Subject: [PATCH] hardening: Provide Kconfig fragments for basic options

From: Kees Cook <[email-masked]>
Date: Thu, 24 Aug 2023 22:06:22 -0700

Inspired by Salvatore Mesoraca's earlier[1] efforts to provide some
in-tree guidance for kernel hardening Kconfig options, add a new fragment
named "hardening-basic.config" (along with some arch-specific fragments)
that enable a basic set of kernel hardening options that have the least
(or no) performance impact and remove a reasonable set of legacy APIs.

Using this fragment is as simple as running "make hardening.config".

More extreme fragments can be added[2] in the future to cover all the
recognized hardening options, and more per-architecture files can be
added too.

For now, document the fragments directly via comments. Perhaps .rst
documentation can be generated from them in the future (rather than the
other way around).

[1] https://lore.kernel.org/kernel-hardening/[email-masked]/
[2] https://github.com/KSPP/linux/issues/14

Cc: Salvatore Mesoraca <[email-masked]>
Cc: [email-masked]
Cc: [email-masked]
Cc: [email-masked]
Cc: [email-masked]
Signed-off-by: Kees Cook <[email-masked]>
---
 MAINTAINERS                         |  2 +
 arch/arm/configs/hardening.config   |  7 ++
 arch/arm64/configs/hardening.config | 22 +++++++
 arch/x86/configs/hardening.config   | 15 +++++
 kernel/configs/hardening.config     | 99 +++++++++++++++++++++++++++++
 5 files changed, 145 insertions(+)
 create mode 100644 arch/arm/configs/hardening.config
 create mode 100644 arch/arm64/configs/hardening.config
 create mode 100644 arch/x86/configs/hardening.config
 create mode 100644 kernel/configs/hardening.config

