# 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) # https://github.com/ClangBuiltLinux/linux/issues/1766 # https://github.com/llvm/llvm-project/issues/59242 depends on !CC_IS_CLANG || CLANG_VERSION > 150006 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 menu "Hardening of kernel data structures" config LIST_HARDENED bool "Check integrity of linked list manipulation" help Minimal integrity checking in the linked-list manipulation routines to catch memory corruptions that are not guaranteed to result in an immediate access fault. If unsure, say N. config BUG_ON_DATA_CORRUPTION bool "Trigger a BUG when data corruption is detected" select LIST_HARDENED help Select this option if the kernel should BUG when it encounters data corruption in kernel memory structures when they get checked for validity. If unsure, say N. endmenu config CC_HAS_RANDSTRUCT def_bool $(cc-option,-frandomize-layout-seed-file=/dev/null) # Randstruct was first added in Clang 15, but it isn't safe to use until # Clang 16 due to https://github.com/llvm/llvm-project/issues/60349 depends on !CC_IS_CLANG || CLANG_VERSION >= 160000 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