linux-kernel - Re: RW and executable hole in page tables on x86

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Date:	Fri, 25 Oct 2013 15:59:23 +0100
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Kees Cook <kees@...flux.net>
CC:	"Yan, Zheng" <zheng.z.yan@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: RW and executable hole in page tables on x86_64

On 10/25/2013 02:34 PM, Kees Cook wrote:
> Hi,
> 
> I've noticed there's a chunk of kernel memory still marked RW and x. See
> 0xffffffff82956000 below...
> 
> ---[ High Kernel Mapping ]---
> 0xffffffff80000000-0xffffffff81000000          16M                           pmd
> 0xffffffff81000000-0xffffffff81a00000          10M     ro         PSE GLB x  pmd
> 0xffffffff81a00000-0xffffffff81e00000           4M     ro         PSE GLB NX pmd
> 0xffffffff81e00000-0xffffffff82200000           4M     RW             GLB NX pte
> 0xffffffff82200000-0xffffffff82800000           6M     RW         PSE GLB NX pmd
> 0xffffffff82800000-0xffffffff82956000        1368K     RW             GLB NX pte
> 0xffffffff82956000-0xffffffff82a00000         680K     RW             GLB x  pte
> 0xffffffff82a00000-0xffffffffa0000000         470M                           pmd
> 
> HPA looked at it for a bit, but it wasn't obvious what was going on. It's
> after the end of bss. I do note that the two adjacent regions add up to
> 2MiB. Is this some kind of leftover mapping? What is this region? Is there
> a sensible place to clean it up?
> 

It looks to be what is left after the 2 MB page for bss is broken up.
It doesn't mean it isn't broken, though.

	-hpa


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