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

[Deepak Gupta]

On Tue, Nov 11, 2025 at 01:58:37PM +0800, Zong Li wrote:
> On Fri, Oct 24, 2025 at 12:51 AM Deepak Gupta via B4 Relay
> <devnull+debug.rivosinc.com@xxxxxxxxxx> wrote:
> > From: Deepak Gupta <debug@xxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
> > ---
> >  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

toolchain that I have from June doesn't require -march=..._zicfilp_zicfiss
for -fcf-protection=full. If that has changed, I think this will need a
revision.

> > +       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