Re: [PATCH] Don't allow the stack to grow into hugetlb reservedregions

[Hugh Dickins]

On Sun, 28 Jan 2007, Ken Chen wrote:
> 
> For ia64, the hugetlb address region is reserved at the top of user
> space address.  Stacks are below that region.  Throw in the mix, we
> have two stacks, one memory stack that grows down and one register
> stack backing store that grows up.  These two stacks are always in
> pair and grow towards each other. And lastly, we have virtual address
> holes in between regions.  It's just impossible to grow any of these
> two stacks into hugetlb region no matter how I played it.
> 
> So, AFAICS this bug doesn't apply to ia64 (and certainly not x86). The
> new check of is_hugepage_only_range() is really a noop for both arches.

Certainly not a problem on x86.

But, never mind hugetlb, you still not quite convinced me that there's
no problem at all with get_user_pages find_extend_vma growing on ia64.

I repeat that ia64_do_page_fault has REGION tests to guard against
expanding either kind of stack across into another region.  ia64_brk,
ia64_mmap_check and arch_get_unmapped_area have RGN_MAP_LIMIT checks.
But where is the equivalent paranoia when ptrace calls get_user_pages
calls find_extend_vma?

If your usual stacks face each other across the same region, they're
not going to pose problem.  But what if someone mmaps MAP_GROWSDOWN
near the base of a region, then uses ptrace to touch an address near
the top of the region below?

Hugh
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