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

From: Nick Desaulniers
Date: Fri Aug 07 2020 - 14:12:44 EST

On Fri, Jul 31, 2020 at 4:08 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> From: Arvind Sankar <nivedita@xxxxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Reviewed-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>
> Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
> Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> 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