| From foo@baz Fri Jan 29 11:33:22 AM CET 2021 |
| From: Arvind Sankar <nivedita@alum.mit.edu> |
| Date: Tue, 11 Aug 2020 20:43:08 -0400 |
| Subject: x86/boot/compressed: Disable relocation relaxation |
| |
| From: Arvind Sankar <nivedita@alum.mit.edu> |
| |
| commit 09e43968db40c33a73e9ddbfd937f46d5c334924 upstream. |
| |
| 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> |
| Signed-off-by: Ingo Molnar <mingo@kernel.org> |
| Tested-by: Nick Desaulniers <ndesaulniers@google.com> |
| Tested-by: Sedat Dilek <sedat.dilek@gmail.com> |
| Acked-by: Ard Biesheuvel <ardb@kernel.org> |
| Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> |
| Cc: stable@vger.kernel.org |
| Link: https://lore.kernel.org/r/20200812004308.1448603-1-nivedita@alum.mit.edu |
| [nc: Backport to 4.14] |
| Signed-off-by: Nathan Chancellor <nathan@kernel.org> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| --- |
| arch/x86/boot/compressed/Makefile | 2 ++ |
| 1 file changed, 2 insertions(+) |
| |
| --- a/arch/x86/boot/compressed/Makefile |
| +++ b/arch/x86/boot/compressed/Makefile |
| @@ -36,6 +36,8 @@ KBUILD_CFLAGS += -mno-mmx -mno-sse |
| KBUILD_CFLAGS += $(call cc-option,-ffreestanding) |
| KBUILD_CFLAGS += $(call cc-option,-fno-stack-protector) |
| KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member) |
| +# Disable relocation relaxation in case the link is not PIE. |
| +KBUILD_CFLAGS += $(call as-option,-Wa$(comma)-mrelax-relocations=no) |
| |
| KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__ |
| GCOV_PROFILE := n |