[PATCH v13 05/26] x86/cet/shstk: Add Kconfig option for user-mode Shadow Stack

[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_SHADOW_STACK_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@intel.com>
---
v13:
- Update help text, and change default to N.
- Change X86_INTEL_* to X86_*.

v10:
- Change SHSTK to shadow stack in the help text.
- Change build-time check to config-time check.
- Change ARCH_HAS_SHSTK to ARCH_HAS_SHADOW_STACK.

 arch/x86/Kconfig                      | 33 +++++++++++++++++++++++++++
 scripts/as-x86_64-has-shadow-stack.sh |  4 ++++
 2 files changed, 37 insertions(+)
 create mode 100755 scripts/as-x86_64-has-shadow-stack.sh

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 7101ac64bb20..415fcc869afc 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1927,6 +1927,39 @@ config X86_INTEL_TSX_MODE_AUTO
 	  side channel attacks- equals the tsx=auto command line parameter.
 endchoice
 
+config AS_HAS_SHADOW_STACK
+	def_bool $(success,$(srctree)/scripts/as-x86_64-has-shadow-stack.sh $(CC))
+	help
+	  Test the assembler for shadow stack instructions.
+
+config X86_CET
+	def_bool n
+
+config ARCH_HAS_SHADOW_STACK
+	def_bool n
+
+config X86_SHADOW_STACK_USER
+	prompt "Intel Shadow Stacks for user-mode"
+	def_bool n
+	depends on CPU_SUP_INTEL && X86_64
+	depends on AS_HAS_SHADOW_STACK
+	select ARCH_USES_HIGH_VMA_FLAGS
+	select X86_CET
+	select ARCH_HAS_SHADOW_STACK
+	help
+	  Shadow Stacks provides protection against program stack
+	  corruption.  It's a hardware feature.  This only matters
+	  if you have the right hardware.  It's a security hardening
+	  feature and apps must be enabled to use it.  You get no
+	  protection "for free" on old userspace.  The hardware can
+	  support user and kernel, but this option is for user space
+	  only.
+	  Support for this feature is only known to be present on
+	  processors released in 2020 or later.  CET features are also
+	  known to increase kernel text size by 3.7 KB.
+
+	  If unsure, say N.
+
 config EFI
 	bool "EFI runtime service support"
 	depends on ACPI
diff --git a/scripts/as-x86_64-has-shadow-stack.sh b/scripts/as-x86_64-has-shadow-stack.sh
new file mode 100755
index 000000000000..fac1d363a1b8
--- /dev/null
+++ b/scripts/as-x86_64-has-shadow-stack.sh
@@ -0,0 +1,4 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+
+echo "wrussq %rax, (%rbx)" | $* -x assembler -c -
-- 
2.21.0