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authorglider@google.com <glider@google.com>2020-06-16 10:34:35 +0200
committerKees Cook <keescook@chromium.org>2020-06-16 02:06:23 -0700
commitf0fe00d4972a8cd4b98cc2c29758615e4d51cdfe (patch)
tree1dd546fb0c485cc457e571626e64a38a2372be8f
parentb3a9e3b9622ae10064826dccb4f7a52bd88c7407 (diff)
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: Alexander Potapenko <glider@google.com> Link: https://lore.kernel.org/r/20200616083435.223038-1-glider@google.com Reviewed-by: Maciej ┼╗enczykowski <maze@google.com> Signed-off-by: Kees Cook <keescook@chromium.org>
-rw-r--r--Makefile13
-rw-r--r--init/main.c12
-rw-r--r--security/Kconfig.hardening29
3 files changed, 43 insertions, 11 deletions
diff --git a/Makefile b/Makefile
index ae5d8220f431..d1cfbe5abc49 100644
--- a/Makefile
+++ b/Makefile
@@ -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
diff --git a/init/main.c b/init/main.c
index 0ead83e86b5a..9127b240fd26 100644
--- a/init/main.c
+++ b/init/main.c
@@ -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";
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index af4c979b38ee..269967c4fc1b 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -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