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

## Copyright (C) 2019 - 2024 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"

## Definitions:
## KSPP=yes: compliant with recommendations by the KSPP
## KSPP=partial: partially compliant with recommendations by the KSPP

## This configuration file is split into 4 sections:
## 1. Kernel Space
## 2. Direct Memory Access
## 3. Entropy
## 4. Networking

## See the documentation below for details on the majority of the selected commands:
## https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html
## https://wiki.archlinux.org/title/Kernel_parameters#GRUB

## 1. Kernel Space:
##
## https://madaidans-insecurities.github.io/guides/linux-hardening.html#boot-parameters
## https://kspp.github.io/Recommended_Settings#kernel-command-line-options

## Disable merging of slabs with similar size.
## Reduces the risk of triggering heap overflows.
## Prevents overwriting objects from merged caches and limits influencing slab cache layout.
##
## https://www.openwall.com/lists/kernel-hardening/2017/06/19/33
## https://www.openwall.com/lists/kernel-hardening/2017/06/20/10
##
## KSPP=yes
## KSPP sets the kernel parameter and does not set CONFIG_SLAB_MERGE_DEFAULT.
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX slab_nomerge"

## Enable sanity checks and red zoning of slabs via debugging options to detect corruption.
## As a by product of debugging, this will implicitly disabling kernel pointer hashing.
## Enabling will therefore leak exact and all kernel memory addresses to root.
## Has the potential to cause a noticeable performance decrease.
##
## https://www.kernel.org/doc/html/latest/mm/slub.html
## https://lore.kernel.org/all/20210601182202.3011020-5-swboyd@chromium.org/T/#u
## https://gitlab.tails.boum.org/tails/tails/-/issues/19613
## https://github.com/Kicksecure/security-misc/issues/253
##
## KSPP=yes
## KSPP sets the kernel parameters and CONFIG_SLUB_DEBUG.
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX slab_debug=FZ"

## Zero memory at allocation time and free time.
## Fills newly allocated pages, freed pages, and heap objects with zeros.
## Mitigates use-after-free exploits by erasing sensitive information in memory.
##
## https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6471384af2a6530696fc0203bafe4de41a23c9ef
##
## KSPP=yes
## KSPP sets the kernel parameters, CONFIG_INIT_ON_ALLOC_DEFAULT_ON=y, and CONFIG_INIT_ON_FREE_DEFAULT_ON=y.
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX init_on_alloc=1"
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX init_on_free=1"

## Enable the kernel page allocator to randomize free lists.
## During early boot, the page allocator has predictable FIFO behavior for physical pages.
## Limits some data exfiltration and ROP attacks that rely on inferring sensitive data location.
## Also improves performance by optimizing memory-side cache utilization.
##
## https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e900a918b0984ec8f2eb150b8477a47b75d17692
## https://en.wikipedia.org/wiki/Return-oriented_programming#Attacks
##
## KSPP=yes
## KSPP sets the kernel parameter and CONFIG_SHUFFLE_PAGE_ALLOCATOR=y.
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX page_alloc.shuffle=1"

## Enable kernel page table isolation to harden against kernel ASLR (KASLR) bypasses.
## Mitigates the Meltdown CPU vulnerability.
##
## https://en.wikipedia.org/wiki/Kernel_page-table_isolation
##
## KSPP=yes
## KSPP sets the kernel parameter and CONFIG_MITIGATION_PAGE_TABLE_ISOLATION=y.
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX pti=on"

## Enable randomization of the kernel stack offset on syscall entries.
## Hardens against memory corruption attacks due to increased entropy.
## Limits attacks relying on deterministic stack addresses or cross-syscall address exposure.
##
## https://lkml.org/lkml/2019/3/18/246
## https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html
##
## KSPP=yes
## KSPP sets the kernel parameter and CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT=y.
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX randomize_kstack_offset=on"

## Disable vsyscalls to reduce attack surface as they have been replaced by vDSO.
## Vulnerable to ROP attacks as vsyscalls are located at fixed addresses in memory.
##
## https://lwn.net/Articles/446528/
## https://en.wikipedia.org/wiki/VDSO
##
## KSPP=yes
## KSPP sets the kernel parameter, CONFIG_LEGACY_VSYSCALL_NONE=y and does not set CONFIG_X86_VSYSCALL_EMULATION.
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX vsyscall=none"

## Restrict access to debugfs by not registering the file system.
## Deactivated since the file system can contain sensitive information.
##
## https://lkml.org/lkml/2020/7/16/122
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX debugfs=off"

