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

From: Daniel Cashman
Date: Wed Nov 04 2015 - 14:31:36 EST

On 11/3/15 5:31 PM, Andrew Morton wrote:
> On Tue, 03 Nov 2015 18:40:31 -0600 ebiederm@xxxxxxxxxxxx (Eric W. Biederman) wrote:
>> 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
>> something.
> Of course, but that's not really very useful.
>> We have seen in the last couple of months on Android how only having 8 bits
>> doesn't help much.
> Now THAT is important. What happened here and how well does the
> proposed fix improve things? How much longer will a brute-force attack
> take to succeed, with a particular set of kernel parameters? Is the
> new duration considered to be sufficiently long and if not, are there
> alternative fixes we should be looking at?
> Stuff like this.
>> Each additional bit doubles the protection (and unfortunately also
>> increases fragmentation of the userspace address space).
> OK, so the benefit comes with a cost and people who are configuring
> systems (and the people who are reviewing this patchset!) need to
> understand the tradeoffs. Please.

The direct motivation here was in response to the libstagefright
vulnerabilities that affected Android, specifically to information
provided by Google's project zero at:

The attack there specifically used the limited randomness used in
generating the mmap base address as part of a brute-force-based exploit.
In this particular case, the attack was against the mediaserver process
on Android, which was limited to respawning every 5 seconds, giving the
attacker an average expected success rate of defeating the mmap ASLR
after over 10 minutes (128 tries at 5 seconds each). With change to the
maximum proposed value of 16 bits, this would change to over 45 hours
(32768 tries), which would make the user of such a system much more
likely to notice such an attack.

I understand the desire for this clarification, and will happily try to
improve the explanation for this change, especially so that those
considering use of this option understand the tradeoffs, but I also view
this as one particular hardening change which is a component of making
attacks such as these harder, rather than the only solution. As for the
clarification itself, where would you like it? I could include a cover
letter for this patch-set, elaborate more in the commit message itself,
add more to the Kconfig help description, or some combination of the above.

Thank You,
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