linux/security/Kconfig.hardening

# SPDX-License-Identifier: GPL-2.0-only
menu "Kernel hardening options"

config GCC_PLUGIN_STRUCTLEAK
	bool
	help
	  While the kernel is built with warnings enabled for any missed
	  stack variable initializations, this warning is silenced for
	  anything passed by reference to another function, under the
	  occasionally misguided assumption that the function will do
	  the initialization. As this regularly leads to exploitable
	  flaws, this plugin is available to identify and zero-initialize
	  such variables, depending on the chosen level of coverage.

	  This plugin was originally ported from grsecurity/PaX. More
	  information at:
	   * https://grsecurity.net/
	   * https://pax.grsecurity.net/

menu "Memory initialization"

config CC_HAS_AUTO_VAR_INIT_PATTERN
	def_bool $(cc-option,-ftrivial-auto-var-init=pattern)

config CC_HAS_AUTO_VAR_INIT_ZERO_BARE
	def_bool $(cc-option,-ftrivial-auto-var-init=zero)

config CC_HAS_AUTO_VAR_INIT_ZERO_ENABLER
	# Clang 16 and later warn about using the -enable flag, but it
	# is required before then.
	def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
	depends on !CC_HAS_AUTO_VAR_INIT_ZERO_BARE

config CC_HAS_AUTO_VAR_INIT_ZERO
	def_bool CC_HAS_AUTO_VAR_INIT_ZERO_BARE || CC_HAS_AUTO_VAR_INIT_ZERO_ENABLER

choice
	prompt "Initialize kernel stack variables at function entry"
	default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
	default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
	default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
	default INIT_STACK_NONE
	help
	  This option enables initialization of stack variables at
	  function entry time. This has the possibility to have the
	  greatest coverage (since all functions can have their
	  variables initialized), but the performance impact depends
	  on the function calling complexity of a given workload's
	  syscalls.

	  This chooses the level of coverage over classes of potentially
	  uninitialized variables. The selected class of variable will be
	  initialized before use in a function.

	config INIT_STACK_NONE
		bool "no automatic stack variable initialization (weakest)"
		help
		  Disable automatic stack variable initialization.
		  This leaves the kernel vulnerable to the standard
		  classes of uninitialized stack variable exploits
		  and information exposures.

	config GCC_PLUGIN_STRUCTLEAK_USER
		bool "zero-init structs marked for userspace (weak)"
		# Plugin can be removed once the kernel only supports GCC 12+
		depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
		select GCC_PLUGIN_STRUCTLEAK
		help
		  Zero-initialize any structures on the stack containing
		  a __user attribute. This can prevent some classes of
		  uninitialized stack variable exploits and information
		  exposures, like CVE-2013-2141:
		  https://git.kernel.org/linus/b9e146d8eb3b9eca

	config GCC_PLUGIN_STRUCTLEAK_BYREF
		bool "zero-init structs passed by reference (strong)"
		# Plugin can be removed once the kernel only supports GCC 12+
		depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
		depends on !(KASAN && KASAN_STACK)
		select GCC_PLUGIN_STRUCTLEAK
		help
		  Zero-initialize any structures on the stack that may
		  be passed by reference and had not already been
		  explicitly initialized. This can prevent most classes
		  of uninitialized stack variable exploits and information
		  exposures, like CVE-2017-1000410:
		  https://git.kernel.org/linus/06e7e776ca4d3654

		  As a side-effect, this keeps a lot of variables on the
		  stack that can otherwise be optimized out, so combining
		  this with CONFIG_KASAN_STACK can lead to a stack overflow
		  and is disallowed.

	config GCC_PLUGIN_STRUCTLEAK_BYREF_ALL
		bool "zero-init everything passed by reference (very strong)"
		# Plugin can be removed once the kernel only supports GCC 12+
		depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
		depends on !(KASAN && KASAN_STACK)
		select GCC_PLUGIN_STRUCTLEAK
		help
		  Zero-initialize any stack variables that may be passed
		  by reference and had not already been explicitly
		  initialized. This is intended to eliminate all classes
		  of uninitialized stack variable exploits and information
		  exposures.

		  As a side-effect, this keeps a lot of variables on the
		  stack that can otherwise be optimized out, so combining
		  this with CONFIG_KASAN_STACK can lead to a stack overflow
		  and is disallowed.

	config INIT_STACK_ALL_PATTERN
		bool "pattern-init everything (strongest)"
		depends on CC_HAS_AUTO_VAR_INIT_PATTERN
		depends on !KMSAN
		help
		  Initializes everything on the stack (including padding)
		  with a specific debug value. This is intended to eliminate
		  all classes of uninitialized stack variable exploits and
		  information exposures, even variables that were warned about
		  having been left uninitialized.

