A vulnerability in the Linux kernel's KVM x86 memory management unit (MMU) handling causes out-of-bounds accesses due to unexpected SS bit settings in nested page fault (#NPF) exit codes when running Hyper-V on Windows 11. The issue arises because the AMD manual is unclear about when the SS bit is set, leading to warnings and undefined behavior sanitizer (UBSAN) reports. The fix involves dropping unexpected bits in error codes while preserving the SS bit when reflecting faults to the first-level hypervisor (L1).