CVE-2025-40026 (GCVE-0-2025-40026)

Vulnerability from cvelistv5 – Published: 2025-10-28 09:32 – Updated: 2026-10-15 22:02
VLAI
Title
KVM: x86: Don't (re)check L1 intercepts when completing userspace I/O
Summary
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: <TASK> 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 </TASK>
Severity
7.9 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H
SSVC
Exploitation: none Automatable: no Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-10-15 21:56 UTC
Assigner
Linux CNA under the mitre root
CNA scorecard D 53/100 over 6996 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < a908eca437789589dd4624da428614c1275064dc (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < 00338255bb1f422642fb2798ebe92e93b6e4209b (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < e0ce3ed1048a47986d15aef1a98ebda25560d257 (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < ba35a5d775799ce5ad60230be97336f2fefd518e (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < 3d3abf3f7e8b1abb082070a343de82d7efc80523 (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < e7177c7e32cb806f348387b7f4faafd4a5b32054 (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < 3a062a5c55adc5507600b9ae6d911e247e2f1d6e (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < 7366830642505683bbe905a2ba5d18d6e4b512b8 (git)
Affected: 8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9 , < e750f85391286a4c8100275516973324b621a269 (git)
guessed Create a notification for this product.
Linux Linux Affected: 3.0
Unaffected: 0 , < 3.0 (semver)
Unaffected: 5.4.301 , ≤ 5.4.* (semver)
Unaffected: 5.10.246 , ≤ 5.10.* (semver)
Unaffected: 5.15.195 , ≤ 5.15.* (semver)
Unaffected: 6.1.157 , ≤ 6.1.* (semver)
Unaffected: 6.6.111 , ≤ 6.6.* (semver)
Unaffected: 6.12.52 , ≤ 6.12.* (semver)
Unaffected: 6.16.12 , ≤ 6.16.* (semver)
Unaffected: 6.17.2 , ≤ 6.17.* (semver)
Unaffected: 6.18 , ≤ * (original_commit_for_fix)
guessed Create a notification for this product.
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          "lang": "en",
          "value": "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"
        }
      ],
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        {
          "cvssV3_1": {
            "baseScore": 7.9,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - Exploitation requires either `KVM_RUN`/`ioctl()` access to `/dev/kvm` on the host or code execution inside a guest running a nested hypervisor; there is no network-reachable path to `x86_emulate_insn()`.\nAC:L - The syzkaller reproducer deterministically toggles the port-I/O bitmap while servicing the `KVM_EXIT_IO` exit, and the guest-side variant only needs a second L1 vCPU to write the bitmap page \u2014 the attacker controls both sides of the window, and nested virt is enabled by default on kvm_intel/kvm_amd.\nPR:L - The attacker needs only unprivileged access to `/dev/kvm` (commonly granted via the `kvm` group or a VMM process), or ordinary code execution inside a guest; no root or CAP_SYS_ADMIN in the init namespace is required.\nUI:N - The vCPU thread reaches the buggy re-check on the normal `KVM_RUN` re-entry path after a userspace I/O exit; no victim action is needed.\nS:C - A guest (L1/L2) triggers corrupted state and a WARN/panic in the host kernel\u0027s KVM module, crossing the guest-to-host security boundary, and the aborted emulation also crosses the L1/L2 authority boundary by delivering an I/O intercept L1 never legitimately received.\nC:L - The stale `pio.count`/`pio.size` makes the next emulated `in` take the completion path and hand back data captured during the earlier I/O read on L2\u0027s behalf, leaking bounded `pio_data` contents across the L1/L2 boundary; the copy is capped at the 1024-byte `read_cache`, so it is not an arbitrary read.\nI:L - KVM injects a nested IOIO VM-exit to L1 for an access it already performed against the host device model (double execution) and subsequently substitutes stale `pio_data` for a real port read, corrupting guest I/O state in a limited, non-arbitrary way.\nA:H - The un-cleared `vcpu-\u003earch.pio.count` trips `WARN_ON_ONCE()` in `emulator_pio_in_out()` in the host kernel \u2014 a full host panic on the widely deployed `panic_on_warn=1` configuration \u2014 and leaves the vCPU with permanently desynchronized emulation state."
