On Wed, 18 Nov 2015 15:20:04 -0800 Daniel Cashman [off-list ref] wrote:

Address Space Layout Randomization (ASLR) provides a barrier to
exploitation of user-space processes in the presence of security
vulnerabilities by making it more difficult to find desired code/data
which could help an attack.  This is done by adding a random offset to the
location of regions in the process address space, with a greater range of
potential offset values corresponding to better protection/a larger
search-space for brute force, but also to greater potential for
fragmentation.

The offset added to the mmap_base address, which provides the basis for
the majority of the mappings for a process, is set once on process exec in
arch_pick_mmap_layout() and is done via hard-coded per-arch values, which
reflect, hopefully, the best compromise for all systems.  The trade-off
between increased entropy in the offset value generation and the
corresponding increased variability in address space fragmentation is not
absolute, however, and some platforms may tolerate higher amounts of
entropy.  This patch introduces both new Kconfig values and a sysctl
interface which may be used to change the amount of entropy used for
offset generation on a system.

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

http://googleprojectzero.blogspot.com/2015/09/stagefrightened.html

The attack presented therein, by Google's project zero, specifically
targeted the limited randomness used to generate the offset added to the
mmap_base address in order to craft a brute-force-based attack. 
Concretely, the attack was against the mediaserver process, which was
limited to respawning every 5 seconds, on an arm device.  The hard-coded 8
bits used resulted in an average expected success rate of defeating the
mmap ASLR after just over 10 minutes (128 tries at 5 seconds a piece). 
With this patch, and an accompanying increase in the entropy value to 16
bits, the same attack would take an average expected time of over 45 hours
(32768 tries), which makes it both less feasible and more likely to be
noticed.

The introduced Kconfig and sysctl options are limited by per-arch minimum
and maximum values, the minimum of which was chosen to match the current
hard-coded value and the maximum of which was chosen so as to give the
greatest flexibility without generating an invalid mmap_base address,
generally a 3-4 bits less than the number of bits in the user-space
accessible virtual address space.

When decided whether or not to change the default value, a system
developer should consider that mmap_base address could be placed anywhere
up to 2^(value) bits away from the non-randomized location, which would
introduce variable-sized areas above and below the mmap_base address such
that the maximum vm_area_struct size may be reduced, preventing very large
allocations. 

Nice, thanks.

mips, powerpc and s390 also implement arch_mmap_rnd().  Are there any
special considerations here, or it just a matter of maintainers wiring
it up and testing it?