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

[Nick Hu]

Hi Deepak

On Thu, Nov 14, 2024 at 11:50 PM Deepak Gupta <debug@xxxxxxxxxxxx> wrote:
>
>
> Hi Nick,
>
> Thanks for reviewing and helping.
>
> On Thu, Nov 14, 2024 at 02:17:30PM +0800, Nick Hu wrote:
> >Hi Deepak
> >
> >On Thu, Nov 14, 2024 at 9:25 AM Deepak Gupta <debug@xxxxxxxxxxxx> wrote:
> >>
> >> On Thu, Nov 14, 2024 at 09:20:14AM +0800, Nick Hu wrote:
> >> >Hi Deepak
> >> >
> >> >On Thu, Nov 14, 2024 at 9:06 AM Deepak Gupta <debug@xxxxxxxxxxxx> wrote:
> >> >> >> 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;
> >> >
> >>
> >> hmm... Why?
> >>
> >> `conf` is pointing to per-hart state in firmware.
> >>
> >> On this incoming cpu, opensbi (or equivalent) firmware must have
> >> ensured that this per-hart state doesn't have lock set.
> >>
> >> Am I missing something?
> >>
> >Current OpenSBI doesn't clear the lock in the warm init of the hotplug path.
> >It seems like we need a patch to address it.
>
> Got it thanks.
> Since you already know what's the problem, can you send a patch to opensbi.
> If you want rather have me do it, let me know. Thanks.
>
No problem. I'll send a patch to opensbi.

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