linux-kernel - Re: [RFC PATCH] Randomization of address chosen by mmap.

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Date:   Wed, 28 Feb 2018 10:33:49 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Ilya Smith <blackzert@...il.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Michal Hocko <mhocko@...e.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jan Kara <jack@...e.cz>, Jerome Glisse <jglisse@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, Helge Deller <deller@....de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC PATCH] Randomization of address chosen by mmap.

On Wed, Feb 28, 2018 at 08:13:00PM +0300, Ilya Smith wrote:
> > It would be worth spelling out the "not recommended" bit some more
> > too: this fragments the mmap space, which has some serious issues on
> > smaller address spaces if you get into a situation where you cannot
> > allocate a hole large enough between the other allocations.
> > 
> 
> I’m agree, that's the point.

Would it be worth randomising the address returned just ever so slightly?
ie instead of allocating exactly the next address, put in a guard hole
of (configurable, by default maybe) 1-15 pages?  Is that enough extra
entropy to foil an interesting number of attacks, or do we need the full
randomise-the-address-space approach in order to be useful?