Merge tag 'hardening-v5.15-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull hardening updates from Kees Cook:

 - Expand lib/test_stackinit to include more initialization styles

 - Improve Kconfig for CLang's auto-var-init feature

 - Introduce support for GCC's zero-call-used-regs feature

* tag 'hardening-v5.15-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  lib/test_stackinit: Add assigned initializers
  lib/test_stackinit: Allow building stand-alone
  lib/test_stackinit: Fix static initializer test
  hardening: Clarify Kconfig text for auto-var-init
  hardening: Introduce CONFIG_ZERO_CALL_USED_REGS

Makefile

@@ -841,6 +841,11 @@ endif
 # for the randomize_kstack_offset feature. Disable it for all compilers.
 KBUILD_CFLAGS	+= $(call cc-option, -fno-stack-clash-protection)
 
+# Clear used registers at func exit (to reduce data lifetime and ROP gadgets).
+ifdef CONFIG_ZERO_CALL_USED_REGS
+KBUILD_CFLAGS	+= -fzero-call-used-regs=used-gpr
+endif
+
 DEBUG_CFLAGS	:=
 
 # Workaround for GCC versions < 5.0

security/Kconfig.hardening

@@ -29,6 +29,7 @@ choice
 	prompt "Initialize kernel stack variables at function entry"
 	default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
 	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_PATTERN
 	default INIT_STACK_NONE
 	help
 	  This option enables initialization of stack variables at
@@ -39,11 +40,11 @@ choice
 	  syscalls.
 
 	  This chooses the level of coverage over classes of potentially
-	  uninitialized variables. The selected class will be
+	  uninitialized variables. The selected class of variable will be
 	  initialized before use in a function.
 
 	config INIT_STACK_NONE
-		bool "no automatic initialization (weakest)"
+		bool "no automatic stack variable initialization (weakest)"
 		help
 		  Disable automatic stack variable initialization.
 		  This leaves the kernel vulnerable to the standard
@@ -80,7 +81,7 @@ choice
 		  and is disallowed.
 
 	config GCC_PLUGIN_STRUCTLEAK_BYREF_ALL
-		bool "zero-init anything passed by reference (very strong)"
+		bool "zero-init everything passed by reference (very strong)"
 		depends on GCC_PLUGINS
 		depends on !(KASAN && KASAN_STACK)
 		select GCC_PLUGIN_STRUCTLEAK
@@ -91,33 +92,44 @@ choice
 		  of uninitialized stack variable exploits and information
 		  exposures.
 
+		  As a side-effect, this keeps a lot of variables on the
+		  stack that can otherwise be optimized out, so combining
+		  this with CONFIG_KASAN_STACK can lead to a stack overflow
+		  and is disallowed.
+
 	config INIT_STACK_ALL_PATTERN
-		bool "0xAA-init everything on the stack (strongest)"
+		bool "pattern-init everything (strongest)"
 		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
-		  of uninitialized stack variable exploits and information
-		  exposures, even variables that were warned to have been
-		  left uninitialized.
+		  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.
+		  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 on the stack (strongest and safest)"
+		bool "zero-init everything (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.
+		  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
 
@@ -217,6 +229,25 @@ config INIT_ON_FREE_DEFAULT_ON
 	  touching "cold" memory areas. Most cases see 3-5% impact. Some
 	  synthetic workloads have measured as high as 8%.
 
+config CC_HAS_ZERO_CALL_USED_REGS
+	def_bool $(cc-option,-fzero-call-used-regs=used-gpr)
+
+config ZERO_CALL_USED_REGS
+	bool "Enable register zeroing on function exit"
+	depends on CC_HAS_ZERO_CALL_USED_REGS
+	help
+	  At the end of functions, always zero any caller-used register
+	  contents. This helps ensure that temporary values are not
+	  leaked beyond the function boundary. This means that register
+	  contents are less likely to be available for side channels
+	  and information exposures. Additionally, this helps reduce the
+	  number of useful ROP gadgets by about 20% (and removes compiler
+	  generated "write-what-where" gadgets) in the resulting kernel
+	  image. This has a less than 1% performance impact on most
+	  workloads. Image size growth depends on architecture, and should
+	  be evaluated for suitability. For example, x86_64 grows by less
+	  than 1%, and arm64 grows by about 5%.
+
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