CVE-2025-40026 KVM: x86: Don't (re)check L1 intercepts when completing userspace I/O

What is CVE-2025-40026?

CVE-2025-40026 is a high-severity denial of service vulnerability in Linux, published on 28 Oct 2025. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

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>

Which versions are affected by CVE-2025-40026?

How do I fix CVE-2025-40026?

Apply the vendor's patch.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2025-40026 being exploited?

There is no known exploitation of CVE-2025-40026 in the wild and no public exploit code.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as BELL-CVE-2025-40026, DEBIAN-CVE-2025-40026, GHSA-3p46-x25w-pgvh, RHSA-2026:68131, RHSA-2026:78843, RHSA-2026:78844 and 6 more.