		  Pattern initialization is known to provoke many existing bugs
		  related to uninitialized locals, e.g. pointers receive
		  non-NULL values, buffer sizes and indices are very big. The
		  pattern is situation-specific; Clang on 64-bit uses 0xAA
		  repeating for all types and padding except float and double
		  which use 0xFF repeating (-NaN). Clang on 32-bit uses 0xFF
		  repeating for all types and padding.

	config INIT_STACK_ALL_ZERO
		bool "zero-init everything (strongest and safest)"
		depends on CC_HAS_AUTO_VAR_INIT_ZERO
		depends on !KMSAN
		help
		  Initializes everything on the stack (including padding)
		  with a zero value. This is intended to eliminate all
		  classes of uninitialized stack variable exploits and
		  information exposures, even variables that were warned
		  about having been left uninitialized.

		  Zero initialization provides safe defaults for strings
		  (immediately NUL-terminated), pointers (NULL), indices
		  (index 0), and sizes (0 length), so it is therefore more
		  suitable as a production security mitigation than pattern
		  initialization.

endchoice

config GCC_PLUGIN_STRUCTLEAK_VERBOSE
	bool "Report forcefully initialized variables"
	depends on GCC_PLUGIN_STRUCTLEAK
	depends on !COMPILE_TEST	# too noisy
	help
	  This option will cause a warning to be printed each time the
	  structleak plugin finds a variable it thinks needs to be
	  initialized. Since not all existing initializers are detected
	  by the plugin, this can produce false positive warnings.

config GCC_PLUGIN_STACKLEAK
	bool "Poison kernel stack before returning from syscalls"
	depends on GCC_PLUGINS
	depends on HAVE_ARCH_STACKLEAK
	help
	  This option makes the kernel erase the kernel stack before
	  returning from system calls. This has the effect of leaving
	  the stack initialized to the poison value, which both reduces
	  the lifetime of any sensitive stack contents and reduces
	  potential for uninitialized stack variable exploits or information
	  exposures (it does not cover functions reaching the same stack
	  depth as prior functions during the same syscall). This blocks
	  most uninitialized stack variable attacks, with the performance
	  impact being driven by the depth of the stack usage, rather than
	  the function calling complexity.

	  The performance impact on a single CPU system kernel compilation
	  sees a 1% slowdown, other systems and workloads may vary and you
	  are advised to test this feature on your expected workload before
	  deploying it.

	  This plugin was ported from grsecurity/PaX. More information at:
	   * https://grsecurity.net/
	   * https://pax.grsecurity.net/

config GCC_PLUGIN_STACKLEAK_VERBOSE
	bool "Report stack depth analysis instrumentation" if EXPERT
	depends on GCC_PLUGIN_STACKLEAK
	depends on !COMPILE_TEST	# too noisy
	help
	  This option will cause a warning to be printed each time the
	  stackleak plugin finds a function it thinks needs to be
	  instrumented. This is useful for comparing coverage between
	  builds.

config STACKLEAK_TRACK_MIN_SIZE
	int "Minimum stack frame size of functions tracked by STACKLEAK"
	default 100
	range 0 4096
	depends on GCC_PLUGIN_STACKLEAK
	help
	  The STACKLEAK gcc plugin instruments the kernel code for tracking
	  the lowest border of the kernel stack (and for some other purposes).
	  It inserts the stackleak_track_stack() call for the functions with
	  a stack frame size greater than or equal to this parameter.
	  If unsure, leave the default value 100.

config STACKLEAK_METRICS
	bool "Show STACKLEAK metrics in the /proc file system"
	depends on GCC_PLUGIN_STACKLEAK
	depends on PROC_FS
	help
	  If this is set, STACKLEAK metrics for every task are available in
	  the /proc file system. In particular, /proc/<pid>/stack_depth
	  shows the maximum kernel stack consumption for the current and
	  previous syscalls. Although this information is not precise, it
	  can be useful for estimating the STACKLEAK performance impact for
	  your workloads.

config STACKLEAK_RUNTIME_DISABLE
	bool "Allow runtime disabling of kernel stack erasing"
	depends on GCC_PLUGIN_STACKLEAK
	help
	  This option provides 'stack_erasing' sysctl, which can be used in
	  runtime to control kernel stack erasing for kernels built with
	  CONFIG_GCC_PLUGIN_STACKLEAK.

config INIT_ON_ALLOC_DEFAULT_ON
	bool "Enable heap memory zeroing on allocation by default"
	depends on !KMSAN
	help
	  This has the effect of setting "init_on_alloc=1" on the kernel
	  command line. This can be disabled with "init_on_alloc=0".
	  When "init_on_alloc" is enabled, all page allocator and slab
	  allocator memory will be zeroed when allocated, eliminating
	  many kinds of "uninitialized heap memory" flaws, especially
	  heap content exposures. The performance impact varies by
	  workload, but most cases see <1% impact. Some synthetic
	  workloads have measured as high as 7%.

