CVE-2025-40300

01/Description

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 ]

02/Affected products

linux: linux kernel; debian: debian linux

03/References10

• git.kernel.org/stable/c/15006289e5c38b2a830e1fba221977a27598176c Patch
• git.kernel.org/stable/c/2f4f2f8f860cb4c3336a7435ebe8dcfded0c9c6e Patch
• git.kernel.org/stable/c/2f8f173413f1cbf52660d04df92d0069c4306d25 Patch
• git.kernel.org/stable/c/34e5667041050711a947e260fc9ebebe08bddee5 Patch
• git.kernel.org/stable/c/459274c77b37ac63b78c928b4b4e748d1f9d05c8 Patch
• git.kernel.org/stable/c/510603f504796c3535f67f55fb0b124a303b44c8 Patch
• git.kernel.org/stable/c/893387c18612bb452336a5881da0d015a7e8f4a2 Patch
• git.kernel.org/stable/c/9c23a90648e831d611152ac08dbcd1283d405e7f Patch
• git.kernel.org/stable/c/ac60717f9a8d21c58617d0b34274babf24135835 Patch
• git.kernel.org/stable/c/c08192b5d6730a914dee6175bc71092ee6a65f14 Patch