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SubjectRe: [PATCH 2/2] mm: restrict access to /proc/meminfo
On Thu, Sep 29, 2011 at 20:18 +0400, Vasiliy Kulikov wrote:
> On Tue, Sep 27, 2011 at 13:33 -0700, David Rientjes wrote:
> > I'd much rather just convert everything to use MB rather than KB so you
> > can't determine things at a page level. I think that gets us much closer
> > to what the patch is intending to restrict. But I also expect some
> > breakage from things that just expect meminfo to be in KB units without
> > parsing what the kernel is exporting.
>
> I'm not convinced with rounding the information to MBs. The attacker
> still may fill slabs with new objects to trigger new slab pages
> allocations. He will be able to see when this MB-granularity barrier is
> overrun thus seeing how many kbs there were before:
>
> old = new - filled_obj_size_sum
>
> As `new' is just increased, it means it is known with KB granularity,
> not MB. By counting used slab objects he learns filled_obj_size_sum.

s/filled_obj_size_sum/old/ of course.

> So, rounding gives us nothing, but obscurity.
>
> Thanks,

--
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments


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