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>

Makefile

@@ -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

@@ -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