## Force the kernel to panic on "oopses".
## Can sometimes potentially indicate and thwart certain kernel exploitation attempts.
## Panics may be due to false-positives such as bad drivers.
##
## https://en.wikipedia.org/wiki/Kernel_panic#Linux
## https://en.wikipedia.org/wiki/Linux_kernel_oops
## https://forums.whonix.org/t/set-oops-panic-kernel-parameter-or-kernel-panic-on-oops-1-sysctl-for-better-security/7713
##
## KSPP=partial
## KSPP sets CONFIG_PANIC_ON_OOPS=y, but also requires CONFIG_PANIC_TIMEOUT=-1.
##
## See /usr/libexec/security-misc/panic-on-oops for implementation.
##
#GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX oops=panic"

## Modify machine check exception handler.
## Can decide whether the system should panic or not based on the occurrence of an exception.
##
## https://www.kernel.org/doc/html/latest/arch/x86/x86_64/machinecheck.html
## https://www.kernel.org/doc/html/latest/arch/x86/x86_64/boot-options.html#machine-check
## https://forums.whonix.org/t/kernel-hardening/7296/494
##
## The default kernel setting will be utilized until provided sufficient evidence to modify.
##
#GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX mce=0"

## Prevent sensitive kernel information leaks in the console during boot.
## Must be used in combination with the kernel.printk sysctl.
## See /usr/lib/sysctl.d/30_silent-kernel-printk.conf for implementation.
##
## https://www.kernel.org/doc/html/latest/core-api/printk-basics.html
## https://wiki.archlinux.org/title/silent_boot
##
## See /etc/default/grub.d/41_quiet_boot.cfg for implementation.
##
#GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX loglevel=0"
#GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX quiet"

## Enable the kernel "Electric-Fence" sampling-based memory safety error detector.
## KFENCE detects heap out-of-bounds access, use-after-free, and invalid-free errors.
## Aims to have very low processing overhead at each sampling interval.
## Sampling interval is set to occur every 100 milliseconds as per KSPP recommendation.
##
## https://www.kernel.org/doc/html/latest/dev-tools/kfence.html
## https://google.github.io/kernel-sanitizers/KFENCE.html
## https://blogs.oracle.com/linux/post/linux-slub-allocator-internals-and-debugging-4
## https://lwn.net/Articles/835542/
##
## KSPP=yes
## KSPP sets the kernel parameter, CONFIG_KFENCE=y, and CONFIG_KFENCE_SAMPLE_INTERVAL=100.
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX kfence.sample_interval=100"

## Disable 32-bit Virtual Dynamic Shared Object (vDSO) mappings.
## Legacy compatibility feature for superseded glibc versions.
##
## https://lore.kernel.org/lkml/20080409082927.BD59E26F992@magilla.localdomain/T/
## https://lists.openwall.net/linux-kernel/2014/03/11/3
##
## KSPP=yes
## KSPP sets the kernel parameter and does not set CONFIG_COMPAT_VDSO.
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX vdso32=0"

## Switch (back) to using kCFI as the default Control Flow Integrity (CFI) implementation.
## The default implementation is FineIBT as of Linux kernel 6.2.
## The Intel-developed IBT (Indirect Branch Tracking) is only used if supported by the CPU.
## kCFI is software-only while FineIBT is a hybrid software/hardware implementation.
## FineIBT may result in some performance benefits as it only performs checking at destinations.
## FineIBT is considered weaker against attacks that can write arbitrary executables into memory.
## Upstream hardening work has provided users the ability to disable FineIBT based on requests.
## Choice of CFI implementation is highly dependent on user threat model as there are pros/cons to both.
## Do not modify from the default setting if unsure of implications.
##
## https://lore.kernel.org/all/20221027092842.699804264@infradead.org/
## https://lore.kernel.org/lkml/202210010918.4918F847C4@keescook/T/#u
## https://lore.kernel.org/lkml/202210182217.486CBA50@keescook/T/
## https://lore.kernel.org/lkml/202407150933.E1871BE@keescook/
## https://isopenbsdsecu.re/mitigations/forward_edge_cfi/
## https://docs.kernel.org/next/x86/shstk.html
## https://source.android.com/docs/security/test/kcfi
## https://lpc.events/event/16/contributions/1315/attachments/1067/2169/cfi.pdf
## https://forums.whonix.org/t/kernel-hardening-security-misc/7296/561
##
## KSPP=yes
## KSPP sets the kernel parameter.
##
## TODO: Debian 13 Trixie
## Applicable when using Linux kernel >= 6.2 (retained here for future-proofing and completeness).
##
#GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX cfi=kcfi"

## Disable support for x86 processes and syscalls.
## Unconditionally disables IA32 emulation to substantially reduce attack surface.
##
## https://lore.kernel.org/all/20230623111409.3047467-7-nik.borisov@suse.com/
##
## KSPP=yes
## KSPP does not set CONFIG_COMPAT, CONFIG_IA32_EMULATION, CONFIG_X86_X32, CONFIG_X86_X32_ABI, and CONFIG_MODIFY_LDT_SYSCALL.
##
## TODO: Debian 13 Trixie
## Applicable when using Linux kernel >= 6.7 (retained here for future-proofing and completeness).
##
#GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX ia32_emulation=0"

