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_BARE
  26         def_bool $(cc-option,-ftrivial-auto-var-init=zero)
  27
  28 config CC_HAS_AUTO_VAR_INIT_ZERO_ENABLER
  29         # Clang 16 and later warn about using the -enable flag, but it
  30         # is required before then.
  31         def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
  32         depends on !CC_HAS_AUTO_VAR_INIT_ZERO_BARE
  33
  34 config CC_HAS_AUTO_VAR_INIT_ZERO
  35         def_bool CC_HAS_AUTO_VAR_INIT_ZERO_BARE || CC_HAS_AUTO_VAR_INIT_ZERO_ENABLER
  36
  37 choice
  38         prompt "Initialize kernel stack variables at function entry"
  39         default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
  40         default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
  41         default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
  42         default INIT_STACK_NONE
  43         help
  44           This option enables initialization of stack variables at
  45           function entry time. This has the possibility to have the
  46           greatest coverage (since all functions can have their
  47           variables initialized), but the performance impact depends
  48           on the function calling complexity of a given workload's
  49           syscalls.
  50
  51           This chooses the level of coverage over classes of potentially
  52           uninitialized variables. The selected class of variable will be
  53           initialized before use in a function.
  54
  55         config INIT_STACK_NONE
  56                 bool "no automatic stack variable initialization (weakest)"
  57                 help
  58                   Disable automatic stack variable initialization.
  59                   This leaves the kernel vulnerable to the standard
  60                   classes of uninitialized stack variable exploits
  61                   and information exposures.
  62
  63         config GCC_PLUGIN_STRUCTLEAK_USER
  64                 bool "zero-init structs marked for userspace (weak)"
  65                 # Plugin can be removed once the kernel only supports GCC 12+
  66                 depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
  67                 select GCC_PLUGIN_STRUCTLEAK
  68                 help
  69                   Zero-initialize any structures on the stack containing
  70                   a __user attribute. This can prevent some classes of
  71                   uninitialized stack variable exploits and information
  72                   exposures, like CVE-2013-2141:
  73                   https://git.kernel.org/linus/b9e146d8eb3b9eca
  74
  75         config GCC_PLUGIN_STRUCTLEAK_BYREF
  76                 bool "zero-init structs passed by reference (strong)"
  77                 # Plugin can be removed once the kernel only supports GCC 12+
  78                 depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
  79                 depends on !(KASAN && KASAN_STACK)
  80                 select GCC_PLUGIN_STRUCTLEAK
  81                 help
  82                   Zero-initialize any structures on the stack that may
  83                   be passed by reference and had not already been
  84                   explicitly initialized. This can prevent most classes
  85                   of uninitialized stack variable exploits and information
  86                   exposures, like CVE-2017-1000410:
  87                   https://git.kernel.org/linus/06e7e776ca4d3654
  88
  89                   As a side-effect, this keeps a lot of variables on the
  90                   stack that can otherwise be optimized out, so combining
  91                   this with CONFIG_KASAN_STACK can lead to a stack overflow
  92                   and is disallowed.
  93
  94         config GCC_PLUGIN_STRUCTLEAK_BYREF_ALL
  95                 bool "zero-init everything passed by reference (very strong)"
  96                 # Plugin can be removed once the kernel only supports GCC 12+
  97                 depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
  98                 depends on !(KASAN && KASAN_STACK)
  99                 select GCC_PLUGIN_STRUCTLEAK
 100                 help
 101                   Zero-initialize any stack variables that may be passed
 102                   by reference and had not already been explicitly
 103                   initialized. This is intended to eliminate all classes
 104                   of uninitialized stack variable exploits and information
 105                   exposures.
 106
 107                   As a side-effect, this keeps a lot of variables on the
 108                   stack that can otherwise be optimized out, so combining
 109                   this with CONFIG_KASAN_STACK can lead to a stack overflow
 110                   and is disallowed.
