x86/boot: Remove run-time relocations from .head.text code

The assembly code in head_{32,64}.S, while meant to be
position-independent, generates run-time relocations because it uses
instructions such as:

	leal	gdt(%edx), %eax

which make the assembler and linker think that the code is using %edx as
an index into gdt, and hence gdt needs to be relocated to its run-time
address.

On 32-bit, with lld Dmitry Golovin reports that this results in a
link-time error with default options (i.e. unless -z notext is
explicitly passed):

  LD      arch/x86/boot/compressed/vmlinux
  ld.lld: error: can't create dynamic relocation R_386_32 against local
  symbol in readonly segment; recompile object files with -fPIC or pass
  '-Wl,-z,notext' to allow text relocations in the output

With the BFD linker, this generates a warning during the build, if
--warn-shared-textrel is enabled, which at least Gentoo enables by
default:

  LD      arch/x86/boot/compressed/vmlinux
  ld: arch/x86/boot/compressed/head_32.o: warning: relocation in read-only section `.head.text'
  ld: warning: creating a DT_TEXTREL in object

On 64-bit, it is not possible to link the kernel as -pie with lld, and
it is only possible with a BFD linker that supports -z noreloc-overflow,
i.e. versions >2.26. This is because these instructions cannot really be
relocated: the displacement field is only 32-bits wide, and thus cannot
be relocated for a 64-bit load address. The -z noreloc-overflow option
simply overrides the linker error, and results in R_X86_64_RELATIVE
relocations that apply a 64-bit relocation to a 32-bit field anyway.
This happens to work because nothing will process these run-time
relocations.

Start fixing this by removing relocations from .head.text:

- On 32-bit, use a base register that holds the address of the GOT and
  reference symbol addresses using @GOTOFF, i.e.
	leal	gdt@GOTOFF(%edx), %eax

- On 64-bit, most of the code can (and already does) use %rip-relative
  addressing, however the .code32 bits can't, and the 64-bit code also
  needs to reference symbol addresses as they will be after moving the
  compressed kernel to the end of the decompression buffer.
  For these cases, reference the symbols as an offset to startup_32 to
  avoid creating relocations, i.e.:

	leal	(gdt-startup_32)(%bp), %eax

  This only works in .head.text as the subtraction cannot be represented
  as a PC-relative relocation unless startup_32 is in the same section
  as the code. Move efi32_pe_entry into .head.text so that it can use
  the same method to avoid relocations.

Reported-by: Dmitry Golovin <dima@golovin.in>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Fangrui Song <maskray@google.com>
Link: https://lore.kernel.org/r/20200731230820.1742553-6-keescook@chromium.org