## 2. Direct Memory Access:
##
## https://madaidans-insecurities.github.io/guides/linux-hardening.html#dma-attacks

## Enable CPU manufacturer-specific IOMMU drivers to mitigate some DMA attacks.
##
## KSPP=yes
## KSPP sets CONFIG_INTEL_IOMMU=y, CONFIG_INTEL_IOMMU_DEFAULT_ON=y, CONFIG_INTEL_IOMMU_SVM=y, CONFIG_AMD_IOMMU=y, and CONFIG_AMD_IOMMU_V2=y.
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX amd_iommu=force_isolation"
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX intel_iommu=on"

## Enable and force use of IOMMU translation to protect against some DMA attacks.
## Strictly force DMA unmap operations to synchronously invalidate IOMMU hardware TLBs.
## Ensures devices will never be able to access stale data contents.
##
## https://en.wikipedia.org/wiki/Input%E2%80%93output_memory_management_unit
## https://en.wikipedia.org/wiki/DMA_attack
## https://lenovopress.lenovo.com/lp1467.pdf
##
## KSPP=yes
## KSPP sets the kernel parameters, CONFIG_IOMMU_SUPPORT=y, CONFIG_IOMMU_DEFAULT_DMA_STRICT=y, and does not set CONFIG_IOMMU_DEFAULT_PASSTHROUGH.
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX iommu=force"
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX iommu.passthrough=0"
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX iommu.strict=1"

## Clear the busmaster bit on all PCI bridges during the EFI hand-off.
## Terminates all existing DMA transactions prior to the kernel's IOMMU setup.
## Forces third party PCI devices to then re-set their busmaster bit in order to perform DMA.
## Assumes that the motherboard chipset and firmware are not malicious.
## May cause complete boot failure on certain hardware with incompatible firmware.
##
## https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=4444f8541dad16fefd9b8807ad1451e806ef1d94
## https://mjg59.dreamwidth.org/54433.html
##
## KSPP=yes
## KSPP sets CONFIG_EFI_DISABLE_PCI_DMA=y.
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX efi=disable_early_pci_dma"

## 3. Entropy:
##
## https://madaidans-insecurities.github.io/guides/linux-hardening.html#rdrand

## Do not credit the CPU or bootloader seeds as entropy sources at boot.
## The RDRAND CPU (RNG) instructions are proprietary and closed-source.
## Numerous implementations of RDRAND have a long history of being defective.
## The RNG seed passed by the bootloader could also potentially be tampered.
## Maximizing the entropy pool at boot is desirable for all cryptographic operations.
## These settings ensure additional entropy is obtained from other sources to initialize the RNG.
## Note that distrusting these (relatively fast) sources of entropy will increase boot time.
##
## https://en.wikipedia.org/wiki/RDRAND#Reception
## https://systemd.io/RANDOM_SEEDS/
## https://www.kicksecure.com/wiki/Dev/Entropy#RDRAND
## https://arstechnica.com/gadgets/2019/10/how-a-months-old-amd-microcode-bug-destroyed-my-weekend/
## https://x.com/pid_eins/status/1149649806056280069
## https://archive.nytimes.com/www.nytimes.com/interactive/2013/09/05/us/documents-reveal-nsa-campaign-against-encryption.html
## https://forums.whonix.org/t/entropy-config-random-trust-cpu-yes-or-no-rng-core-default-quality/8566
## https://github.com/NixOS/nixpkgs/pull/165355
## https://lkml.org/lkml/2022/6/5/271
##
## KSPP=yes
## KSPP sets CONFIG_RANDOM_TRUST_BOOTLOADER=y and CONFIG_RANDOM_TRUST_CPU=y.
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX random.trust_bootloader=off"
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX random.trust_cpu=off"

## Obtain more entropy during boot as the runtime memory allocator is being initialized.
## Entropy will be extracted from up to the first 4GB of RAM.
## Requires the linux-hardened kernel patch.
##
## https://www.kicksecure.com/wiki/Hardened-kernel#linux-hardened
## https://github.com/anthraxx/linux-hardened/commit/c3e7df1dba1eb8105d6d5143079a6a0ad9e9ebc7
## https://github.com/anthraxx/linux-hardened/commit/a04458f97fe1f7e95888c77c0165b646375db9c4
##
GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX extra_latent_entropy"

## 4. Networking
##
## https://madaidans-insecurities.github.io/guides/linux-hardening.html#kasr-boot-parameters

## Disable the entire IPv6 stack functionality.
## Removes attack surface associated with the IPv6 module.
##
## https://www.kernel.org/doc/html/latest/networking/ipv6.html
## https://wiki.archlinux.org/title/IPv6#Disable_IPv6
##
## Enabling makes redundant many network hardening sysctl's in /usr/lib/sysctl.d/990-security-misc.conf.
##
#GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX ipv6.disable=1"