CVE-2025-37957 is a vulnerability identified in the Linux kernel's Kernel-based Virtual Machine (KVM) subsystem, specifically affecting the Secure Virtual Machine (SVM) implementation for AMD processors. The issue arises from improper handling of the System Management Mode (SMM) state during a SHUTDOWN interception event in nested virtualization scenarios. Previously, a related fix addressed use-after-free conditions caused by triple faults in nested mode by forcibly exiting nested mode on virtual CPU (vCPU) reset. However, this fix did not cover the analogous case when the vCPU is in SMM, a highly privileged CPU mode used for low-level system management tasks. The vulnerability manifests when KVM forces a vCPU INIT (initialization) after a SHUTDOWN interception while the vCPU remains in SMM. Architecturally, the CPU blocks INIT commands during SMM, so KVM's current behavior triggers a kernel warning (WARN) to indicate potential emulation bugs. This WARN can be triggered by crafted sequences involving entering SMM via KVM_SMI ioctl, executing invalid instructions to cause exceptions, and eventually inducing a triple fault. The root cause is that KVM does not forcibly exit SMM before issuing INIT on SHUTDOWN, leading to undefined behavior and potential instability or denial of service within the virtualized environment. The Linux kernel developers have addressed this by enhancing the KVM code to forcibly exit SMM mode on SHUTDOWN interception, ensuring the vCPU is in a sane state before INIT is applied. This fix prevents the WARN and stabilizes the behavior under these edge-case conditions. The vulnerability was discovered using Syzkaller, a kernel fuzzing tool, by the Linux Verification Center. The affected versions include several recent Linux kernel commits prior to the fix. No known exploits are reported in the wild as of the publication date. This vulnerability is subtle and primarily impacts environments using nested virtualization with KVM on AMD hardware where SMM is explicitly manipulated. It involves complex CPU modes and virtualization internals, making exploitation non-trivial and requiring specific conditions to trigger the issue.

For European organizations, the impact of CVE-2025-37957 is primarily relevant to those utilizing Linux-based virtualization infrastructures, especially those employing nested virtualization with KVM on AMD platforms. Such environments are common in cloud service providers, data centers, and enterprises running private clouds or virtualized test environments. The vulnerability could lead to kernel warnings and potential instability or crashes in virtual machines, resulting in denial of service conditions. While it does not directly lead to privilege escalation or data leakage, the instability could disrupt critical services, impacting availability. Given the reliance on Linux and KVM in European IT infrastructures, particularly in sectors like finance, telecommunications, and government where virtualization is prevalent, this vulnerability could affect service reliability. However, the requirement for specific conditions (nested virtualization, SMM manipulation) limits the scope to specialized use cases rather than general Linux deployments. The absence of known exploits reduces immediate risk, but unpatched systems remain vulnerable to potential future exploitation or accidental triggering during fuzz testing or malformed workloads. Organizations running virtualized environments with AMD processors should be aware of this issue as it could complicate debugging and stability of nested VMs, potentially affecting development, testing, or production workloads relying on nested virtualization capabilities.

1. Apply the latest Linux kernel patches that address CVE-2025-37957 as soon as they become available. Monitor kernel updates from trusted sources and prioritize updating hypervisor hosts running KVM on AMD hardware. 2. Review and restrict the use of nested virtualization where possible, especially in production environments, to reduce exposure to this and similar vulnerabilities. 3. Limit or disable the use of explicit SMM entry via KVM_SMI ioctl unless absolutely necessary, as this is a key step in triggering the vulnerability. 4. Implement rigorous testing and monitoring of virtualized environments to detect kernel WARNs or instability related to vCPU resets and SMM state transitions. 5. For organizations using automated fuzzing or testing tools like Syzkaller, ensure these tools are run in isolated environments to prevent accidental triggering of the vulnerability in production. 6. Maintain strict access controls and audit logs around virtualization management interfaces to prevent unauthorized or malformed ioctl calls that could exploit this vulnerability. 7. Collaborate with hardware vendors and Linux distribution maintainers to ensure timely dissemination and deployment of patches across all affected systems.