Messages in this thread | | | Date | Fri, 14 Nov 2014 21:04:19 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: RW and executable hole in page tables on x86_64 |
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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, 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.
Thanks,
tglx
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