[RFC PATCH v9 1/7] x86/cet/ibt: Add Kconfig option for user-mode Indirect Branch Tracking

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

Introduce Kconfig option X86_INTEL_BRANCH_TRACKING_USER.

Indirect Branch Tracking (IBT) provides protection against CALL-/JMP-
oriented programming attacks.  It is active when the kernel has this
feature enabled, and the processor and the application support it.
When this feature is enabled, legacy non-IBT applications continue to
work, but without IBT protection.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
 arch/x86/Kconfig  | 15 +++++++++++++++
 arch/x86/Makefile |  7 +++++++
 2 files changed, 22 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index d1447380e02e..563f3c81f323 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1998,6 +1998,21 @@ config X86_INTEL_SHADOW_STACK_USER
 
 	  If unsure, say y.
 
+config X86_INTEL_BRANCH_TRACKING_USER
+	prompt "Intel Indirect Branch Tracking for user-mode"
+	def_bool n
+	depends on CPU_SUP_INTEL && X86_64
+	select X86_INTEL_CET
+	---help---
+	  Indirect Branch Tracking (IBT) provides protection against
+	  CALL-/JMP-oriented programming attacks.  It is active when
+	  the kernel has this feature enabled, and the processor and
+	  the application support it.  When this feature is enabled,
+	  legacy non-IBT applications continue to work, but without
+	  IBT protection.
+
+	  If unsure, say y
+
 config EFI
 	bool "EFI runtime service support"
 	depends on ACPI
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index c34f5befa4c8..f97b2c1c4d04 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -156,6 +156,13 @@ ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
   endif
 endif
 
+# Check compiler ibt support
+ifdef CONFIG_X86_INTEL_BRANCH_TRACKING_USER
+  ifeq ($(call cc-option-yn, -fcf-protection=branch), n)
+      $(error CONFIG_X86_INTEL_BRANCH_TRACKING_USER not supported by compiler)
+  endif
+endif
+
 #
 # If the function graph tracer is used with mcount instead of fentry,
 # '-maccumulate-outgoing-args' is needed to prevent a GCC bug
-- 
2.21.0