config INIT_ON_FREE_DEFAULT_ON
	bool "Enable heap memory zeroing on free by default"
	depends on !KMSAN
	help
	  This has the effect of setting "init_on_free=1" on the kernel
	  command line. This can be disabled with "init_on_free=0".
	  Similar to "init_on_alloc", when "init_on_free" is enabled,
	  all page allocator and slab allocator memory will be zeroed
	  when freed, eliminating many kinds of "uninitialized heap memory"
	  flaws, especially heap content exposures. The primary difference
	  with "init_on_free" is that data lifetime in memory is reduced,
	  as anything freed is wiped immediately, making live forensics or
	  cold boot memory attacks unable to recover freed memory contents.
	  The performance impact varies by workload, but is more expensive
	  than "init_on_alloc" due to the negative cache effects of
	  touching "cold" memory areas. Most cases see 3-5% impact. Some
	  synthetic workloads have measured as high as 8%.

config CC_HAS_ZERO_CALL_USED_REGS
	def_bool $(cc-option,-fzero-call-used-regs=used-gpr)

config ZERO_CALL_USED_REGS
	bool "Enable register zeroing on function exit"
	depends on CC_HAS_ZERO_CALL_USED_REGS
	help
	  At the end of functions, always zero any caller-used register
	  contents. This helps ensure that temporary values are not
	  leaked beyond the function boundary. This means that register
	  contents are less likely to be available for side channels
	  and information exposures. Additionally, this helps reduce the
	  number of useful ROP gadgets by about 20% (and removes compiler
	  generated "write-what-where" gadgets) in the resulting kernel
	  image. This has a less than 1% performance impact on most
	  workloads. Image size growth depends on architecture, and should
	  be evaluated for suitability. For example, x86_64 grows by less
	  than 1%, and arm64 grows by about 5%.

endmenu

config CC_HAS_RANDSTRUCT
	def_bool $(cc-option,-frandomize-layout-seed-file=/dev/null)

choice
	prompt "Randomize layout of sensitive kernel structures"
	default RANDSTRUCT_FULL if COMPILE_TEST && (GCC_PLUGINS || CC_HAS_RANDSTRUCT)
	default RANDSTRUCT_NONE
	help
	  If you enable this, the layouts of structures that are entirely
	  function pointers (and have not been manually annotated with
	  __no_randomize_layout), or structures that have been explicitly
	  marked with __randomize_layout, will be randomized at compile-time.
	  This can introduce the requirement of an additional information
	  exposure vulnerability for exploits targeting these structure
	  types.

	  Enabling this feature will introduce some performance impact,
	  slightly increase memory usage, and prevent the use of forensic
	  tools like Volatility against the system (unless the kernel
	  source tree isn't cleaned after kernel installation).

	  The seed used for compilation is in scripts/basic/randomize.seed.
	  It remains after a "make clean" to allow for external modules to
	  be compiled with the existing seed and will be removed by a
	  "make mrproper" or "make distclean". This file should not be made
	  public, or the structure layout can be determined.

	config RANDSTRUCT_NONE
		bool "Disable structure layout randomization"
		help
		  Build normally: no structure layout randomization.

	config RANDSTRUCT_FULL
		bool "Fully randomize structure layout"
		depends on CC_HAS_RANDSTRUCT || GCC_PLUGINS
		select MODVERSIONS if MODULES
		help
		  Fully randomize the member layout of sensitive
		  structures as much as possible, which may have both a
		  memory size and performance impact.

		  One difference between the Clang and GCC plugin
		  implementations is the handling of bitfields. The GCC
		  plugin treats them as fully separate variables,
		  introducing sometimes significant padding. Clang tries
		  to keep adjacent bitfields together, but with their bit
		  ordering randomized.

	config RANDSTRUCT_PERFORMANCE
		bool "Limit randomization of structure layout to cache-lines"
		depends on GCC_PLUGINS
		select MODVERSIONS if MODULES
		help
		  Randomization of sensitive kernel structures will make a
		  best effort at restricting randomization to cacheline-sized
		  groups of members. It will further not randomize bitfields
		  in structures. This reduces the performance hit of RANDSTRUCT
		  at the cost of weakened randomization.
endchoice

config RANDSTRUCT
	def_bool !RANDSTRUCT_NONE

config GCC_PLUGIN_RANDSTRUCT
	def_bool GCC_PLUGINS && RANDSTRUCT
	help
	  Use GCC plugin to randomize structure layout.

	  This plugin was ported from grsecurity/PaX. More
	  information at:
	   * https://grsecurity.net/
	   * https://pax.grsecurity.net/

endmenu