linux-kernel - Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap

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Message-ID: <87k2pyppfk.fsf@x220.int.ebiederm.org>
Date:	Tue, 03 Nov 2015 18:40:31 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Daniel Cashman <dcashman@...roid.com>,
	linux-kernel@...r.kernel.org, linux@....linux.org.uk,
	keescook@...omium.org, mingo@...nel.org,
	linux-arm-kernel@...ts.infradead.org, corbet@....net,
	dzickus@...hat.com, xypron.glpk@....de, jpoimboe@...hat.com,
	kirill.shutemov@...ux.intel.com, n-horiguchi@...jp.nec.com,
	aarcange@...hat.com, mgorman@...e.de, tglx@...utronix.de,
	rientjes@...gle.com, linux-mm@...ck.org, linux-doc@...r.kernel.org,
	salyzyn@...roid.com, jeffv@...gle.com, nnk@...gle.com,
	dcashman <dcashman@...gle.com>
Subject: Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR.

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Tue,  3 Nov 2015 10:10:03 -0800 Daniel Cashman <dcashman@...roid.com> wrote:
>
>> ASLR currently only uses 8 bits to generate the random offset for the
>> mmap base address on 32 bit architectures. This value was chosen to
>> prevent a poorly chosen value from dividing the address space in such
>> a way as to prevent large allocations. This may not be an issue on all
>> platforms. Allow the specification of a minimum number of bits so that
>> platforms desiring greater ASLR protection may determine where to place
>> the trade-off.
>
> Can we please include a very good description of the motivation for this
> change?  What is inadequate about the current code, what value does the
> enhancement have to our users, what real-world problems are being solved,
> etc.
>
> Because all we have at present is "greater ASLR protection", which doesn't
> really tell anyone anything.

The description seemed clear to me.

More random bits, more entropy, more work needed to brute force.

8 bits only requires 256 tries (or a 1 in 256) chance to brute force
something.

We have seen in the last couple of months on Android how only having 8 bits
doesn't help much.

Each additional bit doubles the protection (and unfortunately also
increases fragmentation of the userspace address space).

Eric

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