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CVE-2026-99178

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: 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.

Timeline

  1. ID reserved

    By Linux

  2. · 33 days later

    Published

  3. · 4 days later

    Record updated

Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.

Scores

No CVSS score yet: neither the CNA, CISA nor NVD has scored this CVE. NVD status: Received.

EPSS (exploitation in 30 days)
0.17% · top 93% of all CVEs (9 Nov 2026)
Loading the EPSS history…

The list uses the assigner's score when there is one, then CISA's, then NVD's: Not scored.

Affected products

  • Linux · Linux

    • affected 631bc4878220932fe67fc46fc7cf7cccdb1ec597 – < fd60b32a99627fda479c9fa1d4a82fa9f7994670
    • affected 631bc4878220932fe67fc46fc7cf7cccdb1ec597 – < b77e9622bbade9812e2090c66d25c372f515e921
    • affected 631bc4878220932fe67fc46fc7cf7cccdb1ec597 – < f45ee1ff6358a22d7e22a4c53cb3148ea97b10d9
    • affected 631bc4878220932fe67fc46fc7cf7cccdb1ec597 – < 81d99abe2bd81379abfb595e403bbbee6fbfc624

  • Linux · Linux

    • affected 2.6.38
    • unaffected < 2.6.38
    • unaffected 6.12.115 – ≤ 6.12.*
    • unaffected 6.18.59 – ≤ 6.18.*
    • unaffected 7.2.13 – ≤ 7.2.*
    • unaffected 7.3 – ≤ *

References

4

Sources

The same CVE in the primary databases and aggregators, to check against.

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Record last updated .