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SubjectRe: RW and executable hole in page tables on x86_64
Continuing a thread from a year ago...

On Fri, Oct 25, 2013 at 03:59:23PM +0100, H. Peter Anvin wrote:
> 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.

It looks like the problem still exists:

---[ High Kernel Mapping ]---
0xffffffff80000000-0xffffffff9ca00000 458M pmd
0xffffffff9ca00000-0xffffffff9d200000 8M ro PSE GLB x pmd
0xffffffff9d200000-0xffffffff9d3f3000 1996K ro GLB x pte
0xffffffff9d3f3000-0xffffffff9d400000 52K ro x pte
0xffffffff9d400000-0xffffffff9d600000 2M ro PSE GLB NX pmd
0xffffffff9d600000-0xffffffff9d7e8000 1952K ro GLB NX pte
0xffffffff9d7e8000-0xffffffff9d800000 96K ro NX pte
0xffffffff9d800000-0xffffffff9d8ff000 1020K RW GLB NX pte
0xffffffff9d8ff000-0xffffffff9da2d000 1208K RW NX pte
0xffffffff9da2d000-0xffffffff9dc00000 1868K RW GLB NX pte
0xffffffff9dc00000-0xffffffff9e600000 10M RW PSE GLB NX pmd
0xffffffff9e600000-0xffffffff9e7f5000 2004K RW GLB NX pte
0xffffffff9e7f5000-0xffffffff9e800000 44K RW GLB x pte
0xffffffff9e800000-0xffffffffc0000000 536M pmd

Still seems to be the bss getting broken up. What is this left-over
memory used for? Any pointers to where it happens? I'd really like to
kill this area.

-Kees

--
Kees Cook @outflux.net


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