[RFC PATCH 11/12] riscv: Kconfig & Makefile for riscv kernel control flow integrity

[Deepak Gupta <debug@rivosinc.com>]

Defines `CONFIG_RISCV_KERNEL_CFI` and selects SHADOW_CALL_STACK
and DYNAMIC_SCS both so that zicfiss can be wired up.

Makefile checks if CONFIG_RISCV_KERNEL_CFI is enabled, then light
up zicfiss and zicfilp compiler flags.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
 arch/riscv/Kconfig  | 36 +++++++++++++++++++++++++++++++++++-
 arch/riscv/Makefile |  6 ++++++
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index be09c8836d56..5276598bb773 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -193,7 +193,7 @@ config GCC_SUPPORTS_DYNAMIC_FTRACE
 	depends on $(cc-option,-fpatchable-function-entry=8)
 
 config HAVE_SHADOW_CALL_STACK
-	def_bool $(cc-option,-fsanitize=shadow-call-stack)
+	def_bool $(cc-option,-fsanitize=shadow-call-stack) || $(cc-option,-mabi=lp64 -march=rv64ima_zicfilp_zicfiss)
 	# https://github.com/riscv-non-isa/riscv-elf-psabi-doc/commit/a484e843e6eeb51f0cb7b8819e50da6d2444d769
 	depends on $(ld-option,--no-relax-gp)
 
@@ -211,6 +211,30 @@ config ARCH_HAS_BROKEN_DWARF5
 	# https://github.com/llvm/llvm-project/commit/7ffabb61a5569444b5ac9322e22e5471cc5e4a77
 	depends on LD_IS_LLD && LLD_VERSION < 180000
 
+config RISCV_KERNEL_CFI
+	def_bool n
+	bool "hw assisted riscv kernel control flow integrity (kcfi)"
+	depends on 64BIT && $(cc-option,-mabi=lp64 -march=rv64ima_zicfilp_zicfiss)
+	select ARCH_SUPPORTS_SHADOW_CALL_STACK
+	select SHADOW_CALL_STACK
+	select DYNAMIC_SCS
+	help
+	  Provides CPU assisted control flow integrity to for riscv kernel.
+	  Control flow integrity is provided by implementing shadow stack for
+	  backward edge and indirect branch tracking for forward edge. Shadow
+	  stack protection is a hardware feature that detects function return
+	  address corruption. This helps mitigate ROP attacks. RISCV_KERNEL_CFI
+	  selects CONFIG_SHADOW_CALL_STACK which uses software based shadow
+	  stack but is unprotected against stray writes. Selecting RISCV_KERNEL_CFI
+	  will select CONFIG_DYNAMIC_SCS and will enable hardware assisted shadow
+	  stack protection against stray writes.
+	  Indirect branch tracking enforces that all indirect branches must land
+	  on a landing pad instruction else CPU will fault. This enables forward
+	  control flow (call/jmp) protection in kernel and restricts all indirect
+	  call or jump in kernel to a landing pad instruction which mostly likely
+	  will be start of the function.
+	  default n
+
 config ARCH_MMAP_RND_BITS_MIN
 	default 18 if 64BIT
 	default 8
@@ -639,6 +663,16 @@ config RISCV_ISA_ZICBOZ
 
 	   If you don't know what to do here, say Y.
 
+config TOOLCHAIN_HAS_ZICFILP
+	bool
+	default y
+	depends on 64BIT && $(cc-option,-mabi=lp64 -march=rv64ima_zicfilp)
+
+config TOOLCHAIN_HAS_ZICFISS
+	bool
+	default y
+	depends on 64BIT && $(cc-option,-mabi=lp64 -march=rv64ima_zicfiss)
+
 config TOOLCHAIN_HAS_ZIHINTPAUSE
 	bool
 	default y
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 5b3115a19852..ae156e37e886 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -58,8 +58,10 @@ else ifeq ($(CONFIG_LTO_CLANG),y)
 endif
 
 ifeq ($(CONFIG_SHADOW_CALL_STACK),y)
+ifndef CONFIG_DYNAMIC_SCS
 	KBUILD_LDFLAGS += --no-relax-gp
 endif
+endif
 
 # ISA string setting
 riscv-march-$(CONFIG_ARCH_RV32I)	:= rv32ima
@@ -78,6 +80,10 @@ endif
 # Check if the toolchain supports Zihintpause extension
 riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZIHINTPAUSE) := $(riscv-march-y)_zihintpause
 
+ifeq ($(CONFIG_RISCV_KERNEL_CFI),y)
+riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZICFILP) := $(riscv-march-y)_zicfilp
+riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZICFISS) := $(riscv-march-y)_zicfiss
+endif
 # Remove F,D,V from isa string for all. Keep extensions between "fd" and "v" by
 # matching non-v and non-multi-letter extensions out with the filter ([^v_]*)
 KBUILD_CFLAGS += -march=$(shell echo $(riscv-march-y) | sed -E 's/(rv32ima|rv64ima)fd([^v_]*)v?/\1\2/')
-- 
2.43.2