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

Deepak Gupta
2024-11-14T01:25:14Z

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@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;
>

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?

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