Re: [PATCH] x86, mm: get ASLR work for hugetlb mappings

[Kirill A. Shutemov]

Dave Hansen wrote:
> On 10/22/2013 06:52 AM, Kirill A. Shutemov wrote:
> > Matthew noticed that hugetlb doesn't participate in ASLR on x86-64.
> > The reason is genereic hugetlb_get_unmapped_area() which is used on
> > x86-64. It doesn't support randomization and use bottom-up unmapped area
> > lookup, instead of usual top-down on x86-64.
> 
> I have to wonder if this was on purpose in order to keep the large and
> small mappings separate.  We don't *have* to keep them separate this, of
> course, but it makes me wonder.

I haven't seen any evidence that it's on purpose, but who knows...

In x86-specific hugetlb_get_unmapped_area() there's explicit check what is
mm->get_unmapped_area top-down or bottom-up, and doing the same.

> > x86 has arch-specific hugetlb_get_unmapped_area(), but it's used only on
> > x86-32.
> > 
> > Let's use arch-specific hugetlb_get_unmapped_area() on x86-64 too.
> > It fixes the issue and make hugetlb use top-down unmapped area lookup.
> 
> Shouldn't we fix the generic code instead of further specializing the
> x86 stuff?

For that we need to modify info.low_limit to mm->mmap_legacy_base (which
is x86 specific, no-go) or switch to top-down and set info.high_limit to
mm->mmap_base.

I don't know how it can affect other architectures.

> In any case, you probably also want to run this through: the
> libhugetlbfs tests:
> 
> http://sourceforge.net/p/libhugetlbfs/code/ci/master/tree/tests/

I've got the same fail list for upstream and patched kernel, so no
regression was found.

********** TEST SUMMARY  
*                      2M            
*                      32-bit 64-bit 
*     Total testcases:   107    110   
*             Skipped:     0      0   
*                PASS:    98    108   
*                FAIL:     2      2   
*    Killed by signal:     7      0   
*   Bad configuration:     0      0   
*       Expected FAIL:     0      0   
*     Unexpected PASS:     0      0   
* Strange test result:     0      0   
**********

-- 
 Kirill A. Shutemov
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