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AL2023 Kernel Hardening

The 6.1 Linux kernel in AL2023 is configured and built with several hardening options and features.

Kernel Hardening options (architecture independent)

Allow ACPI methods to be inserted/replaced at runtime (CONFIG_ACPI_CUSTOM_METHOD)

Amazon Linux disables this option as it allows root users to write to arbitrary kernel memory.

This option is one of the Kernel Self Protection Project Recommended Settings.

Miscellaneous Binary Formats (binfmt_misc)

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends. In AL2023, this feature is optional, and is built as a kernel module.

BUG() support

This option is one of the Kernel Self Protection Project Recommended Settings.

BUG() if kernel encounters data corruption in when checking kernel memory structures for validity

Some parts of the Linux kernel will check the internal consistency of data structures and can BUG() when they detect data corruption.

This option is one of the Kernel Self Protection Project Recommended Settings.

COMPAT_BRK

With this option disabled (which is how Amazon Linux configures the kernel), the randomize_va_space sysctl setting defaults to 2, which also enables heap randomization on top of mmap base, stack, and VDSO page randomization.

This option exists in the kernel to provide compatibility with some ancient libc.so.5 binaries from 1996 and earlier.

This option is one of the Kernel Self Protection Project Recommended Settings.

COMPAT_VDSO

This configuration option is relevant to x86-64 and not aarch64. By setting this to n, the Amazon Linux kernel does not make a 32-bit virtual Dynamic Shared Object (VDSO) visible at a predictable address. The most recent glibc known to be broken by this option being set to n is glibc 2.3.3, from 2004.

This option is one of the Kernel Self Protection Project Recommended Settings.

CONFIG_DEBUG gated hardening

Linux kernel configuration options gated by CONFIG_DEBUG are typically designed for use in kernels built for debugging issues, and things like performance are not a priority. AL2023 enables the CONFIG_DEBUG_LIST hardening option.

Disable DMA for PCI devices in EFI stub before configuring the IOMMU

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends.

Hardening for copying memory between kernel and userspace

When the kernel needs to copy memory to or from userspace, this option enables some checks which can protect against some classes of heap overflow issues.

The CONFIG_HARDENED_USERCOPY_FALLBACK option existed in kernels 4.16 through 5.15 to help kernel developers discover any missing allowlist entries via a WARN(). Because AL2023 ships a 6.1 kernel, this option is no longer relevant to AL2023.

The CONFIG_HARDENED_USERCOPY_PAGESPAN option existed in kernels primarily as a debugging option for developers and no longer applies to the 6.1 kernel in AL2023.

This option is one of the Kernel Self Protection Project Recommended Settings.

Hibernation Support

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends. This option needs to be enabled in order to support the ability to Hibernate your On-Demand Instance, and to support the ability to Hibernate interrupted Spot Instances

Random Number Generation

The AL2023 kernel is configured to ensure adequate entropy is available for usage within EC2.

CONFIG_INET_DIAG

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends. In AL2023, this feature is optional, and is built as a kernel module.

Zero all kernel page and slab allocator memory on allocation and deallocation

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends. These options are disabled in AL2023 due to the possible performance impact of enabling this functionality by default. The CONFIG_INIT_ON_ALLOC_DEFAULT_ON behavior can be enabled by adding init_on_alloc=1 to the kernel command line, and the CONFIG_INIT_ON_FREE_DEFAULT_ON behavior can be enabled by adding init_on_free=1.

Initialize all stack variables as zero (CONFIG_INIT_STACK_ALL_ZERO)

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends. This option requires GCC 12 or higher, while AL2023 ships with GCC 11.

Kernel Module Signing

AL2023 signs and validates the signatures of kernel modules. The CONFIG_MODULE_SIG_FORCE option, which would require modules to have a valid signature is not enabled in order to preserve compatibility for users building third party modules. For users wanting to ensure that all kernel modules are signed, the Lockdown Linux Security Module (LSM) can be configured to enforce this.

kexec

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends. This option is enabled so that kdump functionality can be used.