            }
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        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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        {
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      "title": "KVM: x86: Don\u0027t (re)check L1 intercepts when completing userspace I/O",
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    "dateReserved": "2025-04-16T07:20:57.152Z",
    "dateUpdated": "2026-10-15T22:02:36.361Z",
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                    "lessThan": "3.0",
                    "status": "unaffected",
                    "version": "0",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "5.4.*",
                    "status": "unaffected",
                    "version": "5.4.301",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "5.10.*",
                    "status": "unaffected",
                    "version": "5.10.246",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "5.15.*",
                    "status": "unaffected",
                    "version": "5.15.195",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "6.1.*",
                    "status": "unaffected",
                    "version": "6.1.157",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "6.6.*",
                    "status": "unaffected",
                    "version": "6.6.111",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "6.12.*",
                    "status": "unaffected",
                    "version": "6.12.52",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "6.16.*",
                    "status": "unaffected",
                    "version": "6.16.12",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "6.17.*",
                    "status": "unaffected",
                    "version": "6.17.2",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "*",
                    "status": "unaffected",
                    "version": "6.18",
                    "versionType": "original_commit_for_fix"
                  }
                ]
              }
            ],
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
          }
        ],
        "cveTags": [],
        "descriptions": [
          {
            "lang": "en",
            "value": "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"
          },
          {
            "lang": "es",
            "value": "En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\n\nKVM: x86: No (re)verificar las intercepciones L1 al completar la E/S del espacio de usuario\n\nAl completar la emulaci\u00f3n de una instrucci\u00f3n que gener\u00f3 una salida al espacio de usuario para E/S, no volver a verificar las intercepciones L1 ya que KVM ya ha terminado esa fase de ejecuci\u00f3n de la instrucci\u00f3n, es decir, ya se ha comprometido a permitir que L2 realice E/S. Si L1 (o el espacio de usuario del host) modifica los mapas de bits de permisos de E/S durante la salida al espacio de usuario, KVM tratar\u00e1 el acceso como interceptado a pesar de haber emulado ya el acceso de E/S.\n\nBasarse en EMULTYPE_NO_DECODE para detectar que KVM est\u00e1 completando la emulaci\u00f3n. De los tres usuarios de EMULTYPE_NO_DECODE, solo complete_emulated_io() (el \u0027destinatario\u0027 previsto) puede alcanzar el c\u00f3digo en cuesti\u00f3n. El uso de gp_interception() es mutuamente excluyente con is_guest_mode(), y complete_emulated_insn_gp() empareja incondicionalmente EMULTYPE_NO_DECODE con EMULTYPE_SKIP.\n\nEl comportamiento err\u00f3neo fue detectado por un programa syzkaller que alterna la intercepci\u00f3n de E/S del puerto durante la salida de E/S del espacio de usuario, lo que finalmente result\u00f3 en una ADVERTENCIA (WARN) sobre vcpu-\u003earch.pio.count siendo distinto de cero debido a que KVM no complet\u00f3 la emulaci\u00f3n de la instrucci\u00f3n de E/S.\n\n  ADVERTENCIA: CPU: 23 PID: 1083 en arch/x86/kvm/x86.c:8039 emulator_pio_in_out+0x154/0x170 [kvm]\n  M\u00f3dulos enlazados: kvm_intel kvm irqbypass\n  CPU: 23 UID: 1000 PID: 1083 Comm: repro No contaminado 6.16.0-rc5-c1610d2d66b1-next-vm #74 NINGUNO\n  Nombre del hardware: 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  Traza de llamada:\n   \u003cTAREA\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/TAREA\u003e"
          }
        ],
        "id": "CVE-2025-40026",
        "lastModified": "2026-10-19T21:10:00.160",
        "metrics": {
          "cvssMetricV31": [
            {
              "cvssData": {
                "attackComplexity": "LOW",
                "attackVector": "LOCAL",
                "availabilityImpact": "HIGH",
                "baseScore": 7.9,
                "baseSeverity": "HIGH",
                "confidentialityImpact": "LOW",
                "integrityImpact": "LOW",
                "privilegesRequired": "LOW",
                "scope": "CHANGED",
                "userInteraction": "NONE",
                "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H",
                "version": "3.1"
              },
              "exploitabilityScore": 2.0,
              "impactScore": 5.3,
              "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
              "type": "Secondary"
            }
          ],
          "ssvcV203": [
            {
              "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
              "ssvcData": {
                "id": "CVE-2025-40026",
                "options": [
                  {
                    "exploitation": "none"
                  },
                  {
                    "automatable": "no"
                  },
                  {
                    "technicalImpact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-10-15T21:56:14.274469Z",
                "version": "2.0.3"
              }
            }
          ]
        },
        "published": "2025-10-28T10:15:42.167",
        "references": [
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/00338255bb1f422642fb2798ebe92e93b6e4209b"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/3a062a5c55adc5507600b9ae6d911e247e2f1d6e"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/3d3abf3f7e8b1abb082070a343de82d7efc80523"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/7366830642505683bbe905a2ba5d18d6e4b512b8"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/a908eca437789589dd4624da428614c1275064dc"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/ba35a5d775799ce5ad60230be97336f2fefd518e"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/e0ce3ed1048a47986d15aef1a98ebda25560d257"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/e7177c7e32cb806f348387b7f4faafd4a5b32054"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/e750f85391286a4c8100275516973324b621a269"
          }
        ],
        "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "vulnStatus": "Deferred"
      }
    },
    "redhat_vex": {
      "aggregate_severity": "Moderate",
      "current_release_date": "2026-10-08T23:05:08+00:00",
      "cve": "CVE-2025-40026",
      "id": "CVE-2025-40026",
      "initial_release_date": "2025-10-28T00:00:00+00:00",
      "product_status:fixed": "1813",
      "product_status:known_affected": "100",
      "product_status:known_not_affected": "15",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: KVM: x86: Don\u0027t (re)check L1 intercepts when completing userspace I/O",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2025/cve-2025-40026.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "low",
      "current_release_date": "2026-10-15T16:29:28Z",
      "cve": "CVE-2025-40026",
      "id": "CVE-2025-40026",
      "initial_release_date": "2025-10-29T00:25:45Z",
      "product_status:known_affected": "336",
      "product_status:recommended": "80",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2025-40026",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/2025/cve-2025-40026.json",
      "version": "18"
    }
  }
}



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