Re: [PATCH v3 02/19] x86, boot: Move compressed kernel to end of buffer before decompressing

[Kees Cook]

On Fri, Mar 4, 2016 at 8:25 AM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> From: Yinghai Lu <yinghai@xxxxxxxxxx>
>
> For better understanding clarify what are VO and ZO firstly. They are
> introduced in below commits by hpa.
> 77d1a49 x86, boot: make symbols from the main vmlinux available
> 37ba7ab x86, boot: make kernel_alignment adjustable; new bzImage fields
>
> VO:
> - uncompressed kernel
> - size: VO__end - VO__text
>
> ZO:
> - boot/compressed/vmlinux
> - head text + compressed (VO+relocs) + decompressor code
> - size: ZO__end - ZO_startup_32
>
> And also copy INIT_SIZE definition here for reference. From below macros
> we can see INIT_SIZE is either bigger than VO_INIT_SIZE or equal to
> VO_INIT_SIZE.
>
>  #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
>
> Current code uses extract_offset to decide the copied ZO position, it
> put ZO starting from extract_offset. When INIT_SIZE is bigger than
> VO_INIT_SIZE copied ZO occupies space from extract_offset to the end
> of decompressing buffer, just like below.
>
> 0                       extract_offset                       init_size
> |-----------|---------------|------------------------|---------|
>             |               |                        |         |
>           VO__text      startup_32 of ZO         VO__end     ZO__end
>
> However when INIT_SIZE is euqal to VO_INIT_SIZE there's still space left

typo: "equal"

> from end of ZO to the end of decompressing buffer, like below.
>
> 0                       extract_offset                       init_size
> |-----------|---------------|------------------------|---------|
>             |               |                        |         |
>           VO__text      startup_32 of ZO         ZO__end     VO__end
>
> It's better to always put ZO right in the end of decompressing buffer.
> Anyway, who doesn't like it to be better.

It looks to me like it's just cleaner to always have ZO__end ==
VO__end? Is that the motivation for moving it? What problem is solved
with this change?

-Kees

>
> So in this patch add BP_init_size into asm-offsets.c to make it visible
> to asm modules. And find the starting position of copied ZO by init_size
> and ZO run size, namely (ZO__end - startup_32) when move ZO.
>
> Signed-off-by: Yinghai Lu <yinghai@xxxxxxxxxx>
> ---
> v2->v3:
>     Adjust the patch log.
>
>  arch/x86/boot/compressed/head_32.S     | 11 +++++++++--
>  arch/x86/boot/compressed/head_64.S     |  8 ++++++--
>  arch/x86/boot/compressed/mkpiggy.c     |  3 ---
>  arch/x86/boot/compressed/vmlinux.lds.S |  1 +
>  arch/x86/kernel/asm-offsets.c          |  1 +
>  arch/x86/kernel/vmlinux.lds.S          |  1 +
>  6 files changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
> index 8ef964d..0c140f9 100644
> --- a/arch/x86/boot/compressed/head_32.S
> +++ b/arch/x86/boot/compressed/head_32.S
> @@ -148,7 +148,9 @@ preferred_addr:
>  1:
>
>         /* Target address to relocate to for decompression */
> -       addl    $z_extract_offset, %ebx
> +       movl    BP_init_size(%esi), %eax
> +       subl    $_end, %eax
> +       addl    %eax, %ebx
>
>         /* Set up the stack */
>         leal    boot_stack_end(%ebx), %esp
> @@ -210,8 +212,13 @@ relocated:
>                                 /* 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 */
> -       leal    z_extract_offset_negative(%ebx), %ebp
> +
> +       movl    BP_init_size(%esi), %eax
> +       subl    $_end, %eax
> +       movl    %ebx, %ebp
> +       subl    %eax, %ebp
>         pushl   %ebp            /* output address */
> +
>         pushl   $z_input_len    /* input_len */
>         leal    input_data(%ebx), %eax
>         pushl   %eax            /* input_data */
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index b0c0d16..67dd8d3 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -102,7 +102,9 @@ ENTRY(startup_32)
>  1:
>
>         /* Target address to relocate to for decompression */
> -       addl    $z_extract_offset, %ebx
> +       movl    BP_init_size(%esi), %eax
> +       subl    $_end, %eax
> +       addl    %eax, %ebx
>
>  /*
>   * Prepare for entering 64 bit mode
> @@ -330,7 +332,9 @@ preferred_addr:
>  1:
>
>         /* Target address to relocate to for decompression */
> -       leaq    z_extract_offset(%rbp), %rbx
> +       movl    BP_init_size(%rsi), %ebx
> +       subl    $_end, %ebx
> +       addq    %rbp, %rbx
>
>         /* Set up the stack */
>         leaq    boot_stack_end(%rbx), %rsp
> diff --git a/arch/x86/boot/compressed/mkpiggy.c b/arch/x86/boot/compressed/mkpiggy.c
> index d8222f2..b980046 100644
> --- a/arch/x86/boot/compressed/mkpiggy.c
> +++ b/arch/x86/boot/compressed/mkpiggy.c
> @@ -85,9 +85,6 @@ int main(int argc, char *argv[])
>         printf("z_output_len = %lu\n", (unsigned long)olen);
>         printf(".globl z_extract_offset\n");
>         printf("z_extract_offset = 0x%lx\n", offs);
> -       /* 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);
>
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index 34d047c..e24e0a0 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -70,5 +70,6 @@ SECTIONS
>                 _epgtable = . ;
>         }
>  #endif
> +       . = ALIGN(PAGE_SIZE);   /* keep ZO size page aligned */
>         _end = .;
>  }
> diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
> index 84a7524..506de14 100644
> --- a/arch/x86/kernel/asm-offsets.c
> +++ b/arch/x86/kernel/asm-offsets.c
> @@ -81,6 +81,7 @@ void common(void) {
>         OFFSET(BP_hardware_subarch, boot_params, hdr.hardware_subarch);
>         OFFSET(BP_version, boot_params, hdr.version);
>         OFFSET(BP_kernel_alignment, boot_params, hdr.kernel_alignment);
> +       OFFSET(BP_init_size, boot_params, hdr.init_size);
>         OFFSET(BP_pref_address, boot_params, hdr.pref_address);
>         OFFSET(BP_code32_start, boot_params, hdr.code32_start);
>
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 74e4bf1..217d5b1 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -325,6 +325,7 @@ SECTIONS
>                 __brk_limit = .;
>         }
>
> +       . = ALIGN(PAGE_SIZE);           /* keep VO_INIT_SIZE page aligned */
>         _end = .;
>
>          STABS_DEBUG
> --
> 2.5.0
>



-- 
Kees Cook
Chrome OS & Brillo Security