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

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Message-ID: <20160415090514.GA9314@gmail.com>
Date:	Fri, 15 Apr 2016 11:05:15 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Kees Cook <keescook@...omium.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,
	"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@...aani.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Young <dyoung@...hat.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


* Kees Cook <keescook@...omium.org> wrote:

> When INIT_SIZE is bigger than VO_INIT_SIZE (uncommon but possible),
> the copied ZO occupies the memory from extract_offset to the end of
> decompression buffer. It overlaps with the soon-to-be-uncompressed kernel
> like this:
> 
>                             |-----compressed kernel image------|
>                             V                                  V
> 0                       extract_offset                      +INIT_SIZE
> |-----------|---------------|-------------------------|--------|
>             |               |                         |        |
>           VO__text      startup_32 of ZO          VO__end    ZO__end
>             ^                                         ^
>             |-------uncompressed kernel image---------|
> 
> When INIT_SIZE is equal to VO_INIT_SIZE (likely) there's still space
> left from end of ZO to the end of decompressing buffer, like below.
> 
>                             |-compressed kernel image-|
>                             V                         V
> 0                       extract_offset                      +INIT_SIZE
> |-----------|---------------|-------------------------|--------|
>             |               |                         |        |
>           VO__text      startup_32 of ZO          ZO__end    VO__end
>             ^                                                  ^
>             |------------uncompressed kernel image-------------|
> 
> To simplify calculations and avoid special cases, it is cleaner to
> always place the compressed kernel image in memory so that ZO__end
> is at the end of the decompression buffer, instead of placing that
> start extract_offset as is currently done.

Btw., it would be nice to also put such a visualization (of the current layout of 
these values) into the code itself.

Thanks,

	Ingo