CVE-2025-40300: 
Linux Kernel vulnerability analysis and mitigation

VMSCAPE (CVE-2025-40300) is a critical vulnerability discovered in September 2025 that affects the Linux kernel's virtualization infrastructure. The vulnerability exploits insufficient branch predictor isolation between a guest and a userspace hypervisor like QEMU. This vulnerability affects AMD Zen 1-5 architectures and Intel Coffee Lake processors (AMD Security Bulletin, The Register).

VMSCAPE is a Spectre-BTI (Branch Target Injection) attack that enables a malicious KVM guest to leak arbitrary memory from an unmodified QEMU process. On AMD Zen 4 processors, the attack can extract data at a rate of 32 bytes per second, making it possible to leak sensitive information such as cryptographic keys used for disk encryption and decryption. The vulnerability stems from incomplete isolation of branch predictor state between host and guest domains, allowing the branch predictor to mingle predictions across security boundaries (Security Online).

The vulnerability poses significant risks to cloud computing environments by allowing guest VMs to compromise the hypervisor's user-space processes and undermine the foundational trust model of cloud isolation. Attackers can potentially steal customer data, encryption keys, and infrastructure secrets without exploiting software vulnerabilities or requiring elevated privileges (The Register).

The Linux kernel community has developed mitigations, notably the IBPB-on-VMEXIT (Indirect Branch Prediction Barrier) mechanism, which flushes the branch predictor state when switching from guest to host. For emulated devices, the performance overhead is approximately 10%, while systems using Zen 4 processors experience a marginal 1% overhead post-patch. The mitigation is active for all affected systems, including Zen 5 and recent Intel CPUs (The Register).

AMD has acknowledged the vulnerability and announced the release of a Security Brief, while Intel has stated that existing mitigations on their processors can be used to address this issue. Intel engineers are working with the Linux community to ensure appropriate mitigations are applied to Linux userspace hypervisor software (The Register).