[PATCH 05/29] x86: Base IBT bits

[Peter Zijlstra <peterz@infradead.org>]

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

TODO: clang

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/Kconfig           |   15 ++++++++++++
 arch/x86/Makefile          |    5 +++-
 arch/x86/include/asm/ibt.h |   53 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 72 insertions(+), 1 deletion(-)

--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1861,6 +1861,21 @@ config X86_UMIP
 	  specific cases in protected and virtual-8086 modes. Emulated
 	  results are dummy.
 
+config CC_HAS_IBT
+	# GCC >= 9 and binutils >= 2.29
+	# Retpoline check to work around https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93654
+	def_bool $(cc-option, -fcf-protection=branch -mindirect-branch-register) && $(as-instr,endbr64)
+
+config X86_IBT
+	prompt "Indirect Branch Tracking"
+	bool
+	depends on X86_64 && CC_HAS_IBT
+	help
+	  Build the kernel with support for Indirect Branch Tracking, a
+	  hardware supported CFI scheme. Any indirect call must land on
+	  an ENDBR instruction, as such, the compiler will litter the
+	  code with them to make this happen.
+
 config X86_INTEL_MEMORY_PROTECTION_KEYS
 	prompt "Memory Protection Keys"
 	def_bool y
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -62,8 +62,11 @@ 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 +1,53 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_IBT_H
+#define _ASM_X86_IBT_H
+
+#ifdef CONFIG_X86_IBT
+
+#ifndef __ASSEMBLY__
+
+#ifdef CONFIG_X86_64
+#define ASM_ENDBR	"endbr64\n\t"
+#else
+#define ASM_ENDBR	"endbr32\n\t"
+#endif
+
+#define __noendbr	__attribute__((nocf_check))
+
+/*
+ * A bit convoluted, but matches both endbr32 and endbr64 without
+ * having either as literal in the text.
+ */
+static inline bool is_endbr(const void *addr)
+{
+	unsigned int val = ~*(unsigned int *)addr;
+	val |= 0x01000000U;
+	return val == ~0xfa1e0ff3;
+}
+
+#else /* __ASSEMBLY__ */
+
+#ifdef CONFIG_X86_64
+#define ENDBR	endbr64
+#else
+#define ENDBR	endbr32
+#endif
+
+#endif /* __ASSEMBLY__ */
+
+#else /* !IBT */
+
+#ifndef __ASSEMBLY__
+
+#define ASM_ENDBR
+
+#define __noendbr
+
+#else /* __ASSEMBLY__ */
+
+#define ENDBR
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* CONFIG_X86_IBT */
+#endif /* _ASM_X86_IBT_H */