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

[Nick Hu <nick.hu@sifive.com>]

Hi Deepak

On Thu, Nov 14, 2024 at 9:06 AM Deepak Gupta <debug@rivosinc.com> wrote:
>
> On Thu, Nov 14, 2024 at 12:13:38AM +0800, Nick Hu wrote:
> >Hi Deepak
> >
> >On Tue, Nov 12, 2024 at 5:08 AM Deepak Gupta <debug@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@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?
>
> Hmm...
>
> An incoming hotplug CPU has no features setup on it.
> I see that `sbi_cpu_start` will supply `secondary_start_sbi` as start
> up code for incoming CPU. `secondary_start_sbi` is in head.S which converges
> in `.Lsecondary_start_common`. And thus hotplugged CPU should be
> issuing shadow stack set FWFT sbi as well.
>
> Am I missing something ?
>
This is the correct flow. However the opensbi will deny it due to the
SBI_FWFT_SET_FLAG_LOCK already being set.
So the menvcfg.sse will not set by this flow.

if (conf->flags & SBI_FWFT_SET_FLAG_LOCK)
                return SBI_EDENIED;

Regards,
Nick
> >
> >Regards,
> >Nick
> >>
> >> _______________________________________________
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