[PATCH v16 02/26] x86/cet/shstk: Add Kconfig option for user-mode control-flow protection

[Yu-cheng Yu]

Shadow Stack provides protection against function return address
corruption.  It is active when the processor supports it, the kernel has
CONFIG_X86_CET_USER, 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@xxxxxxxxx>
---
 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 fbf26e0f7a6a..78b4b5bb1272 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1931,6 +1931,28 @@ config X86_INTEL_TSX_MODE_AUTO
 	  side channel attacks- equals the tsx=auto command line parameter.
 endchoice
 
+config ARCH_HAS_SHADOW_STACK
+	def_bool n
+
+config X86_CET_USER
+	prompt "Intel Control-flow protection for user-mode"
+	def_bool n
+	depends on CPU_SUP_INTEL && X86_64
+	depends on AS_WRUSS
+	select ARCH_USES_HIGH_VMA_FLAGS
+	select ARCH_HAS_SHADOW_STACK
+	help
+	  Control-flow protection is a hardware security hardening feature
+	  that detects function-return address or jump target changes by
+	  malicious code.  Applications must be enabled to use it, and old
+	  userspace does not get protection "for free".
+	  Support for this feature is present on 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