Re: [PATCH v12 22/28] riscv: enable kernel access to shadow stack memory via FWFT sbi call

["Radim Krčmář" <rkrcmar@ventanamicro.com>]

2025-03-20T15:42:44-07:00, Deepak Gupta <debug@rivosinc.com>:
> On Thu, Mar 20, 2025 at 3:10 PM Radim Krčmář <rkrcmar@ventanamicro.com> wrote:
>>
>> 2025-03-14T14:39:41-07:00, Deepak Gupta <debug@rivosinc.com>:
>> > 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.
>>
>> Why can't kernel access the user shadow stack through an aliased WR
>> mapping?
>
> It can, although that opens up a can of worms. If this alternating
> mapping is user mode
> then ensuring that another threat in userspace can't write to this
> address in this window
> of signal handling.

Right, it must not be user mode.

>                     A kernel alternate mapping can be created, but
> that can lead to all
> sorts of requirements of ensuring the page is pinned down. IIRC, It
> has been debated
> on x86 shadow stack merge time as well on how a flaky alias mapping approach can
> become and weaken the threat model it is supposed to protect against.

True.

> Simply using `ssamoswap` is simple and serves the purpose. Enabling shadow stack
> access for the kernel doesn't have any adverse effect on the kernel.

Makes sense.  We just depend on an extra feature, because we should
consider the case when M-mode doesn't allow S-mode shadow stack, but
S-mode can enable U-mode shadow stack:

>> > ---
>> > diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
>> > @@ -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
>>
>> I think the ecall can fail even on machines that have Zicfiss, so it
>> would be good to disable user shadow stack if that happens.