# Based on recommendations from:
# http://kernsec.org/wiki/index.php/Kernel_Self_Protection_Project#Recommended_settings
# https://wiki.gentoo.org/wiki/Hardened/Hardened_Kernel_Project
#
# Dangerous features that can be permanently (for the boot session) disabled at
# boot via sysctl or kernel cmdline are left enabled here, for improved
# flexibility.
#
# See also <nixos/modules/profiles/hardened.nix>

{ stdenv, version }:

with stdenv.lib;
with stdenv.lib.kernel;
with (stdenv.lib.kernel.whenHelpers version);

assert (versionAtLeast version "4.9");

optionalAttrs (stdenv.hostPlatform.platform.kernelArch == "x86_64") {
  DEFAULT_MMAP_MIN_ADDR = freeform "65536";  # Prevent allocation of first 64K of memory

  # Note: this config depends on EXPERT y and so will not take effect, hence
  # it is left "optional" for now.
  MODIFY_LDT_SYSCALL = option no;

  # Randomize position of kernel and memory.
  RANDOMIZE_BASE   = yes;
  RANDOMIZE_MEMORY = yes;

  # Disable legacy virtual syscalls by default (modern glibc use vDSO instead).
  #
  # Note that the vanilla default is to *emulate* the legacy vsyscall mechanism,
  # which is supposed to be safer than the native variant (wrt. ret2libc), so
  # disabling it mainly helps reduce surface.
  LEGACY_VSYSCALL_NONE = yes;
} // {
  # Report BUG() conditions and kill the offending process.
  BUG = yes;

  BUG_ON_DATA_CORRUPTION = whenAtLeast "4.10" yes;

  # Safer page access permissions (wrt. code injection).  Default on >=4.11.
  DEBUG_RODATA          = whenOlder "4.11" yes;
  DEBUG_SET_MODULE_RONX = whenOlder "4.11" yes;

  # Mark LSM hooks read-only after init.  SECURITY_WRITABLE_HOOKS n
  # conflicts with SECURITY_SELINUX_DISABLE y; disabling the latter
  # implicitly marks LSM hooks read-only after init.
  #
  # SELinux can only be disabled at boot via selinux=0
  #
  # We set SECURITY_WRITABLE_HOOKS n primarily for documentation purposes; the
  # config builder fails to detect that it has indeed been unset.
  SECURITY_SELINUX_DISABLE = whenAtLeast "4.12" no;
  SECURITY_WRITABLE_HOOKS  = whenAtLeast "4.12" (option no);

  DEBUG_WX = yes; # boot-time warning on RWX mappings
  STRICT_KERNEL_RWX = whenAtLeast "4.11" yes;

  # Stricter /dev/mem
  STRICT_DEVMEM    = option yes;
  IO_STRICT_DEVMEM = option yes;

  # Perform additional validation of commonly targeted structures.
  DEBUG_CREDENTIALS     = yes;
  DEBUG_NOTIFIERS       = yes;
  DEBUG_LIST            = yes;
  DEBUG_PI_LIST         = yes; # doesn't BUG()
  DEBUG_SG              = yes;
  SCHED_STACK_END_CHECK = yes;

  REFCOUNT_FULL = whenAtLeast "4.13" yes;

  # Perform usercopy bounds checking.
  HARDENED_USERCOPY = yes;
  HARDENED_USERCOPY_FALLBACK = whenAtLeast "4.16" no; # for full whitelist enforcement

  # Randomize allocator freelists.
  SLAB_FREELIST_RANDOM = yes;

  SLAB_FREELIST_HARDENED = whenAtLeast "4.14" yes;

  # Randomize page allocator when page_alloc.shuffle=1
  SHUFFLE_PAGE_ALLOCATOR = whenAtLeast "5.2" yes;

  # Allow enabling slub/slab free poisoning with slub_debug=P
  SLUB_DEBUG = yes;

  # Wipe higher-level memory allocations on free() with page_poison=1
  PAGE_POISONING           = yes;
  PAGE_POISONING_NO_SANITY = yes;
  PAGE_POISONING_ZERO      = yes;

  # Enable the SafeSetId LSM
  SECURITY_SAFESETID = whenAtLeast "5.1" yes;

  # Reboot devices immediately if kernel experiences an Oops.
  PANIC_ON_OOPS = yes;
  PANIC_TIMEOUT = freeform "-1";

  GCC_PLUGINS = yes; # Enable gcc plugin options
  # Gather additional entropy at boot time for systems that may = no;ot have appropriate entropy sources.
  GCC_PLUGIN_LATENT_ENTROPY = yes;

  GCC_PLUGIN_STRUCTLEAK = whenAtLeast "4.11" yes; # A port of the PaX structleak plugin
  GCC_PLUGIN_STRUCTLEAK_BYREF_ALL = whenAtLeast "4.14" yes; # Also cover structs passed by address
  GCC_PLUGIN_STACKLEAK = whenAtLeast "4.20" yes; # A port of the PaX stackleak plugin
  GCC_PLUGIN_RANDSTRUCT = whenAtLeast "4.13" yes; # A port of the PaX randstruct plugin
  GCC_PLUGIN_RANDSTRUCT_PERFORMANCE = whenAtLeast "4.13" yes;

  # Disable various dangerous settings
  ACPI_CUSTOM_METHOD = no; # Allows writing directly to physical memory
  PROC_KCORE         = no; # Exposes kernel text image layout
  INET_DIAG          = no; # Has been used for heap based attacks in the past

  # Use -fstack-protector-strong (gcc 4.9+) for best stack canary coverage.
  CC_STACKPROTECTOR_REGULAR = whenOlder "4.18" no;
  CC_STACKPROTECTOR_STRONG  = whenOlder "4.18" yes;

  # Enable compile/run-time buffer overflow detection ala glibc's _FORTIFY_SOURCE
  FORTIFY_SOURCE = whenAtLeast "4.13" yes;

}