Re: [PATCH v3 1/7] x86, kaslr: Use init_size instead of run_size

[Kees Cook <keescook@chromium.org>]

On Sat, Mar 7, 2015 at 2:07 PM, Yinghai Lu <yinghai@kernel.org> wrote:
> commit e6023367d779 ("x86, kaslr: Prevent .bss from overlaping initrd")
> introduced one run_size for kaslr.
> We should use real runtime size (include copy/decompress) aka init_size.
>
> run_size is VO (vmlinux) init size include bss and brk.
> init_size is the size needed for decompress and it is bigger than run_size
> when decompress need more buff.
>
> According to arch/x86/boot/header.S:
> | #define ZO_INIT_SIZE    (ZO__end - ZO_startup_32 + ZO_z_extract_offset)
> | #define VO_INIT_SIZE    (VO__end - VO__text)
> | #if ZO_INIT_SIZE > VO_INIT_SIZE
> | #define INIT_SIZE ZO_INIT_SIZE
> | #else
> | #define INIT_SIZE VO_INIT_SIZE
> | #endif
> | init_size:              .long INIT_SIZE         # kernel initialization size
>
> Bootloader allocate buffer according to init_size in hdr, and load the
> ZO (arch/x86/boot/compressed/vmlinux) from start of that buffer.
> init_size first come from VO (vmlinux) init size. That VO init size
> is from VO _end to VO _end and include VO bss and brk area.
>
> During running of ZO, ZO move itself to the middle of buffer at
> z_extract_offset to make sure that decompressor would not have output
> overwrite input data before input data get consumed.
> But z_extract_offset calculating is based on size of VO (vmlinux) and size
> of compressed VO only at first.
> So need to make sure [z_extra_offset, init_size) will fit ZO, that means
> init_size need to be adjusted according to ZO size.
> That make init_size is always >= run_size.
>
> During aslr buffer searching, we need to make sure the new buffer is big
> enough for decompress at first. So use init_size instead, and kill not
> needed run_size related code.
>
> Fixes: e6023367d779 ("x86, kaslr: Prevent .bss from overlaping initrd")
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Matt Fleming <matt.fleming@intel.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Cc: Junjie Mao <eternal.n08@gmail.com>
> Signed-off-by: Yinghai Lu <yinghai@kernel.org>

