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Date: Wed, 28 Feb 2018 10:33:49 -0800
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [RFC PATCH] Randomization of address chosen by mmap.
Message-ID: <20180228183349.GA16336@bombadil.infradead.org>
References: <20180227131338.3699-1-blackzert@gmail.com>
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To: Ilya Smith <blackzert@gmail.com>
Cc: Kees Cook <keescook@chromium.org>, Andrew Morton <akpm@linux-foundation.org>, Dan Williams <dan.j.williams@intel.com>, Michal Hocko <mhocko@suse.com>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Jan Kara <jack@suse.cz>, Jerome Glisse <jglisse@redhat.com>, Hugh Dickins <hughd@google.com>, Helge Deller <deller@gmx.de>, Andrea Arcangeli <aarcange@redhat.com>, Oleg Nesterov <oleg@redhat.com>, Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, Kernel Hardening <kernel-hardening@lists.openwall.com>

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.
> > 
> 
> Ia??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?

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