Re: RW and executable hole in page tables on x86_64

From: H. Peter Anvin
Date: Fri Oct 25 2013 - 10:59:55 EST

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.


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