linux-kernel - Re: [PATCH v5 04/21] x86, boot: Move compressed kernel to end of decompression buffer

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Date:	Mon, 18 Apr 2016 09:50:23 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Yinghai Lu <yinghai@...nel.org>, Baoquan He <bhe@...hat.com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Matt Redfearn <matt.redfearn@...tec.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Lasse Collin <lasse.collin@...aani.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Young <dyoung@...hat.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 04/21] x86, boot: Move compressed kernel to end of
 decompression buffer

On Fri, Apr 15, 2016 at 1:09 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Kees Cook <keescook@...omium.org> wrote:
>
>> From: Yinghai Lu <yinghai@...nel.org>
>>
>> This change makes later calculations about where the kernel is located
>> easier to reason about. To better understand this change, we must first
>> clarify what VO and ZO are. They were introduced in commits by hpa:
>>
>> 77d1a49 x86, boot: make symbols from the main vmlinux available
>> 37ba7ab x86, boot: make kernel_alignment adjustable; new bzImage fields
>>
>> Specifically:
>>
>> VO:
>> - uncompressed kernel image
>> - size: VO__end - VO__text ("VO_INIT_SIZE" define)
>>
>> ZO:
>> - bootable compressed kernel image (boot/compressed/vmlinux)
>> - head text + compressed kernel (VO and relocs table) + decompressor code
>> - size: ZO__end - ZO_startup_32 ("ZO_INIT_SIZE" define, though see below)
>>
>> The INIT_SIZE definition is used to find the larger of the two image sizes:
>>
>>  #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
>
> Please also harmonize all the prefixes, i.e. use VO__/ZO__ everywhere (rename
> things where necessary), instead of this mixed up VO_/ZO_/VO__/ZO__ mess.

I'm going to leave these as they are: they're auto-generated based on
various functions of interest:

boot/Makefile:sed-zoffset := -e 's/^\([0-9a-fA-F]*\) [ABCDGRSTVW]
\(startup_32\|startup_64\|efi32_stub_entry\|efi64_stub_entry\|efi_pe_entry\|input_data\|_end\|z_.*\)$$/\#define
ZO_\2 0x\1/p'

i.e. ZO__end is _end's location. ZO_input_data is input_data's
position. I think it would further complicate things if we tried to
consolidate consecutive "_"s, or if we eliminated the leading "_"
(ZOinput_data and ZO_end).

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security