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

[Deepak Gupta <debug@rivosinc.com>]

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 ?

>
>Regards,
>Nick
>>
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