[PATCH v21 02/26] x86/cet/shstk: Add Kconfig option for user-mode control-flow protection

[Yu-cheng Yu <yu-cheng.yu@intel.com>]

Shadow Stack provides protection against function return address
corruption.  It is active when the processor supports it, the kernel has
CONFIG_X86_CET enabled, and the application is built for the feature.
This is only implemented for the 64-bit kernel.  When it is enabled, legacy
non-Shadow Stack applications continue to work, but without protection.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
 arch/x86/Kconfig           | 22 ++++++++++++++++++++++
 arch/x86/Kconfig.assembler |  5 +++++
 2 files changed, 27 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 21f851179ff0..c80cead80a49 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -28,6 +28,7 @@ config X86_64
 	select ARCH_HAS_GIGANTIC_PAGE
 	select ARCH_SUPPORTS_INT128 if CC_HAS_INT128
 	select ARCH_USE_CMPXCHG_LOCKREF
+	select ARCH_HAS_SHADOW_STACK
 	select HAVE_ARCH_SOFT_DIRTY
 	select MODULES_USE_ELF_RELA
 	select NEED_DMA_MAP_STATE
@@ -1951,6 +1952,27 @@ config X86_SGX
 
 	  If unsure, say N.
 
+config ARCH_HAS_SHADOW_STACK
+	def_bool n
+
+config X86_CET
+	prompt "Intel Control-flow protection for user-mode"
+	def_bool n
+	depends on AS_WRUSS
+	depends on ARCH_HAS_SHADOW_STACK
+	select ARCH_USES_HIGH_VMA_FLAGS
+	help
+	  Control-flow protection is a set of hardware features which place
+	  additional restrictions on indirect branches.  These help
+	  mitigate ROP attacks.  Applications must be enabled to use it,
+	  and old userspace does not get protection "for free".
+	  Support for this feature is present on Tiger Lake family of
+	  processors released in 2020 or later.  Enabling this feature
+	  increases kernel text size by 3.7 KB.
+	  See Documentation/x86/intel_cet.rst for more information.
+
+	  If unsure, say N.
+
 config EFI
 	bool "EFI runtime service support"
 	depends on ACPI
diff --git a/arch/x86/Kconfig.assembler b/arch/x86/Kconfig.assembler
index 26b8c08e2fc4..00c79dd93651 100644
--- a/arch/x86/Kconfig.assembler
+++ b/arch/x86/Kconfig.assembler
@@ -19,3 +19,8 @@ config AS_TPAUSE
 	def_bool $(as-instr,tpause %ecx)
 	help
 	  Supported by binutils >= 2.31.1 and LLVM integrated assembler >= V7
+
+config AS_WRUSS
+	def_bool $(as-instr,wrussq %rax$(comma)(%rbx))
+	help
+	  Supported by binutils >= 2.31 and LLVM integrated assembler
-- 
2.21.0