[PATCH v18 24/27] 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`,
`ARCH_HAS_USER_SHADOW_STACK` and DYNAMIC_SIGFRAME for riscv.

Reviewed-by: Zong Li <zong.li@sifive.com>
Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
 arch/riscv/Kconfig                  | 21 +++++++++++++++++++++
 arch/riscv/configs/hardening.config |  4 ++++
 2 files changed, 25 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 36061f4732b7..385c3d93e378 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -1161,6 +1161,27 @@ config RANDOMIZE_BASE
 
           If unsure, say N.
 
+config RISCV_USER_CFI
+	def_bool n
+	bool "riscv userspace control flow integrity"
+	depends on 64BIT && $(cc-option,-mabi=lp64 -march=rv64ima_zicfiss)
+	depends on RISCV_ALTERNATIVE
+	select RISCV_SBI
+	select ARCH_HAS_USER_SHADOW_STACK
+	select ARCH_USES_HIGH_VMA_FLAGS
+	select DYNAMIC_SIGFRAME
+	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 n.
+
 endmenu # "Kernel features"
 
 menu "Boot options"
diff --git a/arch/riscv/configs/hardening.config b/arch/riscv/configs/hardening.config
new file mode 100644
index 000000000000..089f4cee82f4
--- /dev/null
+++ b/arch/riscv/configs/hardening.config
@@ -0,0 +1,4 @@
+# RISCV specific kernel hardening options
+
+# Enable control flow integrity support for usermode.
+CONFIG_RISCV_USER_CFI=y

-- 
2.43.0