[PATCH v8 24/29] riscv: enable kernel access to shadow stack memory via FWFT sbi call

[Nick Hu]

Hi Deepak

On Tue, Nov 12, 2024 at 5:08 AM Deepak Gupta <debug at rivosinc.com> wrote:
>
> Kernel will have to perform shadow stack operations on user shadow stack.
> Like during signal delivery and sigreturn, shadow stack token must be
> created and validated respectively. Thus shadow stack access for kernel
> must be enabled.
>
> In future when kernel shadow stacks are enabled for linux kernel, it must
> be enabled as early as possible for better coverage and prevent imbalance
> between regular stack and shadow stack. After `relocate_enable_mmu` has
> been done, this is as early as possible it can enabled.
>
> Signed-off-by: Deepak Gupta <debug at rivosinc.com>
> ---
>  arch/riscv/kernel/asm-offsets.c |  4 ++++
>  arch/riscv/kernel/head.S        | 12 ++++++++++++
>  2 files changed, 16 insertions(+)
>
> diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
> index 766bd33f10cb..a22ab8a41672 100644
> --- a/arch/riscv/kernel/asm-offsets.c
> +++ b/arch/riscv/kernel/asm-offsets.c
> @@ -517,4 +517,8 @@ void asm_offsets(void)
>         DEFINE(FREGS_A6,            offsetof(struct ftrace_regs, a6));
>         DEFINE(FREGS_A7,            offsetof(struct ftrace_regs, a7));
>  #endif
> +       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_SET_FLAG_LOCK, SBI_FWFT_SET_FLAG_LOCK);
>  }
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index 356d5397b2a2..6244408ca917 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -164,6 +164,12 @@ secondary_start_sbi:
>         call relocate_enable_mmu
>  #endif
>         call .Lsetup_trap_vector
> +       li a7, SBI_EXT_FWFT
> +       li a6, SBI_EXT_FWFT_SET
> +       li a0, SBI_FWFT_SHADOW_STACK
> +       li a1, 1 /* enable supervisor to access shadow stack access */
> +       li a2, SBI_FWFT_SET_FLAG_LOCK
> +       ecall
>         scs_load_current
>         call smp_callin
>  #endif /* CONFIG_SMP */
> @@ -320,6 +326,12 @@ SYM_CODE_START(_start_kernel)
>         la tp, init_task
>         la sp, init_thread_union + THREAD_SIZE
>         addi sp, sp, -PT_SIZE_ON_STACK
> +       li a7, SBI_EXT_FWFT
> +       li a6, SBI_EXT_FWFT_SET
> +       li a0, SBI_FWFT_SHADOW_STACK
> +       li a1, 1 /* enable supervisor to access shadow stack access */
> +       li a2, SBI_FWFT_SET_FLAG_LOCK
> +       ecall
>         scs_load_current
>
>  #ifdef CONFIG_KASAN
>
> --
> 2.45.0
>
Should we clear the SBI_FWFT_SET_FLAG_LOCK before the cpu hotplug
otherwise the menvcfg.sse won't be set by the fwft set sbi call when
the hotplug cpu back to kernel?

Regards,
Nick
>
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