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Date:   Thu, 3 Dec 2020 09:42:54 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Topi Miettinen <toiwoton@...il.com>,
        linux-hardening@...r.kernel.org, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Rapoport <rppt@...nel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v5] mm: Optional full ASLR for mmap(), mremap(), vdso and stack


> On Dec 3, 2020, at 9:29 AM, Florian Weimer <fweimer@...hat.com> wrote:
> 
> * Andy Lutomirski:
> 
>> If you want a 4GB allocation to succeed, you can only divide the
>> address space into 32k fragments.  Or, a little more precisely, if you
>> want a randomly selected 4GB region to be empty, any other allocation
>> has a 1/32k chance of being in the way.  (Rough numbers — I’m ignoring
>> effects of the beginning and end of the address space, and I’m
>> ignoring the size of a potential conflicting allocation.).
> 
> I think the probability distribution is way more advantageous than that
> because it is unlikely that 32K allocations are all exactly spaced 4 GB
> apart.  (And with 32K allocations, you are close to the VMA limit anyway.)

I’m assuming the naive algorithm of choosing an address and trying it.  Actually looking for a big enough gap would be more reliable.

I suspect that something much more clever could be done in which the heap is divided up into a few independently randomized sections and heap pages are randomized within the sections might do much better. There should certainly be a lot of room for something between what we have now and a fully randomized scheme.

It might also be worth looking at what other OSes do.

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