linux-kernel - Re: [PATCH v5 7/7] x86/boot: Check that there are no run-time relocations

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Message-ID: <CA+icZUX56cEjdkiuxOXtCgY1F0NWD5QxqMBRQxE5Li3=RUe-7w@mail.gmail.com>
Date:   Wed, 15 Jul 2020 11:00:10 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        Dmitry Golovin <dima@...ovin.in>,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        "H . J . Lu" <hjl@...rceware.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 7/7] x86/boot: Check that there are no run-time relocations

On Wed, Jul 15, 2020 at 2:41 AM Arvind Sankar <nivedita@...m.mit.edu> wrote:
>
> Add a linker script check that there are no run-time relocations, and
> remove the old one that tries to check via looking for specially-named
> sections in the object files.
>
> Drop the tests for -fPIE compiler option and -pie linker option, as they
> are available in all supported gcc and binutils versions (as well as
> clang and lld).
>

Tested-by: Sedat Dilek <sedat.dilek@...il.com>
Reported-by: Sedat Dilek <sedat.dilek@...il.com>

Here I reported the breakage with LLD and tested the move to
LDFLAGS_vmlinux with a previous version of the patch.

- Sedat -

> Reviewed-by: Kees Cook <keescook@...omium.org>
> Reviewed-by: Ard Biesheuvel <ardb@...nel.org>
> Reviewed-by: Fangrui Song <maskray@...gle.com>
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> ---
>  arch/x86/boot/compressed/Makefile      | 28 +++-----------------------
>  arch/x86/boot/compressed/vmlinux.lds.S |  8 ++++++++
>  2 files changed, 11 insertions(+), 25 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index ae2c0dc98a6a..a9e082b8c720 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -29,7 +29,7 @@ targets := vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2 vmlinux.bin.lzma \
>         vmlinux.bin.xz vmlinux.bin.lzo vmlinux.bin.lz4
>
>  KBUILD_CFLAGS := -m$(BITS) -O2
> -KBUILD_CFLAGS += -fno-strict-aliasing $(call cc-option, -fPIE, -fPIC)
> +KBUILD_CFLAGS += -fno-strict-aliasing -fPIE
>  KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
>  cflags-$(CONFIG_X86_32) := -march=i386
>  cflags-$(CONFIG_X86_64) := -mcmodel=small
> @@ -51,7 +51,7 @@ UBSAN_SANITIZE :=n
>  KBUILD_LDFLAGS := -m elf_$(UTS_MACHINE)
>  # Compressed kernel should be built as PIE since it may be loaded at any
>  # address by the bootloader.
> -LDFLAGS_vmlinux := $(call ld-option, -pie) $(call ld-option, --no-dynamic-linker)
> +LDFLAGS_vmlinux := -pie $(call ld-option, --no-dynamic-linker)
>  LDFLAGS_vmlinux += -T
>
>  hostprogs      := mkpiggy
> @@ -86,30 +86,8 @@ vmlinux-objs-$(CONFIG_ACPI) += $(obj)/acpi.o
>  vmlinux-objs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
>  vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_thunk_$(BITS).o
>
> -# The compressed kernel is built with -fPIC/-fPIE so that a boot loader
> -# can place it anywhere in memory and it will still run. However, since
> -# it is executed as-is without any ELF relocation processing performed
> -# (and has already had all relocation sections stripped from the binary),
> -# none of the code can use data relocations (e.g. static assignments of
> -# pointer values), since they will be meaningless at runtime. This check
> -# will refuse to link the vmlinux if any of these relocations are found.
> -quiet_cmd_check_data_rel = DATAREL $@
> -define cmd_check_data_rel
> -       for obj in $(filter %.o,$^); do \
> -               $(READELF) -S $$obj | grep -qF .rel.local && { \
> -                       echo "error: $$obj has data relocations!" >&2; \
> -                       exit 1; \
> -               } || true; \
> -       done
> -endef
> -
> -# We need to run two commands under "if_changed", so merge them into a
> -# single invocation.
> -quiet_cmd_check-and-link-vmlinux = LD      $@
> -      cmd_check-and-link-vmlinux = $(cmd_check_data_rel); $(cmd_ld)
> -
>  $(obj)/vmlinux: $(vmlinux-objs-y) FORCE
> -       $(call if_changed,check-and-link-vmlinux)
> +       $(call if_changed,ld)
>
>  OBJCOPYFLAGS_vmlinux.bin :=  -R .comment -S
>  $(obj)/vmlinux.bin: vmlinux FORCE
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index a4a4a59a2628..29df99b6cc64 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -42,6 +42,12 @@ SECTIONS
>                 *(.rodata.*)
>                 _erodata = . ;
>         }
> +       .rel.dyn : {
> +               *(.rel.*)
> +       }
> +       .rela.dyn : {
> +               *(.rela.*)
> +       }
>         .got : {
>                 *(.got)
>         }
> @@ -85,3 +91,5 @@ ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, "Unexpected GOT/PLT en
>  #else
>  ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0xc, "Unexpected GOT/PLT entries detected!")
>  #endif
> +
> +ASSERT(SIZEOF(.rel.dyn) == 0 && SIZEOF(.rela.dyn) == 0, "Unexpected run-time relocations detected!")
> --
> 2.26.2
>