[PATCH v2 3/5] hardening: support initializing stack variables by default

[Ahmad Fatoum]

Uninitialized stack variables can be abused for information exposures
and may even be leveraged for exploits.

Add options to initialize the stack by default.

Signed-off-by: Ahmad Fatoum <a.fatoum at pengutronix.de>
---
 Makefile              | 14 ++++++++
 lib/Kconfig.hardening | 76 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 90 insertions(+)

diff --git a/Makefile b/Makefile
index cc8655f68a53..bbb5d9409ea8 100644
--- a/Makefile
+++ b/Makefile
@@ -691,6 +691,20 @@ endif
 
 KBUILD_CFLAGS-$(CONFIG_CC_IS_CLANG) += -Wno-gnu
 
+# Initialize all stack variables with a 0xAA pattern.
+KBUILD_CFLAGS-$(CONFIG_INIT_STACK_ALL_PATTERN)	+= -ftrivial-auto-var-init=pattern
+
+# Initialize all stack variables with a zero value.
+ifdef CONFIG_INIT_STACK_ALL_ZERO
+KBUILD_CFLAGS	+= -ftrivial-auto-var-init=zero
+ifdef CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO_ENABLER
+# https://github.com/llvm/llvm-project/issues/44842
+CC_AUTO_VAR_INIT_ZERO_ENABLER := -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
+export CC_AUTO_VAR_INIT_ZERO_ENABLER
+KBUILD_CFLAGS	+= $(CC_AUTO_VAR_INIT_ZERO_ENABLER)
+endif
+endif
+
 KBUILD_CFLAGS-$(CONFIG_WERROR) += -Werror
 
 # This warning generated too much noise in a regular build.
diff --git a/lib/Kconfig.hardening b/lib/Kconfig.hardening
index 7a177840a5a1..84aaab1693e0 100644
--- a/lib/Kconfig.hardening
+++ b/lib/Kconfig.hardening
@@ -12,6 +12,82 @@ config BUG_ON_DATA_CORRUPTION
 
 menu "Memory initialization"
 
+config CC_HAS_AUTO_VAR_INIT_PATTERN
+	def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
+
+config CC_HAS_AUTO_VAR_INIT_ZERO_BARE
+	def_bool $(cc-option,-ftrivial-auto-var-init=zero)
+
+config CC_HAS_AUTO_VAR_INIT_ZERO_ENABLER
+	# Clang 16 and later warn about using the -enable flag, but it
+	# is required before then.
+	def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
+	depends on !CC_HAS_AUTO_VAR_INIT_ZERO_BARE
+
+config CC_HAS_AUTO_VAR_INIT_ZERO
+	def_bool CC_HAS_AUTO_VAR_INIT_ZERO_BARE || CC_HAS_AUTO_VAR_INIT_ZERO_ENABLER
+
+choice
+	prompt "Initialize barebox stack variables at function entry"
+	default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
+	default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
+	default INIT_STACK_NONE
+	help
+	  This option enables initialization of stack variables at
+	  function entry time. This has the possibility to have the
+	  greatest coverage (since all functions can have their
+	  variables initialized), but the performance impact depends
+	  on the function calling complexity of a given workload's
+	  syscalls.
+
+	  This chooses the level of coverage over classes of potentially
+	  uninitialized variables. The selected class of variable will be
+	  initialized before use in a function.
+
+	config INIT_STACK_NONE
+		bool "no automatic stack variable initialization (weakest)"
+		help
+		  Disable automatic stack variable initialization.
+		  This leaves barebox vulnerable to the standard
+		  classes of uninitialized stack variable exploits
+		  and information exposures.
+
+	config INIT_STACK_ALL_PATTERN
+		bool "pattern-init everything (strongest)"
+		depends on CC_HAS_AUTO_VAR_INIT_PATTERN
+		help
+		  Initializes everything on the stack (including padding)
+		  with a specific debug value. This is intended to eliminate
+		  all classes of uninitialized stack variable exploits and
+		  information exposures, even variables that were warned about
+		  having 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. The
+		  pattern is situation-specific; Clang on 64-bit uses 0xAA
+		  repeating for all types and padding except float and double
+		  which use 0xFF repeating (-NaN). Clang on 32-bit uses 0xFF
+		  repeating for all types and padding.
+
+	config INIT_STACK_ALL_ZERO
+		bool "zero-init everything (strongest and safest)"
+		depends on CC_HAS_AUTO_VAR_INIT_ZERO
+		help
+		  Initializes everything on the stack (including padding)
+		  with a zero value. This is intended to eliminate all
+		  classes of uninitialized stack variable exploits and
+		  information exposures, even variables that were warned
+		  about having been left uninitialized.
+
+		  Zero initialization provides safe defaults for strings
+		  (immediately NUL-terminated), pointers (NULL), indices
+		  (index 0), and sizes (0 length), so it is therefore more
+		  suitable as a production security mitigation than pattern
+		  initialization.
+
+endchoice
+
 config INIT_ON_ALLOC_DEFAULT_ON
 	bool "Enable heap memory zeroing on allocation by default"
 	depends on !MALLOC_LIBC
-- 
2.39.5