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

Type: advisorySources: OSV

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+

Products
Kernel
Affected versions
Not specified
Fixed versions
5.10.246, 5.15.195, 5.4.301, 6.1.157, 6.12.52, 6.16.12, 6.17.2, 6.6.111
CVEs
CVE-2025-40026
Confirmed fixed CVEs
Not specified
Updated in this feed
4 Nov 2026, 18:44 UTC · Revision 1

Source evidence

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

core-osv · Original source

Published 28 Oct 2025, 09:32 UTC

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+

osv:CVE-2025-40026

Kernel

Fixed versions: 5.4.301, 5.10.246, 5.15.195, 6.1.157, 6.6.111, 6.12.52, 6.16.12, 6.17.2

Complete source record
{
  "id": "a04f5f37862ff2a09a2d0b8b3e3d30fe95cb3025b1c124b839f1fdfd9c6c8168",
  "url": "https://osv.dev/vulnerability/CVE-2025-40026",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "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+",
    "evidence": "osv:CVE-2025-40026",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "5.4.301",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "5.10.246",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "5.15.195",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.1.157",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.6.111",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.12.52",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.16.12",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.17.2",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "high",
    "structured": {
      "id": "CVE-2025-40026",
      "aliases": [
        "CVE-2025-40026"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "type": "GIT",
              "events": [
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                  "introduced": "8a76d7f25f8f24fc5a328c8e15e4a7313cf141b9"
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                {
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                },
                {
                  "fixed": "00338255bb1f422642fb2798ebe92e93b6e4209b"
                },
                {
                  "fixed": "e0ce3ed1048a47986d15aef1a98ebda25560d257"
                },
                {
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                },
                {
                  "fixed": "3d3abf3f7e8b1abb082070a343de82d7efc80523"
                },
                {
                  "fixed": "e7177c7e32cb806f348387b7f4faafd4a5b32054"
                },
                {
                  "fixed": "3a062a5c55adc5507600b9ae6d911e247e2f1d6e"
                },
                {
                  "fixed": "7366830642505683bbe905a2ba5d18d6e4b512b8"
                },
                {
                  "fixed": "e750f85391286a4c8100275516973324b621a269"
                }
              ]
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2025-40026.json"
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "3.0.0"
                },
                {
                  "fixed": "5.4.301"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "5.5.0"
                },
                {
                  "fixed": "5.10.246"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "5.11.0"
                },
                {
                  "fixed": "5.15.195"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "5.16.0"
                },
                {
                  "fixed": "6.1.157"
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              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.2.0"
                },
                {
                  "fixed": "6.6.111"
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              ]
            },
            {
              "type": "ECOSYSTEM",
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                {
                  "introduced": "6.7.0"
                },
                {
                  "fixed": "6.12.52"
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              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.13.0"
                },
                {
                  "fixed": "6.16.12"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.17.0"
                },
                {
                  "fixed": "6.17.2"
                }
              ]
            }
          ],
          "package": {
            "name": "Kernel",
            "ecosystem": "Linux"
          },
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2025-40026.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:38.304166322Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/00338255bb1f422642fb2798ebe92e93b6e4209b",
        "https://git.kernel.org/stable/c/3a062a5c55adc5507600b9ae6d911e247e2f1d6e",
        "https://git.kernel.org/stable/c/3d3abf3f7e8b1abb082070a343de82d7efc80523",
        "https://git.kernel.org/stable/c/7366830642505683bbe905a2ba5d18d6e4b512b8",
        "https://git.kernel.org/stable/c/a908eca437789589dd4624da428614c1275064dc",
        "https://git.kernel.org/stable/c/ba35a5d775799ce5ad60230be97336f2fefd518e",
        "https://git.kernel.org/stable/c/e0ce3ed1048a47986d15aef1a98ebda25560d257",
        "https://git.kernel.org/stable/c/e7177c7e32cb806f348387b7f4faafd4a5b32054",
        "https://git.kernel.org/stable/c/e750f85391286a4c8100275516973324b621a269",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/40xxx/CVE-2025-40026.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2025-40026",
        "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
      ],
      "provider_metadata": {
        "cna_assigner": "Linux",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/40xxx/CVE-2025-40026.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2025-40026",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 505816,
  "published_at": "2025-10-28T09:32:33.075+00:00"
}