[PATCH] [RFC] security: allow using Clang's zero initialization for stack variables

[glider@google.com]

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.
Performance-wise, the difference between pattern and zero initialization
is usually negligible, although the generated code for zero
initialization is more compact.

The proposed config, CONFIG_USE_CLANG_ZERO_INITIALIZATION, makes
CONFIG_INIT_STACK_ALL use zero initialization if the corresponding flags
are supported by Clang.

Cc: Kees Cook <keescook@chromium.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Alexander Potapenko <glider@google.com>
---
 Makefile                   | 15 ++++++++++++++-
 security/Kconfig.hardening | 16 ++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index fd31992bf918..2860bad7e39a 100644
--- a/Makefile
+++ b/Makefile
@@ -802,9 +802,22 @@ KBUILD_CFLAGS	+= -fomit-frame-pointer
 endif
 endif
 
-# Initialize all stack variables with a pattern, if desired.
+# Initialize all stack variables, if desired.
 ifdef CONFIG_INIT_STACK_ALL
+
+# Use pattern initialization by default.
+ifndef CONFIG_USE_CLANG_ZERO_INITIALIZATION
 KBUILD_CFLAGS	+= -ftrivial-auto-var-init=pattern
+else
+
+ifdef CONFIG_CC_HAS_AUTO_VAR_ZERO_INIT
+# 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 -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
+endif
+
+endif
 endif
 
 DEBUG_CFLAGS	:= $(call cc-option, -fno-var-tracking-assignments)
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index af4c979b38ee..299d27c6d78c 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -22,6 +22,9 @@ menu "Memory initialization"
 config CC_HAS_AUTO_VAR_INIT
 	def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
 
+config CC_HAS_AUTO_VAR_ZERO_INIT
+	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
@@ -100,6 +103,19 @@ choice
 
 endchoice
 
+config USE_CLANG_ZERO_INITIALIZATION
+	bool "Use Clang's zero initialization for local variables"
+	depends on CC_HAS_AUTO_VAR_ZERO_INIT
+	depends on INIT_STACK_ALL
+	help
+	  If set, uses zeros instead of 0xAA to initialize local variables in
+	  INIT_STACK_ALL. Zeroing the stack provides safer defaults for strings,
+	  pointers, indexes, and sizes. The downsides are less-safe defaults for
+	  return values, and exposing fewer bugs where the underlying code
+	  relies on zero initialization.
+	  The corresponding flag isn't officially supported by Clang and may
+	  sooner or later go away or get renamed.
+
 config GCC_PLUGIN_STRUCTLEAK_VERBOSE
 	bool "Report forcefully initialized variables"
 	depends on GCC_PLUGIN_STRUCTLEAK
-- 
2.27.0.290.gba653c62da-goog