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

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Message-ID: <20180302204808.GA671@bombadil.infradead.org>
Date:   Fri, 2 Mar 2018 12:48:08 -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 Fri, Mar 02, 2018 at 11:30:28PM +0300, Ilya Smith wrote:
> This is a really good question. Lets think we choose address with random-length 
> guard hole. This length is limited by some configuration as you described. For 
> instance let it be 1MB. Now according to current implementation, we still may 
> fill this gap with small allocations with size less than 1MB. Attacker will 
> going to build attack base on this predictable behaviour - he jus need to spray 
> with 1 MB chunks (or less, with some expectation). This attack harder but not 
> impossible.

Ah, I didn't mean that.  I was thinking that we can change the
implementation to reserve 1-N pages after the end of the mapping.
So you can't map anything else in there, and any load/store into that
region will segfault.