cve 2025 40026

CVE-2025-40026 is a Linux kernel vulnerability affecting KVM's x86 virtualization paths. The bug arises from an unsafe assumption in a fastpath used when completing userspace I/O, where KVM rechecks L1 intercept state in a context that cannot safely perform the required memory accesses. This can lead to kernel BUGs, vCPU failures, and host instability. The upstream fix changes the control flow to avoid rechecking L1 intercepts from the fastpath, instead completing userspace I/O from a safe context, eliminating the illegal sleep-from-fastpath scenario. This tag covers discussions about the vulnerability, its impact on KVM-based virtualization, and the patch that resolves it.