Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR.

From: Eric W. Biederman
Date: Tue Nov 03 2015 - 19:49:34 EST

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Tue, 3 Nov 2015 10:10:03 -0800 Daniel Cashman <dcashman@xxxxxxxxxxx> 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

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).


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