kernel: KVM: cmpxchg_gpte can write to pfns outside the userspace region

A flaw was found in KVM. When updating a guest's page table entry, vmpgoff was improperly used as the offset to get the page's pfn. As vaddr and vmpgoff are controllable by user-mode processes, this flaw allows unprivileged local users on the host to write outside the userspace region and...