Re: [RFC PATCH v2] x86/sev: enforce RIP-relative accesses in early SEV/SME code

[Kirill A. Shutemov]

On Thu, Jan 11, 2024 at 10:36:50PM +0000, Kevin Loughlin wrote:
> SEV/SME code can execute prior to page table fixups for kernel
> relocation. However, as with global variables accessed in
> __startup_64(), the compiler is not required to generate RIP-relative
> accesses for SEV/SME global variables, causing certain flavors of SEV
> hosts and guests built with clang to crash during boot.
> 
> While an attempt was made to force RIP-relative addressing for certain
> global SEV/SME variables via inline assembly (see snp_cpuid_get_table()
> for example), RIP-relative addressing must be pervasively-enforced for
> SEV/SME global variables when accessed prior to page table fixups.
> 
> __startup_64() already handles this issue for select non-SEV/SME global
> variables using fixup_pointer(), which adjusts the pointer relative to
> a `physaddr` argument. To avoid having to pass around this `physaddr`
> argument across all functions needing to apply pointer fixups, this
> patch introduces the macro GET_RIP_RELATIVE_PTR() (an abstraction of
> the existing snp_cpuid_get_table()), which generates an RIP-relative
> pointer to a passed variable. Similarly, PTR_TO_RIP_RELATIVE_PTR() is
> introduced to fixup an existing pointer value with RIP-relative logic.

Can we replace existing fixup_pointer() (and other fixup_*()) with the new
thing? I don't think we need two confusing things for the same function.

Also, is there any reason why GET_RIP_RELATIVE_PTR() and
PTR_TO_RIP_RELATIVE_PTR() have to be macros? Inline functions would be
cleaner.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov