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

[Kevin Loughlin]

On Mon, Jan 15, 2024 at 12:47 PM Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> 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.
>
> So, about that. If I understand my gcc toolchain folks correctly:
>
> mcmodel=kernel - everything fits into the high 31 bit of the address
> space
>
> -fPIE/PIC - position independent
>
> And supplied both don't make a whole lotta of sense: if you're building
> position-independent, then mcmodel=kernel would be overridden by the
> first.
>
> I have no clue why clang enabled it...
>
> So, *actually* the proper fix here should be not to add this "fixed_up"
> gunk everywhere but remove mcmodel=kernel from the build and simply do
> -fPIE/PIC.

I believe that the key distinction is that using mcmodel=kernel (upper
2 GB of address space) or the similar mcmodel=small (lower 2 GB) means
the compiler *can* use RIP-relative addressing for globals (because
everything is within +/- 2GB of RIP) but is not *required* to do so.
In contrast, fPIE/fPIC *requires* relative addressing but does not
necessarily require a specific 2 GB placement range. Altogether, I do
think there are use cases for both options individually. I can't think
of a reason why gcc wouldn't be able to support mcmodel=kernel in
conjunction with fPIE off the top of my head, but I admittedly haven't
looked into it; I simply observed that the combination is not
currently supported.

RE: compiling the whole x86-64 kernel with fPIE/fPIC, I believe the
required changes would be very extensive (see "[PATCH RFC 00/43]
x86/pie: Make kernel image's virtual address flexible" at
https://lore.kernel.org/lkml/cover.1682673542.git.houwenlong.hwl@xxxxxxxxxxxx/).