CVE-2026-99178 (GCVE-0-2026-99178)

Vulnerability from cvelistv5 – Published: 2026-10-28 10:35 – Updated: 2026-11-01 12:42
VLAI
Title
x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context
Summary
In the Linux kernel, the following vulnerability has been resolved: x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context An async #PF "page not present" event arrives through the #PF vector, and its reason sits in the per-CPU apf_reason.flags until __kvm_handle_async_pf() reads and clears it. If an NMI arrives after the event is injected and before the #PF handler reads the flags, and the NMI handler itself takes a #PF, that fault reads and clears the flags belonging to the #PF it interrupted. It then finds interrupts disabled in the NMI context it interrupted, and panics: Kernel panic - not syncing: Host injected async #PF in interrupt disabled region Call Trace: <NMI> panic+0x52/0x60 __kvm_handle_async_pf+0xb0/0xb0 exc_page_fault+0x71/0xb0 asm_exc_page_fault+0x27/0x30 RIP: 0010:__get_user_nocheck_8+0x6/0x20 perf_callchain_user+0x147/0x1a0 get_perf_callchain+0x12d/0x1c0 ... perf_event_nmi_handler+0x26/0x50 exc_nmi+0xe8/0x110 RIP: 0010:asm_exc_page_fault+0x0/0x30 </NMI> The last RIP line shows the NMI landing on the first instruction of the #PF entry. perf with user call chains takes such a fault: the sampling NMI walks the user stack with __get_user(), which faults when a saved frame pointer is not a valid user address or the walk runs off the top of the stack. The same window exists when KVM, running as an L1 hypervisor with DELIVERY_AS_PF_VMEXIT, reads the flags itself after a #PF VM-exit. The NMI entry code saves and restores CR2 for this nesting, but not the async #PF reason. Andy Lutomirski described the race, with perf as the example, when async #PF delivery was reworked in 2020 [1]. Saving and clearing the flags across the NMI, next to CR2, was discussed there but not merged. Since that rework, Linux has stopped setting KVM_ASYNC_PF_SEND_ALWAYS, so the host delivers "page not present" only for user mode faults and a kernel-mode fault in NMI context (including kernel-mode #MC and #DB) cannot be an async page fault. In that case, return without touching the flags so that the interrupted handler finds them intact. Tested on v7.3-rc5 in a 4-vCPU, 2 GiB guest restored from a snapshot whose memory is served on demand through userfaultfd, so that reading it back takes async #PFs, while every thread samples hardware cycles with user call chains and points its frame pointer at an unmapped page. Without this patch the guest panicked as above in 3 of 3 runs. With it, the guest survived 3 of 3 runs of 60 seconds, and the host counted 148,441 to 153,334 kvm_async_pf_not_present events in the first 20 seconds of each. On 6.18.50 the results were the same: the guest panicked without the patch and survived with it.
Severity
No CVSS data available.
Assigner
Linux CNA under the mitre root
CNA scorecard D 53/100 over 6637 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 631bc4878220932fe67fc46fc7cf7cccdb1ec597 , < fd60b32a99627fda479c9fa1d4a82fa9f7994670 (git)
Affected: 631bc4878220932fe67fc46fc7cf7cccdb1ec597 , < b77e9622bbade9812e2090c66d25c372f515e921 (git)
Affected: 631bc4878220932fe67fc46fc7cf7cccdb1ec597 , < f45ee1ff6358a22d7e22a4c53cb3148ea97b10d9 (git)
Affected: 631bc4878220932fe67fc46fc7cf7cccdb1ec597 , < 81d99abe2bd81379abfb595e403bbbee6fbfc624 (git)
guessed Create a notification for this product.
Linux Linux Affected: 2.6.38
Unaffected: 0 , < 2.6.38 (semver)
Unaffected: 6.12.115 , ≤ 6.12.* (semver)
Unaffected: 6.18.59 , ≤ 6.18.* (semver)
Unaffected: 7.2.13 , ≤ 7.2.* (semver)
Unaffected: 7.3 , ≤ * (original_commit_for_fix)
guessed Create a notification for this product.
Show details on NVD website
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nx86/kvm: Don\u0027t 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   \u003cNMI\u003e\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   \u003c/NMI\u003e\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."
        }
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    "datePublished": "2026-10-28T10:35:31.961Z",
    "dateReserved": "2026-09-25T10:25:14.399Z",
    "dateUpdated": "2026-11-01T12:42:11.076Z",
    "state": "PUBLISHED"
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      "product_status:known_affected": "2",
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      "title": "x86/kvm: Don\u0027t consume async #PF flags on a kernel #PF in NMI context",
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            "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nx86/kvm: Don\u0027t 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   \u003cNMI\u003e\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   \u003c/NMI\u003e\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."
          }
        ],
        "id": "CVE-2026-99178",
        "lastModified": "2026-11-01T13:18:02.150",
        "metrics": {},
        "published": "2026-10-28T11:17:38.503",
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        "vulnStatus": "Received"
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      "aggregate_severity": "Moderate",
      "current_release_date": "2026-10-29T07:18:59+00:00",
      "cve": "CVE-2026-99178",
      "id": "CVE-2026-99178",
      "initial_release_date": "2026-10-28T10:35:31.961000+00:00",
      "product_status:known_not_affected": "278",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: kernel: Denial of Service via race condition in KVM asynchronous page fault handling",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-99178.json",
      "version": "3"
    }
  }
}



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