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

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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: Alexander Potapenko <glider@google.com>
Link: https://lore.kernel.org/r/20200616083435.223038-1-glider@google.com
Reviewed-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
This commit is contained in:
glider@google.com 2020-06-16 10:34:35 +02:00 committed by Kees Cook
parent b3a9e3b962
commit f0fe00d497
3 changed files with 43 additions and 11 deletions
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13
Makefile
View File

@ -802,11 +802,20 @@ KBUILD_CFLAGS += -fomit-frame-pointer
endif endif
endif endif
# Initialize all stack variables with a pattern, if desired. # Initialize all stack variables with a 0xAA pattern.
ifdef CONFIG_INIT_STACK_ALL ifdef CONFIG_INIT_STACK_ALL_PATTERN
KBUILD_CFLAGS += -ftrivial-auto-var-init=pattern KBUILD_CFLAGS += -ftrivial-auto-var-init=pattern
endif 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) DEBUG_CFLAGS := $(call cc-option, -fno-var-tracking-assignments)
ifdef CONFIG_DEBUG_INFO ifdef CONFIG_DEBUG_INFO

12
init/main.c
View File

@ -779,14 +779,16 @@ static void __init report_meminit(void)
{ {
const char *stack; const char *stack;
if (IS_ENABLED(CONFIG_INIT_STACK_ALL)) if (IS_ENABLED(CONFIG_INIT_STACK_ALL_PATTERN))
stack = "all"; 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)) 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)) else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF))
stack = "byref"; stack = "byref(zero)";
else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_USER)) else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_USER))
stack = "__user"; stack = "__user(zero)";
else else
stack = "off"; stack = "off";

29
security/Kconfig.hardening
View File

@ -19,13 +19,16 @@ config GCC_PLUGIN_STRUCTLEAK
menu "Memory initialization" 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) 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 choice
prompt "Initialize kernel stack variables at function entry" prompt "Initialize kernel stack variables at function entry"
default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS 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 default INIT_STACK_NONE
help help
This option enables initialization of stack variables at This option enables initialization of stack variables at
@ -88,9 +91,9 @@ choice
of uninitialized stack variable exploits and information of uninitialized stack variable exploits and information
exposures. exposures.
config INIT_STACK_ALL config INIT_STACK_ALL_PATTERN
bool "0xAA-init everything on the stack (strongest)" bool "0xAA-init everything on the stack (strongest)"
depends on CC_HAS_AUTO_VAR_INIT depends on CC_HAS_AUTO_VAR_INIT_PATTERN
help help
Initializes everything on the stack with a 0xAA Initializes everything on the stack with a 0xAA
pattern. This is intended to eliminate all classes pattern. This is intended to eliminate all classes
@ -98,6 +101,24 @@ choice
exposures, even variables that were warned to have been exposures, even variables that were warned to have been
left uninitialized. 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 endchoice
config GCC_PLUGIN_STRUCTLEAK_VERBOSE config GCC_PLUGIN_STRUCTLEAK_VERBOSE