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 Żenczykowski <maze@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

Makefile

@@ -802,11 +802,20 @@ KBUILD_CFLAGS	+= -fomit-frame-pointer
 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
 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

@@ -779,14 +779,16 @@ static void __init report_meminit(void)
 {
 	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))
-		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

@@ -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