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

> ---
>  arch/x86/boot/compressed/Makefile  |  4 +---
>  arch/x86/boot/compressed/head_32.S |  5 ++---
>  arch/x86/boot/compressed/head_64.S |  5 +----
>  arch/x86/boot/compressed/misc.c    | 15 +++++++-------
>  arch/x86/boot/compressed/mkpiggy.c |  9 ++------
>  arch/x86/tools/calc_run_size.sh    | 42 --------------------------------------
>  6 files changed, 13 insertions(+), 67 deletions(-)
>  delete mode 100644 arch/x86/tools/calc_run_size.sh
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 0a291cd..70cc92c 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -92,10 +92,8 @@ suffix-$(CONFIG_KERNEL_XZ)   := xz
>  suffix-$(CONFIG_KERNEL_LZO)    := lzo
>  suffix-$(CONFIG_KERNEL_LZ4)    := lz4
>
> -RUN_SIZE = $(shell $(OBJDUMP) -h vmlinux | \
> -            $(CONFIG_SHELL) $(srctree)/arch/x86/tools/calc_run_size.sh)
>  quiet_cmd_mkpiggy = MKPIGGY $@
> -      cmd_mkpiggy = $(obj)/mkpiggy $< $(RUN_SIZE) > $@ || ( rm -f $@ ; false )
> +      cmd_mkpiggy = $(obj)/mkpiggy $< > $@ || ( rm -f $@ ; false )
>
>  targets += piggy.S
>  $(obj)/piggy.S: $(obj)/vmlinux.bin.$(suffix-y) $(obj)/mkpiggy FORCE
> diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
> index 1d7fbbc..cbed140 100644
> --- a/arch/x86/boot/compressed/head_32.S
> +++ b/arch/x86/boot/compressed/head_32.S
> @@ -207,8 +207,7 @@ relocated:
>   * Do the decompression, and jump to the new kernel..
>   */
>                                 /* push arguments for decompress_kernel: */
> -       pushl   $z_run_size     /* size of kernel with .bss and .brk */
> -       pushl   $z_output_len   /* decompressed length, end of relocs */
> +       pushl   $z_output_len   /* decompressed length */
>         leal    z_extract_offset_negative(%ebx), %ebp
>         pushl   %ebp            /* output address */
>         pushl   $z_input_len    /* input_len */
> @@ -218,7 +217,7 @@ relocated:
>         pushl   %eax            /* heap area */
>         pushl   %esi            /* real mode pointer */
>         call    decompress_kernel /* returns kernel location in %eax */
> -       addl    $28, %esp
> +       addl    $24, %esp
>
>  /*
>   * Jump to the decompressed kernel.
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 6b1766c..2884e0c 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -402,16 +402,13 @@ relocated:
>   * Do the decompression, and jump to the new kernel..
>   */
>         pushq   %rsi                    /* Save the real mode argument */
> -       movq    $z_run_size, %r9        /* size of kernel with .bss and .brk */
> -       pushq   %r9
>         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 */
>         movq    %rbp, %r8               /* output target address */
> -       movq    $z_output_len, %r9      /* decompressed length, end of relocs */
> +       movq    $z_output_len, %r9      /* decompressed length */
>         call    decompress_kernel       /* returns kernel location in %rax */
> -       popq    %r9
>         popq    %rsi
>
>  /*
> diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> index 5903089..51e9e54 100644
> --- a/arch/x86/boot/compressed/misc.c
> +++ b/arch/x86/boot/compressed/misc.c
> @@ -370,10 +370,10 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
>                                   unsigned char *input_data,
>                                   unsigned long input_len,
>                                   unsigned char *output,
> -                                 unsigned long output_len,
> -                                 unsigned long run_size)
> +                                 unsigned long output_len)
>  {
>         unsigned char *output_orig = output;
> +       unsigned long init_size;
>
>         real_mode = rmode;
>
> @@ -396,15 +396,14 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
>         free_mem_ptr     = heap;        /* Heap */
>         free_mem_end_ptr = heap + BOOT_HEAP_SIZE;
>
> +       init_size = real_mode->hdr.init_size;
> +
>         /*
> -        * The memory hole needed for the kernel is the larger of either
> -        * the entire decompressed kernel plus relocation table, or the
> -        * entire decompressed kernel plus .bss and .brk sections.
> +        * The memory hole needed for the kernel is init_size for running
> +        * and init_size is bigger than output_len always.
>          */
>         output = choose_kernel_location(real_mode, input_data, input_len,
> -                                       output,
> -                                       output_len > run_size ? output_len
> -                                                             : run_size);
> +                                       output, init_size);
>
>         /* Validate memory location choices. */
>         if ((unsigned long)output & (MIN_KERNEL_ALIGN - 1))
> diff --git a/arch/x86/boot/compressed/mkpiggy.c b/arch/x86/boot/compressed/mkpiggy.c
> index d8222f2..b669ab6 100644
> --- a/arch/x86/boot/compressed/mkpiggy.c
> +++ b/arch/x86/boot/compressed/mkpiggy.c
> @@ -36,13 +36,11 @@ int main(int argc, char *argv[])
>         uint32_t olen;
>         long ilen;
>         unsigned long offs;
> -       unsigned long run_size;
>         FILE *f = NULL;
>         int retval = 1;
>
> -       if (argc < 3) {
> -               fprintf(stderr, "Usage: %s compressed_file run_size\n",
> -                               argv[0]);
> +       if (argc < 2) {
> +               fprintf(stderr, "Usage: %s compressed_file\n", argv[0]);
>                 goto bail;
>         }
>
> @@ -76,7 +74,6 @@ int main(int argc, char *argv[])
>         offs += olen >> 12;     /* Add 8 bytes for each 32K block */
>         offs += 64*1024 + 128;  /* Add 64K + 128 bytes slack */
>         offs = (offs+4095) & ~4095; /* Round to a 4K boundary */
> -       run_size = atoi(argv[2]);
>
>         printf(".section \".rodata..compressed\",\"a\",@progbits\n");
>         printf(".globl z_input_len\n");
> @@ -88,8 +85,6 @@ int main(int argc, char *argv[])
>         /* z_extract_offset_negative allows simplification of head_32.S */
>         printf(".globl z_extract_offset_negative\n");
>         printf("z_extract_offset_negative = -0x%lx\n", offs);
> -       printf(".globl z_run_size\n");
> -       printf("z_run_size = %lu\n", run_size);
>
>         printf(".globl input_data, input_data_end\n");
>         printf("input_data:\n");
> diff --git a/arch/x86/tools/calc_run_size.sh b/arch/x86/tools/calc_run_size.sh
> deleted file mode 100644
> index 1a4c17b..0000000
> --- a/arch/x86/tools/calc_run_size.sh
> +++ /dev/null
> @@ -1,42 +0,0 @@
> -#!/bin/sh
> -#
> -# Calculate the amount of space needed to run the kernel, including room for
> -# the .bss and .brk sections.
> -#
> -# Usage:
> -# objdump -h a.out | sh calc_run_size.sh
> -
> -NUM='\([0-9a-fA-F]*[ \t]*\)'
> -OUT=$(sed -n 's/^[ \t0-9]*.b[sr][sk][ \t]*'"$NUM$NUM$NUM$NUM"'.*/\1\4/p')
> -if [ -z "$OUT" ] ; then
> -       echo "Never found .bss or .brk file offset" >&2
> -       exit 1
> -fi
> -
> -OUT=$(echo ${OUT# })
> -sizeA=$(printf "%d" 0x${OUT%% *})
> -OUT=${OUT#* }
> -offsetA=$(printf "%d" 0x${OUT%% *})
> -OUT=${OUT#* }
> -sizeB=$(printf "%d" 0x${OUT%% *})
> -OUT=${OUT#* }
> -offsetB=$(printf "%d" 0x${OUT%% *})
> -
> -run_size=$(( $offsetA + $sizeA + $sizeB ))
> -
> -# BFD linker shows the same file offset in ELF.
> -if [ "$offsetA" -ne "$offsetB" ] ; then
> -       # Gold linker shows them as consecutive.
> -       endB=$(( $offsetB + $sizeB ))
> -       if [ "$endB" != "$run_size" ] ; then
> -               printf "sizeA: 0x%x\n" $sizeA >&2
> -               printf "offsetA: 0x%x\n" $offsetA >&2
> -               printf "sizeB: 0x%x\n" $sizeB >&2
> -               printf "offsetB: 0x%x\n" $offsetB >&2
> -               echo ".bss and .brk are non-contiguous" >&2
> -               exit 1
> -       fi
> -fi
> -
> -printf "%d\n" $run_size
> -exit 0
> --
> 1.8.4.5
>



-- 
Kees Cook
Chrome OS Security