GHSA-3P46-X25W-PGVH

Vulnerability from github – Published: 2025-10-28 12:30 – Updated: 2026-07-30 06:32
VLAI
Details

In the Linux kernel, the following vulnerability has been resolved:

KVM: x86: Don't (re)check L1 intercepts when completing userspace I/O

When completing emulation of instruction that generated a userspace exit for I/O, don't recheck L1 intercepts as KVM has already finished that phase of instruction execution, i.e. has already committed to allowing L2 to perform I/O. If L1 (or host userspace) modifies the I/O permission bitmaps during the exit to userspace, KVM will treat the access as being intercepted despite already having emulated the I/O access.

Pivot on EMULTYPE_NO_DECODE to detect that KVM is completing emulation. Of the three users of EMULTYPE_NO_DECODE, only complete_emulated_io() (the intended "recipient") can reach the code in question. gp_interception()'s use is mutually exclusive with is_guest_mode(), and complete_emulated_insn_gp() unconditionally pairs EMULTYPE_NO_DECODE with EMULTYPE_SKIP.

The bad behavior was detected by a syzkaller program that toggles port I/O interception during the userspace I/O exit, ultimately resulting in a WARN on vcpu->arch.pio.count being non-zero due to KVM no completing emulation of the I/O instruction.

WARNING: CPU: 23 PID: 1083 at arch/x86/kvm/x86.c:8039 emulator_pio_in_out+0x154/0x170 [kvm] Modules linked in: kvm_intel kvm irqbypass CPU: 23 UID: 1000 PID: 1083 Comm: repro Not tainted 6.16.0-rc5-c1610d2d66b1-next-vm #74 NONE Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 RIP: 0010:emulator_pio_in_out+0x154/0x170 [kvm] PKRU: 55555554 Call Trace: kvm_fast_pio+0xd6/0x1d0 [kvm] vmx_handle_exit+0x149/0x610 [kvm_intel] kvm_arch_vcpu_ioctl_run+0xda8/0x1ac0 [kvm] kvm_vcpu_ioctl+0x244/0x8c0 [kvm] __x64_sys_ioctl+0x8a/0xd0 do_syscall_64+0x5d/0xc60 entry_SYSCALL_64_after_hwframe+0x4b/0x53

Severity
7.9 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H
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{
  "affected": [],
  "aliases": [
    "CVE-2025-40026"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2025-10-28T10:15:42Z",
    "severity": "HIGH"
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: x86: Don\u0027t (re)check L1 intercepts when completing userspace I/O\n\nWhen completing emulation of instruction that generated a userspace exit\nfor I/O, don\u0027t recheck L1 intercepts as KVM has already finished that\nphase of instruction execution, i.e. has already committed to allowing L2\nto perform I/O.  If L1 (or host userspace) modifies the I/O permission\nbitmaps during the exit to userspace,  KVM will treat the access as being\nintercepted despite already having emulated the I/O access.\n\nPivot on EMULTYPE_NO_DECODE to detect that KVM is completing emulation.\nOf the three users of EMULTYPE_NO_DECODE, only complete_emulated_io() (the\nintended \"recipient\") can reach the code in question.  gp_interception()\u0027s\nuse is mutually exclusive with is_guest_mode(), and\ncomplete_emulated_insn_gp() unconditionally pairs EMULTYPE_NO_DECODE with\nEMULTYPE_SKIP.\n\nThe bad behavior was detected by a syzkaller program that toggles port I/O\ninterception during the userspace I/O exit, ultimately resulting in a WARN\non vcpu-\u003earch.pio.count being non-zero due to KVM no completing emulation\nof the I/O instruction.\n\n  WARNING: CPU: 23 PID: 1083 at arch/x86/kvm/x86.c:8039 emulator_pio_in_out+0x154/0x170 [kvm]\n  Modules linked in: kvm_intel kvm irqbypass\n  CPU: 23 UID: 1000 PID: 1083 Comm: repro Not tainted 6.16.0-rc5-c1610d2d66b1-next-vm #74 NONE\n  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015\n  RIP: 0010:emulator_pio_in_out+0x154/0x170 [kvm]\n  PKRU: 55555554\n  Call Trace:\n   \u003cTASK\u003e\n   kvm_fast_pio+0xd6/0x1d0 [kvm]\n   vmx_handle_exit+0x149/0x610 [kvm_intel]\n   kvm_arch_vcpu_ioctl_run+0xda8/0x1ac0 [kvm]\n   kvm_vcpu_ioctl+0x244/0x8c0 [kvm]\n   __x64_sys_ioctl+0x8a/0xd0\n   do_syscall_64+0x5d/0xc60\n   entry_SYSCALL_64_after_hwframe+0x4b/0x53\n   \u003c/TASK\u003e",
  "id": "GHSA-3p46-x25w-pgvh",
  "modified": "2026-07-30T06:32:12Z",
  "published": "2025-10-28T12:30:14Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-40026"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/00338255bb1f422642fb2798ebe92e93b6e4209b"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/3a062a5c55adc5507600b9ae6d911e247e2f1d6e"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/3d3abf3f7e8b1abb082070a343de82d7efc80523"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/7366830642505683bbe905a2ba5d18d6e4b512b8"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/a908eca437789589dd4624da428614c1275064dc"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/ba35a5d775799ce5ad60230be97336f2fefd518e"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/e0ce3ed1048a47986d15aef1a98ebda25560d257"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/e7177c7e32cb806f348387b7f4faafd4a5b32054"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/e750f85391286a4c8100275516973324b621a269"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H",
      "type": "CVSS_V3"
    }
  ]
}



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