diff --git a/MAINTAINERS b/MAINTAINERS
index 48abe1a281f2..36a28c132133 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -11304,8 +11304,10 @@ S:	Supported
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git for-next/hardening
 F:	Documentation/ABI/testing/sysfs-kernel-oops_count
 F:	Documentation/ABI/testing/sysfs-kernel-warn_count
+F:	arch/*/configs/hardening.config
 F:	include/linux/overflow.h
 F:	include/linux/randomize_kstack.h
+F:	kernel/configs/hardening.config
 F:	mm/usercopy.c
 K:	\b(add|choose)_random_kstack_offset\b
 K:	\b__check_(object_size|heap_object)\b
diff --git a/arch/arm/configs/hardening.config b/arch/arm/configs/hardening.config
new file mode 100644
index 000000000000..327349ce6377
--- /dev/null
+++ b/arch/arm/configs/hardening.config
@@ -0,0 +1,7 @@
+# Basic kernel hardening options (specific to arm)
+
+# Make sure PXN/PAN emulation is enabled.
+CONFIG_CPU_SW_DOMAIN_PAN=y
+
+# Dangerous; old interfaces and needless additional attack surface.
+# CONFIG_OABI_COMPAT is not set
diff --git a/arch/arm64/configs/hardening.config b/arch/arm64/configs/hardening.config
new file mode 100644
index 000000000000..b0e795208998
--- /dev/null
+++ b/arch/arm64/configs/hardening.config
@@ -0,0 +1,22 @@
+# Basic kernel hardening options (specific to arm64)
+
+# Make sure PAN emulation is enabled.
+CONFIG_ARM64_SW_TTBR0_PAN=y
+
+# Software Shadow Stack or PAC
+CONFIG_SHADOW_CALL_STACK=y
+
+# Pointer authentication (ARMv8.3 and later). If hardware actually supports
+# it, one can turn off CONFIG_STACKPROTECTOR_STRONG with this enabled.
+CONFIG_ARM64_PTR_AUTH=y
+CONFIG_ARM64_PTR_AUTH_KERNEL=y
+
+# Available in ARMv8.5 and later.
+CONFIG_ARM64_BTI=y
+CONFIG_ARM64_BTI_KERNEL=y
+CONFIG_ARM64_MTE=y
+CONFIG_KASAN_HW_TAGS=y
+CONFIG_ARM64_E0PD=y
+
+# Available in ARMv8.7 and later.
+CONFIG_ARM64_EPAN=y
diff --git a/arch/x86/configs/hardening.config b/arch/x86/configs/hardening.config
new file mode 100644
index 000000000000..19bb0c7a7669
--- /dev/null
+++ b/arch/x86/configs/hardening.config
@@ -0,0 +1,15 @@
+# Basic kernel hardening options (specific to x86)
+
+# Modern libc no longer needs a fixed-position mapping in userspace, remove
+# it as a possible target.
+CONFIG_LEGACY_VSYSCALL_NONE=y
+
+# Enable chip-specific IOMMU support.
+CONFIG_INTEL_IOMMU=y
+CONFIG_INTEL_IOMMU_DEFAULT_ON=y
+CONFIG_INTEL_IOMMU_SVM=y
+CONFIG_AMD_IOMMU=y
+CONFIG_AMD_IOMMU_V2=y
+
+# Enable CET Shadow Stack for userspace.
+CONFIG_X86_USER_SHADOW_STACK=y
diff --git a/kernel/configs/hardening.config b/kernel/configs/hardening.config
new file mode 100644
index 000000000000..dbeecdfca917
--- /dev/null
+++ b/kernel/configs/hardening.config
@@ -0,0 +1,99 @@
+# Basic kernel hardening options
+#
+# These are considered the basic kernel hardening, self-protection, and
+# attack surface reduction options. They are expected to have low (or
+# no) performance impact on most workloads, and have a reasonable level
+# of legacy API removals.
+
+# Make sure reporting of various hardening actions is possible.
+CONFIG_BUG=y
+
+# Basic kernel memory permission enforcement.
+CONFIG_STRICT_KERNEL_RWX=y
+CONFIG_STRICT_MODULE_RWX=y
+CONFIG_VMAP_STACK=y
+
+# Kernel image and memory ASLR.
+CONFIG_RANDOMIZE_BASE=y
+CONFIG_RANDOMIZE_MEMORY=y
+
+# Randomize allocator freelists, harden metadata.
+CONFIG_SLAB_FREELIST_RANDOM=y
+CONFIG_SLAB_FREELIST_HARDENED=y
+CONFIG_SHUFFLE_PAGE_ALLOCATOR=y
+CONFIG_RANDOM_KMALLOC_CACHES=y
+
+# Randomize kernel stack offset on syscall entry.
+CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT=y
+
+# Basic stack frame overflow protection.
+CONFIG_STACKPROTECTOR=y
+CONFIG_STACKPROTECTOR_STRONG=y
+
+# Basic buffer length bounds checking.
+CONFIG_HARDENED_USERCOPY=y
+CONFIG_FORTIFY_SOURCE=y
+
+# Basic array index bounds checking.
+CONFIG_UBSAN=y
+CONFIG_UBSAN_TRAP=y
+CONFIG_UBSAN_BOUNDS=y
+# CONFIG_UBSAN_SHIFT is not set
+# CONFIG_UBSAN_DIV_ZERO
+# CONFIG_UBSAN_UNREACHABLE
+# CONFIG_UBSAN_BOOL
+# CONFIG_UBSAN_ENUM
+# CONFIG_UBSAN_ALIGNMENT
+CONFIG_UBSAN_SANITIZE_ALL=y
+
+# Linked list integrity checking.
+CONFIG_LIST_HARDENED=y
+
+# Initialize all heap variables to zero on allocation.
+CONFIG_INIT_ON_ALLOC_DEFAULT_ON=y
+
+# Initialize all stack variables to zero on function entry.
+CONFIG_INIT_STACK_ALL_ZERO=y
+
+# Wipe all caller-used registers on exit from functions: reduces available
+# ROP gadgets and minimizes stale data in registers usable in side channels.
+CONFIG_ZERO_CALL_USED_REGS=y
+
+# Wipe RAM at reboot via EFI. For more details, see:
+# https://trustedcomputinggroup.org/resource/pc-client-work-group-platform-reset-attack-mitigation-specification/
+# https://bugzilla.redhat.com/show_bug.cgi?id=1532058
+CONFIG_RESET_ATTACK_MITIGATION=y
+
+# Disable DMA between EFI hand-off and the kernel's IOMMU setup.
+CONFIG_EFI_DISABLE_PCI_DMA=y
+
+# Force IOMMU TLB invalidation so devices will never be able to access stale
+# data content.
+CONFIG_IOMMU_SUPPORT=y
+CONFIG_IOMMU_DEFAULT_DMA_STRICT=y
+
+# Do not allow direct physical memory access to non-device memory.
+CONFIG_STRICT_DEVMEM=y
+CONFIG_IO_STRICT_DEVMEM=y
+
+# Provide userspace with seccomp BPF API for syscall attack surface reduction.
+CONFIG_SECCOMP=y
+CONFIG_SECCOMP_FILTER=y
+
+# Provides some protections against SYN flooding.
+CONFIG_SYN_COOKIES=y
+
+# Attack surface reduction: do not autoload TTY line disciplines.
+# CONFIG_LDISC_AUTOLOAD is not set
+
+# Dangerous; enabling this disables userspace brk ASLR.
+# CONFIG_COMPAT_BRK is not set
+
+# Dangerous; exposes kernel text image layout.
+# CONFIG_PROC_KCORE is not set
+
+# Dangerous; enabling this disables userspace VDSO ASLR.
+# CONFIG_COMPAT_VDSO is not set
+
+# Attack surface reduction: Use the modern PTY interface (devpts) only.
+# CONFIG_LEGACY_PTYS is not set
-- 
2.34.1

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