IOMMU Support

AL2023 enables IOMMU support. The CONFIG_IOMMU_DEFAULT_DMA_STRICT option is not enabled by default, but this functionality can be configured by adding iommu.passthrough=0 iommu.strict=1 to the kernel command line.

kfence

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends.

Legacy pty Support

AL2023 uses the modern PTY interface (devpts).

This option is one of the Kernel Self Protection Project Recommended Settings.

Lockdown Linux Security Module (LSM)

AL2023 builds the lockdown LSM, which will automatically lock down the kernel when using Secure Boot.

The CONFIG_LOCK_DOWN_KERNEL_FORCE_CONFIDENTIALITY option is not enabled. Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends. When not using Secure Boot, it is possible to enable the lockdown LSM and configure as wanted.

Page Poisoning

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends. Similarly to Zero all kernel page and slab allocator memory on allocation and deallocation , this is disabled in the AL2023 kernel due to the possible impact on performance.

Stack Protector

The AL2023 kernel is built with the stack-protector feature of GCC enabled with the -fstack-protector-strong option.

This option is one of the Kernel Self Protection Project Recommended Settings.

seccomp BPF API

The seccomp hardening feature is used by software such as systemd and container runtimes to harden userspace applications.

This option is one of the Kernel Self Protection Project Recommended Settings.

panic() timeout

The AL2023 kernel is configured with this value set to 0, meaning that the kernel will not reboot after it panics. Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends. This is configurable through sysctl, /proc/sys/kernel/panic, and on the kernel command line.

Security Models

AL2023 enables SELinux in Permissive mode by default. For more information, see Setting SELinux modes for AL2023.

The Lockdown Linux Security Module (LSM) and yama modules are also enabled.

/proc/kcore

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends.

Kernel stack offset randomization on syscall entry

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends. This can be enabled by setting randomize_kstack_offset=on on the kernel command line.

Reference counting checks (CONFIG_REFCOUNT_FULL)

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends. This option is not curretly enabled due to its possible impact on performance.

Scheduler awareness of SMT cores (CONFIG_SCHED_CORE)

The AL2023 kernel is built with CONFIG_SCHED_CORE, which enables userspace applications to use prctl(PR_SCHED_CORE). This option is one of the Kernel Self Protection Project Recommended Settings.

Check for stack corruption on calls to schedule() (CONFIG_SCHED_STACK_END_CHECK)

The AL2023 kernel is built with CONFIG_SCHED_STACK_END_CHECK enabled. This option is one of the Kernel Self Protection Project Recommended Settings.

Memory allocator hardening

The AL2023 kernel enables hardening of the kernel memory allocator with the CONFIG_SHUFFLE_PAGE_ALLOCATOR, CONFIG_SLAB_FREELIST_HARDENED, and CONFIG_SLAB_FREELIST_RANDOM options. This option is one of the Kernel Self Protection Project Recommended Settings.

SLUB debugging support

The AL2023 kernel enables CONFIG_SLUB_DEBUG as this option enables optional debugging features for the allocator that can be enabled on the kernel command line. This option is one of the Kernel Self Protection Project Recommended Settings.

CONFIG_STATIC_USERMODEHELPER

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends. This is because CONFIG_STATIC_USERMODEHELPER requires special support from the distribution, which is not currently present in Amazon Linux.

Read-Only kernel text and rodata (CONFIG_STRICT_KERNEL_RWX and CONFIG_STRICT_MODULE_RWX)

The AL2023 kernel is configured to mark kernel and kernel module text and rodata memory as read-only, and non-text memory marked as not executable. This option is one of the Kernel Self Protection Project Recommended Settings.

TCP syncookie support (CONFIG_SYN_COOKIES)

The AL2023 kernel is built with support for TCP syncookies. This option is one of the Kernel Self Protection Project Recommended Settings.

Virtually mapped stack with guard pages (CONFIG_VMAP_STACK)

The AL2023 kernel is built with CONFIG_VMAP_STACK, enabling virtually mapped kernel stacks with guard pages. This option is one of the Kernel Self Protection Project Recommended Settings.

Build with compiler warnings as errors (CONFIG_WERROR)

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends.

