linux-kernel - Re: [PATCH v2 01/15] x86, kaslr: Use init_size instead of run

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Message-ID: <CAE9FiQXzv7TPMRA+RqSdV0ERiW3KYA-2KOyDKBrZDn3Zd5RcrQ@mail.gmail.com>
Date:	Fri, 6 Mar 2015 10:44:29 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Matt Fleming <matt.fleming@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Jiri Kosina <jkosina@...e.cz>,
	Borislav Petkov <bp@...e.de>, Baoquan He <bhe@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Junjie Mao <eternal.n08@...il.com>
Subject: Re: [PATCH v2 01/15] x86, kaslr: Use init_size instead of run_size

On Fri, Mar 6, 2015 at 5:55 AM, Borislav Petkov <bp@...en8.de> 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.
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