[PATCH 2/9] x86/cet: Add Kconfig option for user-mode shadow stack

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

Introduce Kconfig option X86_INTEL_SHADOW_STACK_USER.

An application has shadow stack protection when all the following are
true:

  (1) The kernel has X86_INTEL_SHADOW_STACK_USER enabled,
  (2) The running processor supports the shadow stack,
  (3) The application is built with shadow stack enabled tools & libs
      and, and at runtime, all dependent shared libs can support shadow
      stack.

If this kernel config option is enabled, but (2) or (3) above is not
true, the application runs without the shadow stack protection.
Existing legacy applications will continue to work without the shadow
stack protection.

The user-mode shadow stack protection is only implemented for the
64-bit kernel.  Thirty-two bit applications are supported under the
compatibility mode.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
 arch/x86/Kconfig | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c07f492b871a..dd580d4910fc 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1925,6 +1925,30 @@ config X86_INTEL_MEMORY_PROTECTION_KEYS
 
 	  If unsure, say y.
 
+config X86_INTEL_CET
+	def_bool n
+
+config ARCH_HAS_SHSTK
+	def_bool n
+
+config X86_INTEL_SHADOW_STACK_USER
+	prompt "Intel Shadow Stack for user-mode"
+	def_bool n
+	depends on CPU_SUP_INTEL && X86_64
+	select X86_INTEL_CET
+	select ARCH_HAS_SHSTK
+	---help---
+	  Shadow stack provides hardware protection against program stack
+	  corruption.  Only when all the following are true will an application
+	  have the shadow stack protection: the kernel supports it (i.e. this
+	  feature is enabled), the application is compiled and linked with
+	  shadow stack enabled, and the processor supports this feature.
+	  When the kernel has this configuration enabled, existing non shadow
+	  stack applications will continue to work, but without shadow stack
+	  protection.
+
+	  If unsure, say y.
+
 config EFI
 	bool "EFI runtime service support"
 	depends on ACPI
-- 
2.15.1