Merge tag 'var-init-v5.9-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull automatic variable initialization updates from Kees Cook:
 "This adds the "zero" init option from Clang, which is being used
  widely in production builds of Android and Chrome OS (though it also
  keeps the "pattern" init, which is better for debug builds).

   - Introduce CONFIG_INIT_STACK_ALL_ZERO (Alexander Potapenko)"

* tag 'var-init-v5.9-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  security: allow using Clang's zero initialization for stack variables

3 files changed, 43 insertions, 11 deletions

diff --git a/Makefile b/Makefile
index aef9ca61caf0..09c1bd1fcb64 100644
--- a/Makefile
+++ b/Makefile
@@ -803,11 +803,20 @@ 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