[PATCH v4 27/30] riscv: create a config for shadow stack and landing pad instr support

[Deepak Gupta <debug@rivosinc.com>]

This patch creates a config for shadow stack support and landing pad instr
support. Shadow stack support and landing instr support can be enabled by
selecting `CONFIG_RISCV_USER_CFI`. Selecting `CONFIG_RISCV_USER_CFI` wires
up path to enumerate CPU support and if cpu support exists, kernel will
support cpu assisted user mode cfi.

If CONFIG_RISCV_USER_CFI is selected, select `ARCH_USES_HIGH_VMA_FLAGS`
and `ARCH_HAS_USER_SHADOW_STACK` for riscv.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
 arch/riscv/Kconfig | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index d1d629a3eb91..24bf08c905d2 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -231,6 +231,25 @@ config ARCH_HAS_BROKEN_DWARF5
 	# https://github.com/llvm/llvm-project/commit/7ffabb61a5569444b5ac9322e22e5471cc5e4a77
 	depends on LD_IS_LLD && LLD_VERSION < 180000
 
+config RISCV_USER_CFI
+	def_bool y
+	bool "riscv userspace control flow integrity"
+	depends on 64BIT && $(cc-option,-mabi=lp64 -march=rv64ima_zicfiss)
+	depends on RISCV_ALTERNATIVE
+	select ARCH_HAS_USER_SHADOW_STACK
+	select ARCH_USES_HIGH_VMA_FLAGS
+	help
+	  Provides CPU assisted control flow integrity to userspace tasks.
+	  Control flow integrity is provided by implementing shadow stack for
+	  backward edge and indirect branch tracking for forward edge in program.
+	  Shadow stack protection is a hardware feature that detects function
+	  return address corruption. This helps mitigate ROP attacks.
+	  Indirect branch tracking enforces that all indirect branches must land
+	  on a landing pad instruction else CPU will fault. This mitigates against
+	  JOP / COP attacks. Applications must be enabled to use it, and old user-
+	  space does not get protection "for free".
+	  default y
+
 config ARCH_MMAP_RND_BITS_MIN
 	default 18 if 64BIT
 	default 8
-- 
2.45.0