Commit f0fe00d4 authored by glider@google.com's avatar glider@google.com Committed by Kees Cook
Browse files

security: allow using Clang's zero initialization for stack variables 

In addition to -ftrivial-auto-var-init=pattern (used by
CONFIG_INIT_STACK_ALL now) Clang also supports zero initialization for
locals enabled by -ftrivial-auto-var-init=zero. The future of this flag
is still being debated (see https://bugs.llvm.org/show_bug.cgi?id=45497

).
Right now it is guarded by another flag,
-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang,
which means it may not be supported by future Clang releases. Another
possible resolution is that -ftrivial-auto-var-init=zero will persist
(as certain users have already started depending on it), but the name
of the guard flag will change.

In the meantime, zero initialization has proven itself as a good
production mitigation measure against uninitialized locals. Unlike pattern
initialization, which has a higher chance of triggering existing bugs,
zero initialization provides safe defaults for strings, pointers, indexes,
and sizes. On the other hand, pattern initialization remains safer for
return values. Chrome OS and Android are moving to using zero
initialization for production builds.

Performance-wise, the difference between pattern and zero initialization
is usually negligible, although the generated code for zero
initialization is more compact.

This patch renames CONFIG_INIT_STACK_ALL to CONFIG_INIT_STACK_ALL_PATTERN
and introduces another config option, CONFIG_INIT_STACK_ALL_ZERO, that
enables zero initialization for locals if the corresponding flags are
supported by Clang.

Cc: Kees Cook <keescook@chromium.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: default avatarAlexander Potapenko <glider@google.com>
Link: https://lore.kernel.org/r/20200616083435.223038-1-glider@google.com


Reviewed-by: default avatarMaciej Żenczykowski <maze@google.com>
Signed-off-by: default avatarKees Cook <keescook@chromium.org>
parent b3a9e3b9

Makefile

+11 −2
Original line number Diff line number Diff line
@@ -802,11 +802,20 @@ KBUILD_CFLAGS += -fomit-frame-pointer 
endif

endif



# Initialize all stack variables with a pattern, if desired.

ifdef CONFIG_INIT_STACK_ALL

# Initialize all stack variables with a 0xAA pattern.

ifdef CONFIG_INIT_STACK_ALL_PATTERN

KBUILD_CFLAGS	+= -ftrivial-auto-var-init=pattern

endif



# Initialize all stack variables with a zero value.

ifdef CONFIG_INIT_STACK_ALL_ZERO

# Future support for zero initialization is still being debated, see

# https://bugs.llvm.org/show_bug.cgi?id=45497. These flags are subject to being

# renamed or dropped.

KBUILD_CFLAGS	+= -ftrivial-auto-var-init=zero

KBUILD_CFLAGS	+= -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang

endif



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



ifdef CONFIG_DEBUG_INFO

init/main.c

+7 −5
Original line number Diff line number Diff line
@@ -779,14 +779,16 @@ static void __init report_meminit(void) 
{

	const char *stack;



	if (IS_ENABLED(CONFIG_INIT_STACK_ALL))

		stack = "all";

	if (IS_ENABLED(CONFIG_INIT_STACK_ALL_PATTERN))

		stack = "all(pattern)";

	else if (IS_ENABLED(CONFIG_INIT_STACK_ALL_ZERO))

		stack = "all(zero)";

	else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL))

		stack = "byref_all";

		stack = "byref_all(zero)";

	else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF))

		stack = "byref";

		stack = "byref(zero)";

	else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_USER))

		stack = "__user";

		stack = "__user(zero)";

	else

		stack = "off";

security/Kconfig.hardening

+25 −4
Original line number Diff line number Diff line
@@ -19,13 +19,16 @@ config GCC_PLUGIN_STRUCTLEAK 


menu "Memory initialization"



config CC_HAS_AUTO_VAR_INIT

config CC_HAS_AUTO_VAR_INIT_PATTERN

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



config CC_HAS_AUTO_VAR_INIT_ZERO

	def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)



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_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN

	default INIT_STACK_NONE

	help

	  This option enables initialization of stack variables at
@@ -88,9 +91,9 @@ choice 
		  of uninitialized stack variable exploits and information

		  exposures.



	config INIT_STACK_ALL

	config INIT_STACK_ALL_PATTERN

		bool "0xAA-init everything on the stack (strongest)"

		depends on CC_HAS_AUTO_VAR_INIT

		depends on CC_HAS_AUTO_VAR_INIT_PATTERN

		help

		  Initializes everything on the stack with a 0xAA

		  pattern. This is intended to eliminate all classes
@@ -98,6 +101,24 @@ choice 
		  exposures, even variables that were warned to have 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.



	config INIT_STACK_ALL_ZERO

		bool "zero-init everything on the stack (strongest and safest)"

		depends on CC_HAS_AUTO_VAR_INIT_ZERO

		help

		  Initializes everything on the stack with a zero

		  value. 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.



		  Zero initialization provides safe defaults for strings,

		  pointers, indices and sizes, and is therefore

		  more suitable as a security mitigation measure.



endchoice



config GCC_PLUGIN_STRUCTLEAK_VERBOSE

CRA Git | Maintained and supported by SUSTech CRA and CCSE