Re: [PATCH 05/11] riscv: enable landing pad enforcement

[Heinrich Schuchardt]

On 25.07.25 16:20, Deepak Gupta wrote:
> On Fri, Jul 25, 2025 at 08:33:46AM +0200, Heinrich Schuchardt wrote:
> > On 25.07.25 01:36, Deepak Gupta wrote:
> > > Enables landing pad enforcement by invoking a SBI FWFT call.
> > >
> > > Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
> > > ---
> > >  arch/riscv/kernel/asm-offsets.c |  1 +
> > >  arch/riscv/kernel/head.S        | 19 +++++++++++++++++++
> > >  2 files changed, 20 insertions(+)
> > >
> > > diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm- 
> > > offsets.c
> > > index e4d55126dc3e..e6a9fad86fae 100644
> > > --- a/arch/riscv/kernel/asm-offsets.c
> > > +++ b/arch/riscv/kernel/asm-offsets.c
> > > @@ -536,6 +536,7 @@ void asm_offsets(void)
> > >      DEFINE(SBI_EXT_FWFT, SBI_EXT_FWFT);
> > >      DEFINE(SBI_EXT_FWFT_SET, SBI_EXT_FWFT_SET);
> > >      DEFINE(SBI_FWFT_SHADOW_STACK, SBI_FWFT_SHADOW_STACK);
> > > +    DEFINE(SBI_FWFT_LANDING_PAD, SBI_FWFT_LANDING_PAD);
> > >      DEFINE(SBI_FWFT_SET_FLAG_LOCK, SBI_FWFT_SET_FLAG_LOCK);
> > >  #endif
> > >  }
> > > diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> > > index 9c99c5ad6fe8..59af044bf85c 100644
> > > --- a/arch/riscv/kernel/head.S
> > > +++ b/arch/riscv/kernel/head.S
> > > @@ -185,6 +185,16 @@ secondary_start_sbi:
> > >  1:
> > >  #endif
> > >      scs_load_current
> > > +
> > > +#if defined(CONFIG_RISCV_SBI) && defined(CONFIG_RISCV_KERNEL_CFI)
> > > +    li a7, SBI_EXT_FWFT
> > > +    li a6, SBI_EXT_FWFT_SET
> > > +    li a0, SBI_FWFT_LANDING_PAD
> > > +    li a1, 1 /* enable landing pad for supervisor */
> > > +    li a2, SBI_FWFT_SET_FLAG_LOCK
> > > +    ecall    /* check for error condition and take appropriate 
> > > action */
> > > +#endif
> > > +
> > >      call smp_callin
> > >  #endif /* CONFIG_SMP */
> > > @@ -359,6 +369,15 @@ SYM_CODE_START(_start_kernel)
> > >  #endif
> > >      scs_load_current
> > > +#if defined(CONFIG_RISCV_SBI) && defined(CONFIG_RISCV_KERNEL_CFI)
> > > +    li a7, SBI_EXT_FWFT
> > > +    li a6, SBI_EXT_FWFT_SET
> > > +    li a0, SBI_FWFT_LANDING_PAD
> > > +    li a1, 1 /* enable landing pad for supervisor */
> >
> > The SBI specification calls BIT(0) "LOCK".
> > Shouldn't we define a constant for the lock bit instead of using a 
> > magic value?
>
> See below `li a2, SBI_FWFT_SET_FLAG_LOCK`.
>
> "li a1, 1 /* enable landing pad for supervisor */>" --> this is enabling.
> Had we done "li a1, 0 /* enable landing pad for supervisor */" --> this is
> asking firmware to disable the feature (turn off the bit in menvcfg CSR)

So we lack constants for

"Enable landing pad for supervisor-mode"
"Disable landing pad for supervisor-mode"

Best regards

Heinrich

> > > +    li a2, SBI_FWFT_SET_FLAG_LOCK
> > > +    ecall    /* check for error condition and take appropriate 
> > > action */
> > > +#endif
> > > +
> > >  #ifdef CONFIG_KASAN
> > >      call kasan_early_init
> > >  #endif