\nthe flags so that the interrupted handler finds them intact.\n\nTested on v7.3-rc5 in a 4-vCPU, 2 GiB guest restored from a snapshot\nwhose memory is served on demand through userfaultfd, so that reading\nit back takes async #PFs, while every thread samples hardware cycles\nwith user call chains and points its frame pointer at an unmapped page.\nWithout this patch the guest panicked as above in 3 of 3 runs. With it,\nthe guest survived 3 of 3 runs of 60 seconds, and the host counted\n148,441 to 153,334 kvm_async_pf_not_present events in the first 20\nseconds of each. On 6.18.50 the results were the same: the guest\npanicked without the patch and survived with it."}],"providerMetadata":{"dateUpdated":"2026-10-28T10:35:31.961Z","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","shortName":"Linux"},"references":[{"url":"https://git.kernel.org/stable/c/b77e9622bbade9812e2090c66d25c372f515e921"},{"url":"https://git.kernel.org/stable/c/f45ee1ff6358a22d7e22a4c53cb3148ea97b10d9"},{"url":"https://git.kernel.org/stable/c/81d99abe2bd81379abfb595e403bbbee6fbfc624"}],"title":"x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context","x_generator":{"engine":"bippy-1.2.0"}}},"cveMetadata":{"assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","assignerShortName":"Linux","cveId":"CVE-2026-99178","datePublished":"2026-10-28T10:35:31.961Z","dateReserved":"2026-09-25T10:25:14.399Z","dateUpdated":"2026-10-28T10:35:31.961Z","state":"PUBLISHED"},"dataType":"CVE_RECORD","dataVersion":"5.2"},"nvd":{"publishedDate":"2026-10-28 11:17:38","lastModifiedDate":"2026-10-28 11:17:38","problem_types":[],"metrics":[],"configurations":[]},"legacy_mitre":{"record":{"CveYear":"2026","CveId":"99178","Ordinal":"1","Title":"x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI co","CVE":"CVE-2026-99178","Year":"2026"},"notes":[{"CveYear":"2026","CveId":"99178","Ordinal":"1","NoteData":"In the Linux kernel, the following vulnerability has been resolved:\n\nx86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context\n\nAn async #PF \"page not present\" event arrives through the #PF vector,\nand its reason sits in the per-CPU apf_reason.flags until\n__kvm_handle_async_pf() reads and clears it. If an NMI arrives after\nthe event is injected and before the #PF handler reads the flags, and\nthe NMI handler itself takes a #PF, that fault reads and clears the\nflags belonging to the #PF it interrupted. It then finds interrupts\ndisabled in the NMI context it interrupted, and panics:\n\n  Kernel panic - not syncing: Host injected async #PF in interrupt\ndisabled region\n  Call Trace:\n   <NMI>\n   panic+0x52/0x60\n   __kvm_handle_async_pf+0xb0/0xb0\n   exc_page_fault+0x71/0xb0\n   asm_exc_page_fault+0x27/0x30\n  RIP: 0010:__get_user_nocheck_8+0x6/0x20\n   perf_callchain_user+0x147/0x1a0\n   get_perf_callchain+0x12d/0x1c0\n   ...\n   perf_event_nmi_handler+0x26/0x50\n   exc_nmi+0xe8/0x110\n  RIP: 0010:asm_exc_page_fault+0x0/0x30\n   </NMI>\n\nThe last RIP line shows the NMI landing on the first instruction of\nthe #PF entry. perf with user call chains takes such a fault: the\nsampling NMI walks the user stack with __get_user(), which faults when\na saved frame pointer is not a valid user address or the walk runs off\nthe top of the stack. The same window exists when KVM, running as an\nL1 hypervisor with DELIVERY_AS_PF_VMEXIT, reads the flags itself after\na #PF VM-exit.\n\nThe NMI entry code saves and restores CR2 for this nesting, but not the\nasync #PF reason. Andy Lutomirski described the race, with perf as the\nexample, when async #PF delivery was reworked in 2020 [1]. Saving and\nclearing the flags across the NMI, next to CR2, was discussed there but\nnot merged.\n\nSince that rework, Linux has stopped setting KVM_ASYNC_PF_SEND_ALWAYS,\nso the host delivers \"page not present\" only for user mode faults and\na kernel-mode fault in NMI context (including kernel-mode #MC and #DB)\ncannot be an async page fault.  In that case, return without touching\nthe flags so that the interrupted handler finds them intact.\n\nTested on v7.3-rc5 in a 4-vCPU, 2 GiB guest restored from a snapshot\nwhose memory is served on demand through userfaultfd, so that reading\nit back takes async #PFs, while every thread samples hardware cycles\nwith user call chains and points its frame pointer at an unmapped page.\nWithout this patch the guest panicked as above in 3 of 3 runs. With it,\nthe guest survived 3 of 3 runs of 60 seconds, and the host counted\n148,441 to 153,334 kvm_async_pf_not_present events in the first 20\nseconds of each. On 6.18.50 the results were the same: the guest\npanicked without the patch and survived with it.","Type":"Description","Title":"x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI co"}]}}}