CVE-2025-40300: x86/vmscape: Add conditional IBPB mitigation
In the Linux kernel, the following vulnerability has been resolved:
x86/vmscape: Add conditional IBPB mitigation
VMSCAPE is a vulnerability that exploits insufficient branch predictor isolation between a guest and a userspace hypervisor (like QEMU). Existing mitigations already protect kernel/KVM from a malicious guest. Userspace can additionally be protected by flushing the branch predictors after a VMexit.
Since it is the userspace that consumes the poisoned branch predictors, conditionally issue an IBPB after a VMexit and before returning to userspace. Workloads that frequently switch between hypervisor and userspace will incur the most overhead from the new IBPB.
This new IBPB is not integrated with the existing IBPB sites. For instance, a task can use the existing speculation control prctl() to get an IBPB at context switch time. With this implementation, the IBPB is doubled up: one at context switch and another before running userspace.
The intent is to integrate and optimize these cases post-embargo.
[ dhansen: elaborate on suboptimal IBPB solution ]
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2025-40300?
CVE-2025-40300 has a high severity rating due to its potential to compromise guest-host isolation on affected systems.
How do I fix CVE-2025-40300?
To fix CVE-2025-40300, it is recommended to apply the latest updates or patches provided for the Linux kernel.
Which systems are affected by CVE-2025-40300?
CVE-2025-40300 affects the Linux kernel, particularly systems running virtualized environments that utilize insufficient branch predictor isolation.
What potential impact does CVE-2025-40300 have on users?
CVE-2025-40300 could allow an attacker to break guest-host isolation, leading to unauthorized access to sensitive data across virtual machines.
Is CVE-2025-40300 related to previous vulnerabilities?
Yes, CVE-2025-40300 is related to earlier vulnerabilities that impacted branch prediction and system isolation, particularly in virtualized environments.