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

[Mark Brown]

On Wed, Jan 24, 2024 at 10:21:40PM -0800, debug at rivosinc.com 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

> --- 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.
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