Re: [RFC PATCH] x86/sev: x86/sev: enforce PC-relative addressing in clang

[Kirill A. Shutemov]

On Wed, Jan 10, 2024 at 01:26:39AM +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(), clang does not currently generate PC-relative accesses
> for SEV/SME global variables, causing certain flavors of SEV hosts and
> guests to crash.
> 
> While an attempt was made to force PC-relative addressing for certain
> global SEV/SME variables via inline assembly (see snp_cpuid_get_table()
> for example), PC-relative addressing must be pervasively-enforced for
> SEV/SME global variables that can be accessed prior to page table
> fixups.
> 
> To avoid the error-prone approach of manually referencing each SEV/SME
> global variable via a general form of snp_cpuid_get_table(), it is
> preferable to use compiler flags for position-independent code (ex:
> `-fPIE`) that result in PC-relative accesses. While architecture-
> specific code for Linux can be pervasively compiled as position-
> independent on select architectures (ex: RISC-V), this is not currently
> the case for x86-64 and would require extensive changes (see "[PATCH
> RFC 00/43] x86/pie: Make kernel image's virtual address flexible" for
> example).
> 
> Fortunately, the relevant files for SEV/SME code do indeed support
> position-independent clang compilation, so we can use this technique to
> ensure all global variables in these files are accessed via PC-relative
> addressing.
> 
> Unlike clang, gcc does not currently allow `-fPIE` in conjunction with
> `mcmodel=kernel`. Thus, to preserve existing gcc behavior, this patch
> does not remove the (otherwise unnecessary) inline assembly that
> already enforces PC-relative addressing for select SEV/SME globals
> (mentioned above). If gcc supports these joint options in the future,
> we can remove such inline assembly and also apply this patch to gcc
> builds.
> 
> Tested by successful boot of SEV-SNP guest built with clang, alongside
> Adam Dunlap's necessary "[PATCH v2] x86/asm: Force native_apic_mem_read
> to use mov".
> 

Similar issues was fixed before with fixup_pointer() tricks. Have you
tried looking this direction.

Relevant thread starting with:

https://lore.kernel.org/all/20210920192341.maue7db4lcbdn46x@xxxxxxxxxxxxxxxxx

-- 
  Kiryl Shutsemau / Kirill A. Shutemov