Re: [PATCH 2.6.14-rc2-mm2] core remove PageReserved

[Nick Piggin <nickpiggin@yahoo.com.au>]

Hugh Dickins wrote:
> On Tue, 11 Oct 2005, Nick Piggin wrote:
>
> > Alan Cox wrote:
> >
> > > 32000 processes each with 2G mapped as zero pages appears to allow the
> > > refcount to overflow ?
> >
> > That's right (though I count only 8192 required with 4K page size) -
> > close to impossible on 32-bit architectures, though not so the 64-bit
> > ones, which still use 32-bits for count and mapcount.
>
>
> It needs 16GB of page table space to get the mapcount to go negative,
> or if we refine the atomic_add_negative test, 32GB of page table space
> to wrap (I'm assuming an 8-byte PAE page table entries for each unit
> of mapcount, since we're well beyond the 4GB non-PAE limit).
>
> Do we actually need to worry about i386 above 32GB in the x86_64 era?

Considering we already run on 64-bit systems with terabytes of
memory and people don't seem to be breaking down your door for
a fix (that I know of), I'd say no :)

> > I was a bit worried about this too, but Hugh didn't think it was a
> > really big a deal - I guess because the real solution for the refcount
> > overflow on 64-bit is to expand the refcount type.
>
>
> Yes, and I'm imagining some scheme of sharing _count and _mapcount in
> a single atomic64, since we don't want to expand struct page for this.
> Implement that shortly, unless we find a way to eliminate _mapcount
> instead.
>
> But Alan's overflow issue is not new, it's not brought on refcounting
> the ZERO_PAGE: sys_remap_file_pages already allows a file page to be
> mapped multiple times in single process, without the constriction of
> needing lots of vm_area_struct space.  ZERO_PAGE is just more obvious.

Right. As a security issue it is nothing new, though probably it will
be eaiser for big 64-bit systems to _unintentionally_ wrap the ZERO_PAGE
refcount.

--
SUSE Labs, Novell Inc.

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