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

From: Kees Cook
Date: Fri Mar 06 2015 - 14:56:57 EST

On Fri, Mar 6, 2015 at 11:28 AM, Yinghai Lu <yinghai@xxxxxxxxxx> wrote:
> On Fri, Mar 6, 2015 at 10:55 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Fri, Mar 6, 2015 at 10:44 AM, Yinghai Lu <yinghai@xxxxxxxxxx> wrote:
>> 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. :)
> VO (vlinux) init size aka VO_INIT_SIZE already inlude that.
> Please check update version.
> 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)
> | #else
> | #endif
> | init_size: .long INIT_SIZE # kernel initialization size

Okay, I've proven this to myself now. :) I think it would be valuable
to call out that brk and bss are included in the _end calculation. For

$ objdump -h vmlinux | egrep '\.(bss|brk)'
25 .bss 00da7000 ffffffff82436000 0000000002436000 01836000 2**12
26 .brk 00026000 ffffffff831dd000 00000000031dd000 01836000 2**0
$ nm vmlinux | egrep ' _(text|end)'
ffffffff83203000 B _end
ffffffff81000000 T _text
$ objdump -h vmlinux | bash arch/x86/tools/
$ bc
81000000 + 2603000

So, _end - _text does equal _text + bss offset + bss size + brk size

Thanks! It'll be nice to lose the run_size hack. Adding some
documentation to the code here would help others in the future trying
to find this value, I think. :)


> 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 should 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 buffer is bigger
> enough for decompress at first. So use init_size instead, and kill not
> needed run_size related code.

Kees Cook
Chrome OS Security
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