[RFC][PATCH 2/6] x86: Base IBT bits

[Peter Zijlstra <peterz@infradead.org>]

Add Kconfig, Makefile and basic instruction support for x86 IBT.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/Kconfig           |   10 ++++++++++
 arch/x86/Makefile          |    5 ++++-
 arch/x86/include/asm/ibt.h |   40 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 54 insertions(+), 1 deletion(-)

--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1848,6 +1848,16 @@ config X86_UMIP
 	  specific cases in protected and virtual-8086 modes. Emulated
 	  results are dummy.
 
+config CC_HAS_IBT
+	def_bool $(cc-option, -fcf-protection=branch)
+
+config X86_IBT
+	prompt "Indirect Branch Tracking"
+	bool
+	depends on X86_64 && CC_HAS_IBT
+	help
+	  Increase kernel text size for giggles
+
 config X86_INTEL_MEMORY_PROTECTION_KEYS
 	prompt "Memory Protection Keys"
 	def_bool y
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -50,8 +50,11 @@ export BITS
 #
 KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
 
-# Intel CET isn't enabled in the kernel
+ifeq ($(CONFIG_X86_IBT),y)
+KBUILD_CFLAGS += $(call cc-option,-fcf-protection=branch)
+else
 KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
+endif
 
 ifeq ($(CONFIG_X86_32),y)
         BITS := 32
--- /dev/null
+++ b/arch/x86/include/asm/ibt.h
@@ -0,0 +1,40 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_IBT_H
+#define _ASM_X86_IBT_H
+
+#ifdef CONFIG_X86_IBT
+
+#ifndef __ASSEMBLY__
+
+// XXX note about GAS version required
+
+#ifdef CONFIG_X86_64
+#define ASM_ENDBR	".byte 0xf3, 0x0f, 0x1e, 0xfa\n\t"
+#else
+#define ASM_ENDBR	".byte 0xf3, 0x0f, 0x1e, 0xfb\n\t"
+#endif
+
+#else /* __ASSEMBLY__ */
+
+#ifdef CONFIG_X86_64
+#define ENDBR	.byte 0xf3, 0x0f, 0x1e, 0xfa
+#else
+#define ENDBR	.byte 0xf3, 0x0f, 0x1e, 0xfb
+#endif
+
+#endif /* __ASSEMBLY__ */
+
+#else /* !IBT */
+
+#ifndef __ASSEMBLY__
+
+#define ASM_ENDBR
+
+#else /* __ASSEMBLY__ */
+
+#define ENDBR
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* CONFIG_X86_IBT */
+#endif /* _ASM_X86_IBT_H */