[RFC PATCH 08/12] riscv: dynamic (zicfiss) shadow call stack support

[Sami Tolvanen]

On Thu, Apr 11, 2024 at 5:30 PM Deepak Gupta <debug at rivosinc.com> wrote:
>
> On Thu, Apr 11, 2024 at 05:05:38PM +0000, Sami Tolvanen wrote:
> >Hi Deepak,
> >
> >Thanks for the patches!
> >
> >On Tue, Apr 9, 2024 at 6:12 AM Deepak Gupta <debug at rivosinc.com> wrote:
> >>
> >> Adding support for dynamic shadow call stack on riscv. zicfiss ISA extn.
> >> enables protection for shadow stack against stray writes. This patch
> >> enables scs_* macros to use zicfiss shadow stack pointer (CSR_SSP) instead
> >> of relying on `gp`.
> >
> >CONFIG_DYNAMIC_SCS implies that runtime patching is used to select
> >between software SCS and an alternative hardware implementation (in
> >arm64's case, PAC instead of hardware shadow stacks). I understand
> >this series is still an RFC, but I didn't see runtime patching
> >support. Are you planning on implementing this later?
>
> Since I didn't see any example on selecting PAC when `CONFIG_DYNAMIC_SCS`
> is selected. So I had that confusion but wasn't sure. I thought of doing it
> but I don't know how to binary rewrite all the functions. It might be too much.
> So I went ahead with using `CONFIG_DYNAMIC_SCS` in this RFC series.
>
> Question:
> If arm64 were to use PAC with CONFIG_DYNAMIC_SCS, how would it fixup the code
> sequences already setup by compiler for shadow stack push and pop in runtime?
> You expect this to be some offline process using some object editing tool or
> a runtime decision?

We use unwind tables for locating instructions to patch, look for
UNWIND_PATCH_PAC_INTO_SCS. The actual patching code is in
arch/arm64/kernel/pi/patch-scs.c. I suspect this is going to be a bit
trickier when patching between two shadow stack options though.

> >If there's no plan to actually patch between Zicfiss and SCS at
> >runtime, CONFIG_DYNAMIC_SCS doesn't seem like the appropriate choice
> >and we might need a separate config option that still allows you to
> >reuse most of the software SCS code.
>
> I wanted to avoid "#ifdef RISCV_SPECIFIC_HW_SHSTK" in arch agnostic scs code.
> And that's why went with CONFIG_DYNAMIC_SCS which sets dynamic static key once.
> And then I use `is_dynamic` everywhere else in arch agnostic scs code.

We could define arch_ functions for any architecture-specific code
(with a weak default implementation), and maybe add a config option
for specifying which way the shadow stack grows?

Sami