For nearly the past decade has been the kernel lockdown mode for tightening up kernel access from user-space such as when UEFI Secure Boot is enabled.
The kernel lockdown mode restricts PCI BAR access, no writing to /dev/mem, and other restrictions so user-space can't can't modify the running kernel or access sensitive kernel memory.
An oversight has allowed legacy I/O and memory interfaces via sysfs to remain open in lockdown mode, but a fix is on the way...