{"id":"CVE-2026-99178","state":"PUBLISHED","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.","publishedAt":"2026-10-28T10:35:31.961Z","updatedAt":"2026-10-30T12:42:41.245Z","severity":null,"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"1da177e4c3f41524e886b7f1b8a0c1fc7321cac2","status":"affected","lessThan":"fd60b32a99627fda479c9fa1d4a82fa9f7994670","versionType":"git"},{"version":"1da177e4c3f41524e886b7f1b8a0c1fc7321cac2","status":"affected","lessThan":"b77e9622bbade9812e2090c66d25c372f515e921","versionType":"git"},{"version":"1da177e4c3f41524e886b7f1b8a0c1fc7321cac2","status":"affected","lessThan":"f45ee1ff6358a22d7e22a4c53cb3148ea97b10d9","versionType":"git"},{"version":"1da177e4c3f41524e886b7f1b8a0c1fc7321cac2","status":"affected","lessThan":"81d99abe2bd81379abfb595e403bbbee6fbfc624","versionType":"git"},{"version":"0","status":"affected","lessThan":"6.12.115","versionType":"semver"},{"version":"0","status":"affected","lessThan":"6.18.59","versionType":"semver"},{"version":"0","status":"affected","lessThan":"7.2.13","versionType":"semver"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["arch/x86/kernel/kvm.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"6.12.115","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.59","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.2.13","status":"unaffected","lessThanOrEqual":"7.2.*","versionType":"semver"},{"version":"7.3","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["arch/x86/kernel/kvm.c"]}],"references":["https://git.kernel.org/stable/c/fd60b32a99627fda479c9fa1d4a82fa9f7994670","https://git.kernel.org/stable/c/b77e9622bbade9812e2090c66d25c372f515e921","https://git.kernel.org/stable/c/f45ee1ff6358a22d7e22a4c53cb3148ea97b10d9","https://git.kernel.org/stable/c/81d99abe2bd81379abfb595e403bbbee6fbfc624"],"weaknesses":[],"ingestedAt":"2026-10-30T13:02:04.640Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/99xxx/CVE-2026-99178.json","commit":"cb14bf1b43b0c9f05f7f40eae2246263fd430780","baselineCommit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-99178","state":"PUBLISHED","datePublished":"2026-10-28T10:35:31.961Z","dateUpdated":"2026-10-30T12:42:41.245Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2026-09-25T10:25:14.399Z"},"containers":{"cna":{"title":"x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context","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."}],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"1da177e4c3f41524e886b7f1b8a0c1fc7321cac2","status":"affected","lessThan":"fd60b32a99627fda479c9fa1d4a82fa9f7994670","versionType":"git"},{"version":"1da177e4c3f41524e886b7f1b8a0c1fc7321cac2","status":"affected","lessThan":"b77e9622bbade9812e2090c66d25c372f515e921","versionType":"git"},{"version":"1da177e4c3f41524e886b7f1b8a0c1fc7321cac2","status":"affected","lessThan":"f45ee1ff6358a22d7e22a4c53cb3148ea97b10d9","versionType":"git"},{"version":"1da177e4c3f41524e886b7f1b8a0c1fc7321cac2","status":"affected","lessThan":"81d99abe2bd81379abfb595e403bbbee6fbfc624","versionType":"git"},{"version":"0","status":"affected","lessThan":"6.12.115","versionType":"semver"},{"version":"0","status":"affected","lessThan":"6.18.59","versionType":"semver"},{"version":"0","status":"affected","lessThan":"7.2.13","versionType":"semver"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["arch/x86/kernel/kvm.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"6.12.115","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.59","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.2.13","status":"unaffected","lessThanOrEqual":"7.2.*","versionType":"semver"},{"version":"7.3","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["arch/x86/kernel/kvm.c"]}],"references":[{"url":"https://git.kernel.org/stable/c/fd60b32a99627fda479c9fa1d4a82fa9f7994670"},{"url":"https://git.kernel.org/stable/c/b77e9622bbade9812e2090c66d25c372f515e921"},{"url":"https://git.kernel.org/stable/c/f45ee1ff6358a22d7e22a4c53cb3148ea97b10d9"},{"url":"https://git.kernel.org/stable/c/81d99abe2bd81379abfb595e403bbbee6fbfc624"}],"providerMetadata":{"shortName":"Linux","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateUpdated":"2026-10-30T12:42:41.245Z"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionEndExcluding":"6.12.115"},{"vulnerable":true,"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"}]}]}],"x_generator":{"engine":"bippy-1.2.0"}}}}}