 111
 112         config INIT_STACK_ALL_PATTERN
 113                 bool "pattern-init everything (strongest)"
 114                 depends on CC_HAS_AUTO_VAR_INIT_PATTERN
 115                 depends on !KMSAN
 116                 help
 117                   Initializes everything on the stack (including padding)
 118                   with a specific debug value. This is intended to eliminate
 119                   all classes of uninitialized stack variable exploits and
 120                   information exposures, even variables that were warned about
 121                   having been left uninitialized.
 122
 123                   Pattern initialization is known to provoke many existing bugs
 124                   related to uninitialized locals, e.g. pointers receive
 125                   non-NULL values, buffer sizes and indices are very big. The
 126                   pattern is situation-specific; Clang on 64-bit uses 0xAA
 127                   repeating for all types and padding except float and double
 128                   which use 0xFF repeating (-NaN). Clang on 32-bit uses 0xFF
 129                   repeating for all types and padding.
 130
 131         config INIT_STACK_ALL_ZERO
 132                 bool "zero-init everything (strongest and safest)"
 133                 depends on CC_HAS_AUTO_VAR_INIT_ZERO
 134                 depends on !KMSAN
 135                 help
 136                   Initializes everything on the stack (including padding)
 137                   with a zero value. This is intended to eliminate all
 138                   classes of uninitialized stack variable exploits and
 139                   information exposures, even variables that were warned
 140                   about having been left uninitialized.
 141
 142                   Zero initialization provides safe defaults for strings
 143                   (immediately NUL-terminated), pointers (NULL), indices
 144                   (index 0), and sizes (0 length), so it is therefore more
 145                   suitable as a production security mitigation than pattern
 146                   initialization.
 147
 148 endchoice
 149
 150 config GCC_PLUGIN_STRUCTLEAK_VERBOSE
 151         bool "Report forcefully initialized variables"
 152         depends on GCC_PLUGIN_STRUCTLEAK
 153         depends on !COMPILE_TEST        # too noisy
 154         help
 155           This option will cause a warning to be printed each time the
 156           structleak plugin finds a variable it thinks needs to be
 157           initialized. Since not all existing initializers are detected
 158           by the plugin, this can produce false positive warnings.
 159
 160 config GCC_PLUGIN_STACKLEAK
 161         bool "Poison kernel stack before returning from syscalls"
 162         depends on GCC_PLUGINS
 163         depends on HAVE_ARCH_STACKLEAK
 164         help
 165           This option makes the kernel erase the kernel stack before
 166           returning from system calls. This has the effect of leaving
 167           the stack initialized to the poison value, which both reduces
 168           the lifetime of any sensitive stack contents and reduces
 169           potential for uninitialized stack variable exploits or information
 170           exposures (it does not cover functions reaching the same stack
 171           depth as prior functions during the same syscall). This blocks
 172           most uninitialized stack variable attacks, with the performance
 173           impact being driven by the depth of the stack usage, rather than
 174           the function calling complexity.
 175
 176           The performance impact on a single CPU system kernel compilation
 177           sees a 1% slowdown, other systems and workloads may vary and you
 178           are advised to test this feature on your expected workload before
 179           deploying it.
 180
 181           This plugin was ported from grsecurity/PaX. More information at:
 182            * https://grsecurity.net/
 183            * https://pax.grsecurity.net/
 184
 185 config GCC_PLUGIN_STACKLEAK_VERBOSE
 186         bool "Report stack depth analysis instrumentation" if EXPERT
 187         depends on GCC_PLUGIN_STACKLEAK
 188         depends on !COMPILE_TEST        # too noisy
 189         help
 190           This option will cause a warning to be printed each time the
 191           stackleak plugin finds a function it thinks needs to be
 192           instrumented. This is useful for comparing coverage between
 193           builds.
 194
 195 config STACKLEAK_TRACK_MIN_SIZE
 196         int "Minimum stack frame size of functions tracked by STACKLEAK"
 197         default 100
 198         range 0 4096
 199         depends on GCC_PLUGIN_STACKLEAK
 200         help
 201           The STACKLEAK gcc plugin instruments the kernel code for tracking
 202           the lowest border of the kernel stack (and for some other purposes).
 203           It inserts the stackleak_track_stack() call for the functions with
 204           a stack frame size greater than or equal to this parameter.
 205           If unsure, leave the default value 100.
 206
 207 config STACKLEAK_METRICS
 208         bool "Show STACKLEAK metrics in the /proc file system"
 209         depends on GCC_PLUGIN_STACKLEAK
 210         depends on PROC_FS
 211         help
 212           If this is set, STACKLEAK metrics for every task are available in
 213           the /proc file system. In particular, /proc/<pid>/stack_depth
 214           shows the maximum kernel stack consumption for the current and
 215           previous syscalls. Although this information is not precise, it
 216           can be useful for estimating the STACKLEAK performance impact for
 217           your workloads.
 218
 219 config STACKLEAK_RUNTIME_DISABLE
 220         bool "Allow runtime disabling of kernel stack erasing"
 221         depends on GCC_PLUGIN_STACKLEAK
 222         help
 223           This option provides 'stack_erasing' sysctl, which can be used in
 224           runtime to control kernel stack erasing for kernels built with
 225           CONFIG_GCC_PLUGIN_STACKLEAK.
 226
 227 config INIT_ON_ALLOC_DEFAULT_ON
 228         bool "Enable heap memory zeroing on allocation by default"
 229         depends on !KMSAN
 230         help
 231           This has the effect of setting "init_on_alloc=1" on the kernel
 232           command line. This can be disabled with "init_on_alloc=0".
 233           When "init_on_alloc" is enabled, all page allocator and slab
 234           allocator memory will be zeroed when allocated, eliminating
 235           many kinds of "uninitialized heap memory" flaws, especially
 236           heap content exposures. The performance impact varies by
 237           workload, but most cases see <1% impact. Some synthetic
 238           workloads have measured as high as 7%.
 239
 240 config INIT_ON_FREE_DEFAULT_ON
 241         bool "Enable heap memory zeroing on free by default"
 242         depends on !KMSAN
 243         help
 244           This has the effect of setting "init_on_free=1" on the kernel
 245           command line. This can be disabled with "init_on_free=0".
 246           Similar to "init_on_alloc", when "init_on_free" is enabled,
 247           all page allocator and slab allocator memory will be zeroed
 248           when freed, eliminating many kinds of "uninitialized heap memory"
 249           flaws, especially heap content exposures. The primary difference
 250           with "init_on_free" is that data lifetime in memory is reduced,
 251           as anything freed is wiped immediately, making live forensics or
 252           cold boot memory attacks unable to recover freed memory contents.
 253           The performance impact varies by workload, but is more expensive
 254           than "init_on_alloc" due to the negative cache effects of
 255           touching "cold" memory areas. Most cases see 3-5% impact. Some
 256           synthetic workloads have measured as high as 8%.
 257
 258 config CC_HAS_ZERO_CALL_USED_REGS
 259         def_bool $(cc-option,-fzero-call-used-regs=used-gpr)
 260         # https://github.com/ClangBuiltLinux/linux/issues/1766
 261         # https://github.com/llvm/llvm-project/issues/59242
 262         depends on !CC_IS_CLANG || CLANG_VERSION > 150006
 263
 264 config ZERO_CALL_USED_REGS
 265         bool "Enable register zeroing on function exit"
 266         depends on CC_HAS_ZERO_CALL_USED_REGS
 267         help
 268           At the end of functions, always zero any caller-used register
 269           contents. This helps ensure that temporary values are not
 270           leaked beyond the function boundary. This means that register
 271           contents are less likely to be available for side channels
 272           and information exposures. Additionally, this helps reduce the
 273           number of useful ROP gadgets by about 20% (and removes compiler
 274           generated "write-what-where" gadgets) in the resulting kernel
 275           image. This has a less than 1% performance impact on most
 276           workloads. Image size growth depends on architecture, and should
 277           be evaluated for suitability. For example, x86_64 grows by less
 278           than 1%, and arm64 grows by about 5%.
