security-misc/etc/default/grub.d/40_kernel_hardening.cfg

## Copyright (C) 2019 - 2021 ENCRYPTED SUPPORT LP <adrelanos@whonix.org>
## See the file COPYING for copying conditions.

kpkg="linux-image-$(dpkg --print-architecture)" || true
kver="$(dpkg-query --show --showformat='${Version}' "$kpkg")" 2>/dev/null || true
#echo "## kver: $kver"

## Disables the merging of slabs of similar sizes.
## Sometimes a slab can be used in a vulnerable way which an attacker can exploit.
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX slab_nomerge"

if dpkg --compare-versions "$kver" ge "5.3"; then
  ## Enables sanity checks (F) and redzoning (Z).
  GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX slub_debug=FZ"

  #echo "## $kver grater or equal 5.3: yes"
  ## Zero memory at allocation and free time.
  GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX init_on_alloc=1 init_on_free=1"
else
  #echo "## $kver grater or equal 5.3: no"
  ## SLUB poisoning and page poisoning is used if the kernel
  ## does not yet support init_on_{,alloc,free}.
  GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX slub_debug=FZP"

  if command -v "qubesdb-read" >/dev/null 2>&1 ; then
     ## https://github.com/QubesOS/qubes-issues/issues/5212#issuecomment-533873012
     true "skip adding page_poison=1 in Qubes"
  else
     GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX page_poison=1"
  fi
fi

## Makes the kernel panic on uncorrectable errors in ECC memory that an attacker could exploit.
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX mce=0"

## Enables Kernel Page Table Isolation which mitigates Meltdown and improves KASLR.
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX pti=on"

## Vsyscalls are obsolete, are at fixed addresses and are a target for ROP.
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX vsyscall=none"

## Enables page allocator freelist randomization.
if dpkg --compare-versions "${kver}" ge "5.2"; then
  GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX page_alloc.shuffle=1"
fi

## Enables kernel lockdown.
##
## Disabled for now as it enforces module signature verification which breaks
## too many things.
## https://forums.whonix.org/t/enforce-kernel-module-software-signature-verification-module-signing-disallow-kernel-module-loading-by-default/7880
##
#if dpkg --compare-versions "${kver}" ge "5.4"; then
#  GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX lockdown=confidentiality"
#fi

## Gather more entropy during boot.
##
## Requires linux-hardened kernel patch.
## https://github.com/anthraxx/linux-hardened
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX extra_latent_entropy"

## Restrict access to debugfs since it can contain a lot of sensitive information.
## https://lkml.org/lkml/2020/7/16/122
## https://github.com/torvalds/linux/blob/fb1201aececc59990b75ef59fca93ae4aa1e1444/Documentation/admin-guide/kernel-parameters.txt#L835-L848
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX debugfs=off"