CVE-2025-39815: 
Linux Debian vulnerability analysis and mitigation

A stack buffer overflow vulnerability (CVE-2025-39815) was discovered in the Linux kernel's RISC-V KVM implementation, specifically in the vlenb loading functionality. The vulnerability was disclosed on September 16, 2025. The issue affects the Linux kernel's RISC-V architecture implementation where userspace can put up to 2048 bits into an xlen bit stack buffer, potentially leading to a stack overflow condition (NVD, Ubuntu).

The vulnerability exists in the KVM (Kernel-based Virtual Machine) implementation for RISC-V architecture. The issue occurs during the loading of vlenb (vector length in bytes) where the userspace can write more data than the stack buffer can handle - specifically up to 2048 bits into a buffer sized for xlen bits. This mismatch between input size and buffer capacity creates a potential stack overflow condition (NVD).

The vulnerability affects various Linux distributions including Ubuntu 24.04 LTS, 25.04, and their derivatives. Multiple kernel variants including linux-aws, linux-azure, linux-gcp, and linux-oracle are impacted. The issue is particularly concerning for systems running RISC-V architecture with KVM virtualization enabled (Ubuntu).

The vulnerability has been patched in various Linux distributions. Ubuntu has marked this as a medium priority issue and provided updates for affected versions. Debian has included fixes for this vulnerability in version 6.12.48-1 of the Linux kernel package (Debian).