Re: [PATCH v5 06/36] x86/boot: Remove run-time relocations from head_{32,64}.S

[Nick Desaulniers <ndesaulniers@google.com>]

On Fri, Jul 31, 2020 at 4:08 PM Kees Cook <keescook@chromium.org> wrote:
>
> From: Arvind Sankar <nivedita@alum.mit.edu>
>
> The BFD linker generates run-time relocations for z_input_len and
> z_output_len, even though they are absolute symbols.
>
> This is fixed for binutils-2.35 [1]. Work around this for earlier
> versions by defining two variables input_len and output_len in addition
> to the symbols, and use them via position-independent references.
>
> This eliminates the last two run-time relocations in the head code and
> allows us to drop the -z noreloc-overflow flag to the linker.
>
> Move the -pie and --no-dynamic-linker LDFLAGS to LDFLAGS_vmlinux instead
> of KBUILD_LDFLAGS. There shouldn't be anything else getting linked, but
> this is the more logical location for these flags, and modversions might
> call the linker if an EXPORT_SYMBOL is left over accidentally in one of
> the decompressors.
>
> [1] https://sourceware.org/bugzilla/show_bug.cgi?id=25754
>
> Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
> Reviewed-by: Fangrui Song <maskray@google.com>
> Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/x86/boot/compressed/Makefile  | 12 ++----------
>  arch/x86/boot/compressed/head_32.S | 17 ++++++++---------
>  arch/x86/boot/compressed/head_64.S |  4 ++--
>  arch/x86/boot/compressed/mkpiggy.c |  6 ++++++
>  4 files changed, 18 insertions(+), 21 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 489fea16bcfb..7db0102a573d 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -51,16 +51,8 @@ UBSAN_SANITIZE :=n
>  KBUILD_LDFLAGS := -m elf_$(UTS_MACHINE)
>  # Compressed kernel should be built as PIE since it may be loaded at any
>  # address by the bootloader.
> -ifeq ($(CONFIG_X86_32),y)
> -KBUILD_LDFLAGS += $(call ld-option, -pie) $(call ld-option, --no-dynamic-linker)
> -else
> -# To build 64-bit compressed kernel as PIE, we disable relocation
> -# overflow check to avoid relocation overflow error with a new linker
> -# command-line option, -z noreloc-overflow.
> -KBUILD_LDFLAGS += $(shell $(LD) --help 2>&1 | grep -q "\-z noreloc-overflow" \
> -       && echo "-z noreloc-overflow -pie --no-dynamic-linker")
> -endif
> -LDFLAGS_vmlinux := -T
> +LDFLAGS_vmlinux := $(call ld-option, -pie) $(call ld-option, --no-dynamic-linker)

Oh, do these still need ld-option?  bfd and lld both support these
flags. (Though in their --help, they mention single hyphen and double
hyphen respectively.  Also, if we don't build this as PIE because the
linker doesn't support the option, we probably want to fail the build?

> +LDFLAGS_vmlinux += -T
>
>  hostprogs      := mkpiggy
>  HOST_EXTRACFLAGS += -I$(srctree)/tools/include
> diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
> index 8c1a4f5610f5..659fad53ca82 100644
> --- a/arch/x86/boot/compressed/head_32.S
> +++ b/arch/x86/boot/compressed/head_32.S
> @@ -178,18 +178,17 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
>  /*
>   * Do the extraction, and jump to the new kernel..
>   */
> -                               /* push arguments for extract_kernel: */
> -       pushl   $z_output_len   /* decompressed length, end of relocs */
> +       /* push arguments for extract_kernel: */
>
> -       pushl   %ebp            /* output address */
> -
> -       pushl   $z_input_len    /* input_len */
> +       pushl   output_len@GOTOFF(%ebx) /* decompressed length, end of relocs */
> +       pushl   %ebp                    /* output address */
> +       pushl   input_len@GOTOFF(%ebx)  /* input_len */
>         leal    input_data@GOTOFF(%ebx), %eax
> -       pushl   %eax            /* input_data */
> +       pushl   %eax                    /* input_data */
>         leal    boot_heap@GOTOFF(%ebx), %eax
> -       pushl   %eax            /* heap area */
> -       pushl   %esi            /* real mode pointer */
> -       call    extract_kernel  /* returns kernel location in %eax */
> +       pushl   %eax                    /* heap area */
> +       pushl   %esi                    /* real mode pointer */
> +       call    extract_kernel          /* returns kernel location in %eax */
>         addl    $24, %esp
>
>  /*
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 11429092c224..9e46729cf162 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -534,9 +534,9 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
>         movq    %rsi, %rdi              /* real mode address */
>         leaq    boot_heap(%rip), %rsi   /* malloc area for uncompression */
>         leaq    input_data(%rip), %rdx  /* input_data */
> -       movl    $z_input_len, %ecx      /* input_len */
> +       movl    input_len(%rip), %ecx   /* input_len */
>         movq    %rbp, %r8               /* output target address */
> -       movl    $z_output_len, %r9d     /* decompressed length, end of relocs */
> +       movl    output_len(%rip), %r9d  /* decompressed length, end of relocs */
>         call    extract_kernel          /* returns kernel location in %rax */
>         popq    %rsi
>
> diff --git a/arch/x86/boot/compressed/mkpiggy.c b/arch/x86/boot/compressed/mkpiggy.c
> index 7e01248765b2..52aa56cdbacc 100644
> --- a/arch/x86/boot/compressed/mkpiggy.c
> +++ b/arch/x86/boot/compressed/mkpiggy.c
> @@ -60,6 +60,12 @@ int main(int argc, char *argv[])
>         printf(".incbin \"%s\"\n", argv[1]);
>         printf("input_data_end:\n");
>
> +       printf(".section \".rodata\",\"a\",@progbits\n");
> +       printf(".globl input_len\n");
> +       printf("input_len:\n\t.long %lu\n", ilen);
> +       printf(".globl output_len\n");
> +       printf("output_len:\n\t.long %lu\n", (unsigned long)olen);
> +
>         retval = 0;
>  bail:
>         if (f)
> --
> 2.25.1
>
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-- 
Thanks,
~Nick Desaulniers