CVE-2025-40102: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-40102 is a vulnerability discovered in the Linux kernel's KVM (Kernel-based Virtual Machine) component, specifically affecting the ARM64 architecture. The vulnerability was disclosed on October 30, 2025, and involves improper access control to vCPU events before initialization (NVD).

The vulnerability occurs when KVM erroneously allows userspace to pend vCPU events for a vCPU that hasn't been initialized yet. This leads to KVM interpreting uninitialized garbage data for routing and injecting exceptions. In one documented case, the injection code and hypervisor disagree on whether the vCPU has a 32-bit EL1, resulting in the vCPU being put into an illegal mode for AArch64, which triggers a BUG() in exceptiontargetel(). The vulnerability has been assigned a CVSS v3.1 Base Score of 5.5, indicating moderate severity (Red Hat).

When exploited, this vulnerability can cause system instability and potential crashes due to the kernel interpreting uninitialized data. The impact is primarily focused on system availability, as it can trigger kernel panics through the BUG() mechanism in the ARM64 KVM implementation (NVD).

The fix involves rejecting KVM ioctls outright before KVMARMVCPU_INIT is called, as no legitimate VMM would call these operations before initialization. The patch has been implemented in the Linux kernel version 6.17.6-1 (Debian).