Re: [RFC PATCH v1 15/28] riscv/mm: Implement map_shadow_stack() syscall

[Deepak Gupta]

On Tue, Feb 06, 2024 at 04:01:28PM +0000, Mark Brown wrote:
> On Wed, Jan 24, 2024 at 10:21:40PM -0800, debug@xxxxxxxxxxxx wrote:
>
> > As discussed extensively in the changelog for the addition of this
> > syscall on x86 ("x86/shstk: Introduce map_shadow_stack syscall") the
> > existing mmap() and madvise() syscalls do not map entirely well onto the
> > security requirements for guarded control stacks since they lead to
> > windows where memory is allocated but not yet protected or stacks which
> > are not properly and safely initialised. Instead a new syscall
> > map_shadow_stack() has been defined which allocates and initialises a
> > shadow stack page.
>
> While I agree that this is very well written you probably want to update
> the references to guarded control stacks to whatever the RISC-V term is :P

Noted. I'll do that in next patchset.

> > --- a/include/uapi/asm-generic/mman.h
> > +++ b/include/uapi/asm-generic/mman.h
> > @@ -19,4 +19,5 @@
> >  #define MCL_FUTURE	2		/* lock all future mappings */
> >  #define MCL_ONFAULT	4		/* lock all pages that are faulted in */
> >
> > +#define SHADOW_STACK_SET_TOKEN (1ULL << 0)     /* Set up a restore token in the shadow stack */
> >  #endif /* __ASM_GENERIC_MMAN_H */
>
> For arm64 I also added a SHADOW_STACK_SET_MARKER for adding a top of
> stack marker, did you have any thoughts on that for RISC-V?  I think x86
> were considering adding it too, it'd be good if we could get things
> consistent.

Please correct me on this. A token at the top which can't be consumed to restore
but *just* purely as marker, right?
It's a good design basic with not a lot of cost.

I think risc-v should be able to converge on that.