Register zeroing on function exit (CONFIG_ZERO_CALL_USED_REGS)

Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends.

Minimum address for userspace allocation

This hardening option can help reduce the impact of kernel NULL pointer bugs. This option is one of the Kernel Self Protection Project Recommended Settings.

clang specific hardening options

The AL2023 kernel is built with GCC rather than clang, so the CONFIG_CFI_CLANG hardening option cannot be enabled, which also makes CONFIG_CFI_PERMISSIVE not applicable. Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends.

x86-64 specific Kernel Hardening options

x86-64 Support

Base x86-64 support includes the Physical Address Extension (PAE) and no-execute (NX) bit support. This option is one of the Kernel Self Protection Project Recommended Settings.

AMD and Intel IOMMU support

The AL2023 kernel builds with support for the AMD and Intel IOMMUs. This option is one of the Kernel Self Protection Project Recommended Settings.

The CONFIG_INTEL_IOMMU_DEFAULT_ON option is not set, but can be enabled by passing intel_iommu=on to the kernel command line. Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends.

The CONFIG_INTEL_IOMMU_SVM option is not currently enabled in AL2023. Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends.

Support for 32bit userspace

To support running 32bit userspace applications, AL2023 does not enable the CONFIG_X86_VSYSCALL_EMULATION option, and enables the CONFIG_IA32_EMULATION, CONFIG_COMPAT, and CONFIG_X86_VSYSCALL_EMULATION options. Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends.

The x32 native 32-bit ABI for 64-bit processors is not enabled (CONFIG_X86_X32 and CONFIG_X86_X32_ABI). This option is one of the Kernel Self Protection Project Recommended Settings.

x86 Model Specific Register (MSR) support

The CONFIG_X86_MSR option is enabled in order to support turbostat. Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends.

modify_ldt syscall

AL2023 does not allow user programs to modify the x86 Local Descriptor Table (LDT) with the modify_ldt syscall. This call is required to run 16-bit or segmented code, and its absence may break software such as dosemu, running some programs under WINE, and some very old threading libraries. This option is one of the Kernel Self Protection Project Recommended Settings.

Remove kernel mapping in user mode

AL2023 configures the kernel so that the majority of kernel addresses are not mapped into userspace. This option is one of the Kernel Self Protection Project Recommended Settings.

Randomize kernel memory sections

AL2023 configures the kernel to randomize the base virtual addresses of kernel memory sections. This option is one of the Kernel Self Protection Project Recommended Settings.

aarch64 specific Kernel Hardening options

Branch Target Identification

The AL2023 kernel enables support for Branch Target Identification (CONFIG_ARM64_BTI). This option is one of the Kernel Self Protection Project Recommended Settings.

The CONFIG_ARM64_BTI_KERNEL option is not enabled in AL2023 as it is built with GCC, and support for building the kernel with this option is currently disabled in the upstream kernel due to a gcc bug. Although this option is one of the Kernel Self Protection Project (KSPP) Recommended Settings, AL2023 does not set this configuration option to what KSPP recommends.

Pointer Authentication (CONFIG_ARM64_PTR_AUTH)

The AL2023 kernel is built with support for the Pointer Authentication extension (part of the ARMv8.3 Extensions), which can be used to help mitigate Return Oriented Programming (ROP) techniques. The required hardware support for pointer authentication on Graviton was introduced with Graviton 3.

The CONFIG_ARM64_PTR_AUTH option is enabled and provides support for pointer authentication for userspace. Because the CONFIG_ARM64_PTR_AUTH_KERNEL option is also enabled, the AL2023 kernel is able to use the return address protection for itself.

This option is one of the Kernel Self Protection Project Recommended Settings.

Emulate Privileged Access Never using TTBR0_EL1 switching

This option prevents the kernel from accessing userspace memory directly, with TTBR0_EL1 being only temporarily set to a valid value by the user access routines.

This option is one of the Kernel Self Protection Project Recommended Settings.

Unmap kernel when running in userspace

The AL2023 kernel is configured to unmap the kernel when running in userspace (CONFIG_UNMAP_KERNEL_AT_EL0). This option is one of the Kernel Self Protection Project Recommended Settings.