Re: [PATCH v2 01/28] x86/boot/compressed: Disable relocation relaxation

Kees Cook
2020-09-03T21:44:51Z

On Thu, Sep 03, 2020 at 01:30:26PM -0700, Sami Tolvanen wrote:
> From: Arvind Sankar <nivedita@alum.mit.edu>
>
> The x86-64 psABI [0] specifies special relocation types
> (R_X86_64_[REX_]GOTPCRELX) for indirection through the Global Offset
> Table, semantically equivalent to R_X86_64_GOTPCREL, which the linker
> can take advantage of for optimization (relaxation) at link time. This
> is supported by LLD and binutils versions 2.26 onwards.
>
> The compressed kernel is position-independent code, however, when using
> LLD or binutils versions before 2.27, it must be linked without the -pie
> option. In this case, the linker may optimize certain instructions into
> a non-position-independent form, by converting foo@GOTPCREL(%rip) to $foo.
>
> This potential issue has been present with LLD and binutils-2.26 for a
> long time, but it has never manifested itself before now:
> - LLD and binutils-2.26 only relax
> movq foo@GOTPCREL(%rip), %reg
> to
> leaq foo(%rip), %reg
> which is still position-independent, rather than
> mov $foo, %reg
> which is permitted by the psABI when -pie is not enabled.
> - gcc happens to only generate GOTPCREL relocations on mov instructions.
> - clang does generate GOTPCREL relocations on non-mov instructions, but
> when building the compressed kernel, it uses its integrated assembler
> (due to the redefinition of KBUILD_CFLAGS dropping -no-integrated-as),
> which has so far defaulted to not generating the GOTPCRELX
> relocations.
>
> Nick Desaulniers reports [1,2]:
> A recent change [3] to a default value of configuration variable
> (ENABLE_X86_RELAX_RELOCATIONS OFF -> ON) in LLVM now causes Clang's
> integrated assembler to emit R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX
> relocations. LLD will relax instructions with these relocations based
> on whether the image is being linked as position independent or not.
> When not, then LLD will relax these instructions to use absolute
> addressing mode (R_RELAX_GOT_PC_NOPIC). This causes kernels built with
> Clang and linked with LLD to fail to boot.
>
> Patch series [4] is a solution to allow the compressed kernel to be
> linked with -pie unconditionally, but even if merged is unlikely to be
> backported. As a simple solution that can be applied to stable as well,
> prevent the assembler from generating the relaxed relocation types using
> the -mrelax-relocations=no option. For ease of backporting, do this
> unconditionally.
>
> [0] https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/linker-optimization.tex#L65
> [1] https://lore.kernel.org/lkml/20200807194100.3570838-1-ndesaulniers@google.com/
> [2] https://github.com/ClangBuiltLinux/linux/issues/1121
> [3] https://reviews.llvm.org/rGc41a18cf61790fc898dcda1055c3efbf442c14c0
> [4] https://lore.kernel.org/lkml/20200731202738.2577854-1-nivedita@alum.mit.edu/
>
> Reported-by: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>

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

--
Kees Cook