 279
 280 endmenu
 281
 282 menu "Hardening of kernel data structures"
 283
 284 config LIST_HARDENED
 285         bool "Check integrity of linked list manipulation"
 286         help
 287           Minimal integrity checking in the linked-list manipulation routines
 288           to catch memory corruptions that are not guaranteed to result in an
 289           immediate access fault.
 290
 291           If unsure, say N.
 292
 293 config BUG_ON_DATA_CORRUPTION
 294         bool "Trigger a BUG when data corruption is detected"
 295         select LIST_HARDENED
 296         help
 297           Select this option if the kernel should BUG when it encounters
 298           data corruption in kernel memory structures when they get checked
 299           for validity.
 300
 301           If unsure, say N.
 302
 303 endmenu
 304
 305 config CC_HAS_RANDSTRUCT
 306         def_bool $(cc-option,-frandomize-layout-seed-file=/dev/null)
 307         # Randstruct was first added in Clang 15, but it isn't safe to use until
 308         # Clang 16 due to https://github.com/llvm/llvm-project/issues/60349
 309         depends on !CC_IS_CLANG || CLANG_VERSION >= 160000
 310
 311 choice
 312         prompt "Randomize layout of sensitive kernel structures"
 313         default RANDSTRUCT_FULL if COMPILE_TEST && (GCC_PLUGINS || CC_HAS_RANDSTRUCT)
 314         default RANDSTRUCT_NONE
 315         help
 316           If you enable this, the layouts of structures that are entirely
 317           function pointers (and have not been manually annotated with
 318           __no_randomize_layout), or structures that have been explicitly
 319           marked with __randomize_layout, will be randomized at compile-time.
 320           This can introduce the requirement of an additional information
 321           exposure vulnerability for exploits targeting these structure
 322           types.
 323
 324           Enabling this feature will introduce some performance impact,
 325           slightly increase memory usage, and prevent the use of forensic
 326           tools like Volatility against the system (unless the kernel
 327           source tree isn't cleaned after kernel installation).
 328
 329           The seed used for compilation is in scripts/basic/randomize.seed.
 330           It remains after a "make clean" to allow for external modules to
 331           be compiled with the existing seed and will be removed by a
 332           "make mrproper" or "make distclean". This file should not be made
 333           public, or the structure layout can be determined.
 334
 335         config RANDSTRUCT_NONE
 336                 bool "Disable structure layout randomization"
 337                 help
 338                   Build normally: no structure layout randomization.
 339
 340         config RANDSTRUCT_FULL
 341                 bool "Fully randomize structure layout"
 342                 depends on CC_HAS_RANDSTRUCT || GCC_PLUGINS
 343                 select MODVERSIONS if MODULES
 344                 help
 345                   Fully randomize the member layout of sensitive
 346                   structures as much as possible, which may have both a
 347                   memory size and performance impact.
 348
 349                   One difference between the Clang and GCC plugin
 350                   implementations is the handling of bitfields. The GCC
 351                   plugin treats them as fully separate variables,
 352                   introducing sometimes significant padding. Clang tries
 353                   to keep adjacent bitfields together, but with their bit
 354                   ordering randomized.
 355
 356         config RANDSTRUCT_PERFORMANCE
 357                 bool "Limit randomization of structure layout to cache-lines"
 358                 depends on GCC_PLUGINS
 359                 select MODVERSIONS if MODULES
 360                 help
 361                   Randomization of sensitive kernel structures will make a
 362                   best effort at restricting randomization to cacheline-sized
 363                   groups of members. It will further not randomize bitfields
 364                   in structures. This reduces the performance hit of RANDSTRUCT
 365                   at the cost of weakened randomization.
 366 endchoice
 367
 368 config RANDSTRUCT
 369         def_bool !RANDSTRUCT_NONE
 370
 371 config GCC_PLUGIN_RANDSTRUCT
 372         def_bool GCC_PLUGINS && RANDSTRUCT
 373         help
 374           Use GCC plugin to randomize structure layout.
 375
 376           This plugin was ported from grsecurity/PaX. More
 377           information at:
 378            * https://grsecurity.net/
 379            * https://pax.grsecurity.net/
 380
 381 endmenu