[PATCH v22 25/28] riscv: create a config for shadow stack and landing pad instr support

[Zong Li]

On Fri, Oct 24, 2025 at 12:51 AM Deepak Gupta via B4 Relay
<devnull+debug.rivosinc.com at kernel.org> wrote:
>
> From: Deepak Gupta <debug at 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 at sifive.com>
> Signed-off-by: Deepak Gupta <debug at rivosinc.com>
> ---
>  arch/riscv/Kconfig                  | 22 ++++++++++++++++++++++
>  arch/riscv/configs/hardening.config |  4 ++++
>  2 files changed, 26 insertions(+)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 0c6038dc5dfd..4f9f9358e6e3 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -1146,6 +1146,28 @@ config RANDOMIZE_BASE
>
>            If unsure, say N.
>
> +config RISCV_USER_CFI
> +       def_bool y
> +       bool "riscv userspace control flow integrity"
> +       depends on 64BIT && $(cc-option,-mabi=lp64 -march=rv64ima_zicfiss) && \
> +                           $(cc-option,-fcf-protection=full)

Hi Deepak,
I noticed that you added a $(cc-option,-fcf-protection=full) check in
this version. I think this check will fail by a cc1 warning when using
a newer toolchain, because -fcf-protection cannot be used alone, it
must be specified together with the appropriate -march option.
For example:
  1. -fcf-protection=branch requires -march=..._zicfilp
  2. -fcf-protection=return requires -march=..._zicfiss
  3. -fcf-protection=full requires -march=..._zicfilp_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 y.
> +
>  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
>
>