security/Kconfig.hardening

   1 # SPDX-License-Identifier: GPL-2.0-only
   2 menu "Kernel hardening options"
   3
   4 config GCC_PLUGIN_STRUCTLEAK
   5         bool
   6         help
   7           While the kernel is built with warnings enabled for any missed
   8           stack variable initializations, this warning is silenced for
   9           anything passed by reference to another function, under the
  10           occasionally misguided assumption that the function will do
  11           the initialization. As this regularly leads to exploitable
  12           flaws, this plugin is available to identify and zero-initialize
  13           such variables, depending on the chosen level of coverage.
  14
  15           This plugin was originally ported from grsecurity/PaX. More
  16           information at:
  17            * https://grsecurity.net/
  18            * https://pax.grsecurity.net/
  19
  20 menu "Memory initialization"
  21
  22 config CC_HAS_AUTO_VAR_INIT_PATTERN
  23         def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
  24
  25 config CC_HAS_AUTO_VAR_INIT_ZERO
  26         # GCC ignores the -enable flag, so we can test for the feature with
  27         # a single invocation using the flag, but drop it as appropriate in
  28         # the Makefile, depending on the presence of Clang.
  29         def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
  30
  31 choice
  32         prompt "Initialize kernel stack variables at function entry"
  33         default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
  34         default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
  35         default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
  36         default INIT_STACK_NONE
  37         help
  38           This option enables initialization of stack variables at
  39           function entry time. This has the possibility to have the
  40           greatest coverage (since all functions can have their
  41           variables initialized), but the performance impact depends
  42           on the function calling complexity of a given workload's
  43           syscalls.
  44
  45           This chooses the level of coverage over classes of potentially
  46           uninitialized variables. The selected class of variable will be
  47           initialized before use in a function.
  48
  49         config INIT_STACK_NONE
  50                 bool "no automatic stack variable initialization (weakest)"
  51                 help
  52                   Disable automatic stack variable initialization.
  53                   This leaves the kernel vulnerable to the standard
  54                   classes of uninitialized stack variable exploits
  55                   and information exposures.
  56
  57         config GCC_PLUGIN_STRUCTLEAK_USER
  58                 bool "zero-init structs marked for userspace (weak)"
  59                 # Plugin can be removed once the kernel only supports GCC 12+
  60                 depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
  61                 select GCC_PLUGIN_STRUCTLEAK
  62                 help
  63                   Zero-initialize any structures on the stack containing
  64                   a __user attribute. This can prevent some classes of
  65                   uninitialized stack variable exploits and information
  66                   exposures, like CVE-2013-2141:
  67                   https://git.kernel.org/linus/b9e146d8eb3b9eca
  68
  69         config GCC_PLUGIN_STRUCTLEAK_BYREF
  70                 bool "zero-init structs passed by reference (strong)"
  71                 # Plugin can be removed once the kernel only supports GCC 12+
  72                 depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
  73                 depends on !(KASAN && KASAN_STACK)
  74                 select GCC_PLUGIN_STRUCTLEAK
  75                 help
  76                   Zero-initialize any structures on the stack that may
  77                   be passed by reference and had not already been
  78                   explicitly initialized. This can prevent most classes
  79                   of uninitialized stack variable exploits and information
  80                   exposures, like CVE-2017-1000410:
  81                   https://git.kernel.org/linus/06e7e776ca4d3654
  82
  83                   As a side-effect, this keeps a lot of variables on the
  84                   stack that can otherwise be optimized out, so combining
  85                   this with CONFIG_KASAN_STACK can lead to a stack overflow
  86                   and is disallowed.
  87
  88         config GCC_PLUGIN_STRUCTLEAK_BYREF_ALL
  89                 bool "zero-init everything passed by reference (very strong)"
  90                 # Plugin can be removed once the kernel only supports GCC 12+
  91                 depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
  92                 depends on !(KASAN && KASAN_STACK)
  93                 select GCC_PLUGIN_STRUCTLEAK
  94                 help
  95                   Zero-initialize any stack variables that may be passed
  96                   by reference and had not already been explicitly
  97                   initialized. This is intended to eliminate all classes
  98                   of uninitialized stack variable exploits and information
  99                   exposures.
 100
 101                   As a side-effect, this keeps a lot of variables on the
 102                   stack that can otherwise be optimized out, so combining
 103                   this with CONFIG_KASAN_STACK can lead to a stack overflow
 104                   and is disallowed.
 105
 106         config INIT_STACK_ALL_PATTERN
 107                 bool "pattern-init everything (strongest)"
 108                 depends on CC_HAS_AUTO_VAR_INIT_PATTERN
 109                 help
 110                   Initializes everything on the stack (including padding)
 111                   with a specific debug value. This is intended to eliminate
 112                   all classes of uninitialized stack variable exploits and
 113                   information exposures, even variables that were warned about
 114                   having been left uninitialized.
 115
 116                   Pattern initialization is known to provoke many existing bugs
 117                   related to uninitialized locals, e.g. pointers receive
 118                   non-NULL values, buffer sizes and indices are very big. The
 119                   pattern is situation-specific; Clang on 64-bit uses 0xAA
 120                   repeating for all types and padding except float and double
 121                   which use 0xFF repeating (-NaN). Clang on 32-bit uses 0xFF
 122                   repeating for all types and padding.
