Commit 709a972e authored Apr 10, 2019 by Kees Cook

security: Implement Clang's stack initialization

CONFIG_INIT_STACK_ALL turns on stack initialization based on
-ftrivial-auto-var-init in Clang builds, which has greater coverage
than CONFIG_GCC_PLUGINS_STRUCTLEAK_BYREF_ALL.

-ftrivial-auto-var-init Clang option provides trivial initializers for
uninitialized local variables, variable fields and padding.

It has three possible values:
  pattern - uninitialized locals are filled with a fixed pattern
    (mostly 0xAA on 64-bit platforms, see https://reviews.llvm.org/D54604


    for more details, but 0x000000AA for 32-bit pointers) likely to cause
    crashes when uninitialized value is used;
  zero (it's still debated whether this flag makes it to the official
    Clang release) - uninitialized locals are filled with zeroes;
  uninitialized (default) - uninitialized locals are left intact.

This patch uses only the "pattern" mode when CONFIG_INIT_STACK_ALL is
enabled.

Developers have the possibility to opt-out of this feature on a
per-variable basis by using __attribute__((uninitialized)), but such
use should be well justified in comments.

Co-developed-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Alexander Potapenko <glider@google.com>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>

parent b6a6a377

Makefile

+5 −0

Original line number	Diff line number	Diff line
		@@ -745,6 +745,11 @@ KBUILD_CFLAGS += -fomit-frame-pointer
		endif
		endif

		# Initialize all stack variables with a pattern, if desired.
		ifdef CONFIG_INIT_STACK_ALL
		KBUILD_CFLAGS += -ftrivial-auto-var-init=pattern
		endif

		DEBUG_CFLAGS := $(call cc-option, -fno-var-tracking-assignments)

		ifdef CONFIG_DEBUG_INFO

security/Kconfig.hardening

+14 −0

Original line number	Diff line number	Diff line
		@@ -18,9 +18,13 @@ config GCC_PLUGIN_STRUCTLEAK

		menu "Memory initialization"

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

		choice
		prompt "Initialize kernel stack variables at function entry"
		default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
		default INIT_STACK_ALL if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT
		default INIT_STACK_NONE
		help
		This option enables initialization of stack variables at
		@@ -76,6 +80,16 @@ choice
		of uninitialized stack variable exploits and information
		exposures.

		config INIT_STACK_ALL
		bool "0xAA-init everything on the stack (strongest)"
		depends on CC_HAS_AUTO_VAR_INIT
		help
		Initializes everything on the stack with a 0xAA
		pattern. This is intended to eliminate all classes
		of uninitialized stack variable exploits and information
		exposures, even variables that were warned to have been
		left uninitialized.

		endchoice

		config GCC_PLUGIN_STRUCTLEAK_VERBOSE

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