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name : 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)" depends on GCC_PLUGINS 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)" depends on GCC_PLUGINS 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)" depends on GCC_PLUGINS 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 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 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 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" 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" 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 endmenu