 123
 124         config INIT_STACK_ALL_ZERO
 125                 bool "zero-init everything (strongest and safest)"
 126                 depends on CC_HAS_AUTO_VAR_INIT_ZERO
 127                 help
 128                   Initializes everything on the stack (including padding)
 129                   with a zero value. This is intended to eliminate all
 130                   classes of uninitialized stack variable exploits and
 131                   information exposures, even variables that were warned
 132                   about having been left uninitialized.
 133
 134                   Zero initialization provides safe defaults for strings
 135                   (immediately NUL-terminated), pointers (NULL), indices
 136                   (index 0), and sizes (0 length), so it is therefore more
 137                   suitable as a production security mitigation than pattern
 138                   initialization.
 139
 140 endchoice
 141
 142 config GCC_PLUGIN_STRUCTLEAK_VERBOSE
 143         bool "Report forcefully initialized variables"
 144         depends on GCC_PLUGIN_STRUCTLEAK
 145         depends on !COMPILE_TEST        # too noisy
 146         help
 147           This option will cause a warning to be printed each time the
 148           structleak plugin finds a variable it thinks needs to be
 149           initialized. Since not all existing initializers are detected
 150           by the plugin, this can produce false positive warnings.
 151
 152 config GCC_PLUGIN_STACKLEAK
 153         bool "Poison kernel stack before returning from syscalls"
 154         depends on GCC_PLUGINS
 155         depends on HAVE_ARCH_STACKLEAK
 156         help
 157           This option makes the kernel erase the kernel stack before
 158           returning from system calls. This has the effect of leaving
 159           the stack initialized to the poison value, which both reduces
 160           the lifetime of any sensitive stack contents and reduces
 161           potential for uninitialized stack variable exploits or information
 162           exposures (it does not cover functions reaching the same stack
 163           depth as prior functions during the same syscall). This blocks
 164           most uninitialized stack variable attacks, with the performance
 165           impact being driven by the depth of the stack usage, rather than
 166           the function calling complexity.
 167
 168           The performance impact on a single CPU system kernel compilation
 169           sees a 1% slowdown, other systems and workloads may vary and you
 170           are advised to test this feature on your expected workload before
 171           deploying it.
 172
 173           This plugin was ported from grsecurity/PaX. More information at:
 174            * https://grsecurity.net/
 175            * https://pax.grsecurity.net/
 176
 177 config GCC_PLUGIN_STACKLEAK_VERBOSE
 178         bool "Report stack depth analysis instrumentation" if EXPERT
 179         depends on GCC_PLUGIN_STACKLEAK
 180         depends on !COMPILE_TEST        # too noisy
 181         help
 182           This option will cause a warning to be printed each time the
 183           stackleak plugin finds a function it thinks needs to be
 184           instrumented. This is useful for comparing coverage between
 185           builds.
 186
 187 config STACKLEAK_TRACK_MIN_SIZE
 188         int "Minimum stack frame size of functions tracked by STACKLEAK"
 189         default 100
 190         range 0 4096
 191         depends on GCC_PLUGIN_STACKLEAK
 192         help
 193           The STACKLEAK gcc plugin instruments the kernel code for tracking
 194           the lowest border of the kernel stack (and for some other purposes).
 195           It inserts the stackleak_track_stack() call for the functions with
 196           a stack frame size greater than or equal to this parameter.
 197           If unsure, leave the default value 100.
 198
 199 config STACKLEAK_METRICS
 200         bool "Show STACKLEAK metrics in the /proc file system"
 201         depends on GCC_PLUGIN_STACKLEAK
 202         depends on PROC_FS
 203         help
 204           If this is set, STACKLEAK metrics for every task are available in
 205           the /proc file system. In particular, /proc/<pid>/stack_depth
 206           shows the maximum kernel stack consumption for the current and
 207           previous syscalls. Although this information is not precise, it
 208           can be useful for estimating the STACKLEAK performance impact for
 209           your workloads.
 210
 211 config STACKLEAK_RUNTIME_DISABLE
 212         bool "Allow runtime disabling of kernel stack erasing"
 213         depends on GCC_PLUGIN_STACKLEAK
 214         help
 215           This option provides 'stack_erasing' sysctl, which can be used in
 216           runtime to control kernel stack erasing for kernels built with
 217           CONFIG_GCC_PLUGIN_STACKLEAK.
 218
 219 config INIT_ON_ALLOC_DEFAULT_ON
 220         bool "Enable heap memory zeroing on allocation by default"
 221         help
 222           This has the effect of setting "init_on_alloc=1" on the kernel
 223           command line. This can be disabled with "init_on_alloc=0".
 224           When "init_on_alloc" is enabled, all page allocator and slab
 225           allocator memory will be zeroed when allocated, eliminating
 226           many kinds of "uninitialized heap memory" flaws, especially
 227           heap content exposures. The performance impact varies by
 228           workload, but most cases see <1% impact. Some synthetic
 229           workloads have measured as high as 7%.
