linux-kernel - Re: [PATCH v3 16/19] x86, kaslr: Randomize physical and virtual address of kernel separately

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Message-ID: <20160309134009.GC2555@x1.redhat.com>
Date:	Wed, 9 Mar 2016 21:40:09 +0800
From:	Baoquan He <bhe@...hat.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Yinghai Lu <yinghai@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Vivek Goyal <vgoyal@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...nel.org>, lasse.collin@...aani.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Young <dyoung@...hat.com>
Subject: Re: [PATCH v3 16/19] x86, kaslr: Randomize physical and virtual
 address of kernel separately

On 03/08/16 at 10:24am, Kees Cook wrote:
> On Mon, Mar 7, 2016 at 9:34 PM, Baoquan He <bhe@...hat.com> wrote:
> >> It seems like CONFIG_RANDOMIZE_BASE_MAX_OFFSET should have been
> >> eliminated when the on-demand page table code was added. Once that was
> >> added, there's no physical max any more. And virtual randomization
> >> should have no max at all.
> > For physically random, yes, CONFIG_RANDOMIZE_BASE_MAX_OFFSET is not
> > needed anymore. But for virtually random,
> > CONFIG_RANDOMIZE_BASE_MAX_OFFSET is still mandatory since kernel text
> > mapping and kernel module mapping share the 2G virtual address space as
> > follows. Though kaslr increase kernel text mapping from 512M to 1G, it's
> > still limited, can't exceed CONFIG_RANDOMIZE_BASE_MAX_OFFSET.
> >
> > [0xffffffff80000000, 0xffffffffffffffff]
> >
> > But now as you suggested, I would like to change
> > CONFIG_RANDOMIZE_BASE_MAX_OFFSET to another name because it's only
> > valid for virtual randomization. A more specific name is better.
> 
> Yes, right, the virtual has a 1G max, but I meant that it doesn't need
> to be a CONFIG item any more. Physical can use physical memory max as
> its max, and virtual max can now be calculated from the existing text
> mapping size.

Got it, it should be KERNEL_IMAGE_SIZE instead, right?