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

Yinghai Lu
2015-03-06T19:28:31Z

On Fri, Mar 6, 2015 at 10:55 AM, Kees Cook <keescook@chromium.org> wrote:
> On Fri, Mar 6, 2015 at 10:44 AM, Yinghai Lu <yinghai@kernel.org> 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)
| #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 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.