Re: [PATCH RFC/RFT v2 2/2] kernel: converge common shadow stack flow agnostic to arch

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Wed, 2024-10-16 at 14:57 -0700, Deepak Gupta wrote:
> -/*
> - * VM_SHADOW_STACK will have a guard page. This helps userspace protect
> - * itself from attacks. The reasoning is as follows:
> - *
> - * The shadow stack pointer(SSP) is moved by CALL, RET, and INCSSPQ. The
> - * INCSSP instruction can increment the shadow stack pointer. It is the
> - * shadow stack analog of an instruction like:
> - *
> - *   addq $0x80, %rsp
> - *
> - * However, there is one important difference between an ADD on %rsp
> - * and INCSSP. In addition to modifying SSP, INCSSP also reads from the
> - * memory of the first and last elements that were "popped". It can be
> - * thought of as acting like this:
> - *
> - * READ_ONCE(ssp);       // read+discard top element on stack
> - * ssp += nr_to_pop * 8; // move the shadow stack
> - * READ_ONCE(ssp-8);     // read+discard last popped stack element
> - *
> - * The maximum distance INCSSP can move the SSP is 2040 bytes, before
> - * it would read the memory. Therefore a single page gap will be enough
> - * to prevent any operation from shifting the SSP to an adjacent stack,
> - * since it would have to land in the gap at least once, causing a
> - * fault.
> - */

I want to take a deeper look at this series once I can apply and test it, but
can we maybe make this comment more generic and keep it? I think it is similar
reasoning for arm (?), is there anything situation like this for risc-v? Or
rather, why does risc-v have the guard gaps?