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

[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. 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.

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.

>
> > 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.
> >
> > Reviewed-by: Zong Li <zong.li@sifive.com>
> > Signed-off-by: Deepak Gupta <debug@rivosinc.com>
> > ---
> > 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.