Re: [PATCH v2 01/15] x86, kaslr: Use init_size instead of run_size

[Kees Cook]

On Fri, Mar 6, 2015 at 10:44 AM, Yinghai Lu <yinghai@xxxxxxxxxx> wrote:
> On Fri, Mar 6, 2015 at 5:55 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
>> On Wed, Mar 04, 2015 at 12:00:34AM -0800, Yinghai Lu wrote:
>>> commit e6023367d779 ("x86, kaslr: Prevent .bss from overlaping initrd")
>>>
>>> introduced one run_size for kaslr.
>>>
>>> We do not need to have home grown run_size.
>>>
>>> We should use real runtime size (include copy/decompress) aka init_size
>>
>> Why?
>
> New change log:
>
> Subject: [PATCH] x86, kaslr: Use init_size instead of run_size
>
> 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 size of VO (vmlinux).
> 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.
> 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 [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 buffer is bigger
> enough for decompress at first. So use init_size instead, and kill not
> needed run_size related code.

I don't see how bss and brk are related to these sizes. Can you
explain how bss, brk, and initrd factor into these sizes? Those were
what run_size was created to represent. I don't want to accidentally
start stomping on bss and brk again. :)

-Kees

-- 
Kees Cook
Chrome OS Security
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