 230
 231 config INIT_ON_FREE_DEFAULT_ON
 232         bool "Enable heap memory zeroing on free by default"
 233         help
 234           This has the effect of setting "init_on_free=1" on the kernel
 235           command line. This can be disabled with "init_on_free=0".
 236           Similar to "init_on_alloc", when "init_on_free" is enabled,
 237           all page allocator and slab allocator memory will be zeroed
 238           when freed, eliminating many kinds of "uninitialized heap memory"
 239           flaws, especially heap content exposures. The primary difference
 240           with "init_on_free" is that data lifetime in memory is reduced,
 241           as anything freed is wiped immediately, making live forensics or
 242           cold boot memory attacks unable to recover freed memory contents.
 243           The performance impact varies by workload, but is more expensive
 244           than "init_on_alloc" due to the negative cache effects of
 245           touching "cold" memory areas. Most cases see 3-5% impact. Some
 246           synthetic workloads have measured as high as 8%.
 247
 248 config CC_HAS_ZERO_CALL_USED_REGS
 249         def_bool $(cc-option,-fzero-call-used-regs=used-gpr)
 250
 251 config ZERO_CALL_USED_REGS
 252         bool "Enable register zeroing on function exit"
 253         depends on CC_HAS_ZERO_CALL_USED_REGS
 254         help
 255           At the end of functions, always zero any caller-used register
 256           contents. This helps ensure that temporary values are not
 257           leaked beyond the function boundary. This means that register
 258           contents are less likely to be available for side channels
 259           and information exposures. Additionally, this helps reduce the
 260           number of useful ROP gadgets by about 20% (and removes compiler
 261           generated "write-what-where" gadgets) in the resulting kernel
 262           image. This has a less than 1% performance impact on most
 263           workloads. Image size growth depends on architecture, and should
 264           be evaluated for suitability. For example, x86_64 grows by less
 265           than 1%, and arm64 grows by about 5%.
 266
 267 endmenu
 268
 269 config CC_HAS_RANDSTRUCT
 270         def_bool $(cc-option,-frandomize-layout-seed-file=/dev/null)
 271
 272 choice
 273         prompt "Randomize layout of sensitive kernel structures"
 274         default RANDSTRUCT_FULL if COMPILE_TEST && (GCC_PLUGINS || CC_HAS_RANDSTRUCT)
 275         default RANDSTRUCT_NONE
 276         help
 277           If you enable this, the layouts of structures that are entirely
 278           function pointers (and have not been manually annotated with
 279           __no_randomize_layout), or structures that have been explicitly
 280           marked with __randomize_layout, will be randomized at compile-time.
 281           This can introduce the requirement of an additional information
 282           exposure vulnerability for exploits targeting these structure
 283           types.
 284
 285           Enabling this feature will introduce some performance impact,
 286           slightly increase memory usage, and prevent the use of forensic
 287           tools like Volatility against the system (unless the kernel
 288           source tree isn't cleaned after kernel installation).
 289
 290           The seed used for compilation is in scripts/basic/randomize.seed.
 291           It remains after a "make clean" to allow for external modules to
 292           be compiled with the existing seed and will be removed by a
 293           "make mrproper" or "make distclean". This file should not be made
 294           public, or the structure layout can be determined.
 295
 296         config RANDSTRUCT_NONE
 297                 bool "Disable structure layout randomization"
 298                 help
 299                   Build normally: no structure layout randomization.
 300
 301         config RANDSTRUCT_FULL
 302                 bool "Fully randomize structure layout"
 303                 depends on CC_HAS_RANDSTRUCT || GCC_PLUGINS
 304                 select MODVERSIONS if MODULES
 305                 help
 306                   Fully randomize the member layout of sensitive
 307                   structures as much as possible, which may have both a
 308                   memory size and performance impact.
 309
 310                   One difference between the Clang and GCC plugin
 311                   implementations is the handling of bitfields. The GCC
 312                   plugin treats them as fully separate variables,
 313                   introducing sometimes significant padding. Clang tries
 314                   to keep adjacent bitfields together, but with their bit
 315                   ordering randomized.
 316
 317         config RANDSTRUCT_PERFORMANCE
 318                 bool "Limit randomization of structure layout to cache-lines"
 319                 depends on GCC_PLUGINS
 320                 select MODVERSIONS if MODULES
 321                 help
 322                   Randomization of sensitive kernel structures will make a
 323                   best effort at restricting randomization to cacheline-sized
 324                   groups of members. It will further not randomize bitfields
 325                   in structures. This reduces the performance hit of RANDSTRUCT
 326                   at the cost of weakened randomization.
 327 endchoice
 328
 329 config RANDSTRUCT
 330         def_bool !RANDSTRUCT_NONE
 331
 332 config GCC_PLUGIN_RANDSTRUCT
 333         def_bool GCC_PLUGINS && RANDSTRUCT
 334         help
 335           Use GCC plugin to randomize structure layout.
 336
 337           This plugin was ported from grsecurity/PaX. More
 338           information at:
 339            * https://grsecurity.net/
 340            * https://pax.grsecurity.net/
 341
 342 endmenu