Re: [PATCH v3 5/7] x86/boot: Remove run-time relocations from .head.text code

Kees Cook
2020-06-29T19:43:19Z

On Mon, Jun 29, 2020 at 10:09:26AM -0400, Arvind Sankar wrote:
> 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.

Are any of Thomas Garnier's PIE fixes useful here too? He had a lot of
fixes to make changes for PC-relative addressing in the various
assembly bits:
https://lore.kernel.org/lkml/20200228000105.165012-9-thgarnie@chromium.org/

>
> 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.
>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
> Reviewed-by: Fangrui Song <maskray@google.com>

Nice.

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook