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SubjectRe: RW and executable hole in page tables on x86_64
Hi Thomas,

On Fri, Nov 14, 2014 at 09:04:19PM +0100, Thomas Gleixner wrote:
> On Fri, 14 Nov 2014, Kees Cook wrote:
> > 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.
>
> mark_rodata_ro()
>
> set_memory_nx(rodata_start, (all_end - rodata_start) >> PAGE_SHIFT);
>
> all_end is _end which is the end of the the __brk section.
>
> That looks indeed like a 2MB page which is split by set_memory_nx().
>
> That reminder of the 2MB is a leftover from cleanup_highmap()
>
> * We limit the mappings to the region from _text to _brk_end. _brk_end
> * is rounded up to the 2MB boundary.
>
> So what you see is the reminder between _brk_end and the 2MB boundary.
>
> Now the simple solution is to round up all_end in mark_rodata_ro() to
> the 2MB boundary,

Thanks! I just managed to work these things out from Peter's earlier hints,
just before you sent this. Good to have my guess confirmed. :)

> but we should probably get rid of the mapping
> completely.
>
> Something like:
>
> end = roundup((unsigned long)all_end, PMD_SIZE) - 1;
>
> free_init_pages(all_end + 1, end)
>
> should do the trick.

Ah-ha! Okay, I'll send a new patch that does this. Thanks!

-Kees

--
Kees Cook @outflux.net


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