Hi Daniel,

On Fri, Jul 29, 2016 at 06:10:02AM -0400, Daniel Micay wrote:

quoted

quoted

In the Project Zero Stagefright post
(http://googleprojectzero.blogspot.com/2015/09/stagefrightened.html)
, we see that the linear allocation of memory combined with the
low number of bits in the initial mmap offset resulted in a much
more predictable layout which aided the attacker. The initial
random mmap base range was increased by Daniel Cashman in
d07e22597d1d355829b7b18ac19afa912cf758d1, but we've done nothing
to address page relative attacks.

Inter-mmap randomization will decrease the predictability of later
mmap() allocations, which should help make data structures harder
to find in memory. In addition, this patch will also introduce
unmapped gaps between pages, preventing linear overruns from one
mapping to another another mapping. I am unable to quantify how
much this will improve security, but it should be > 0.

One person calls "unmapped gaps between pages" a feature, others
call it a mess. ;-)

It's very hard to quantify the benefits of fine-grained randomization,

?  N = # of possible addresses.  The bigger N is, the more chances the
attacker will trip up before finding what they were looking for.

but there are other useful guarantees you could provide. It would be
quite helpful for the kernel to expose the option to force a PROT_NONE
mapping after every allocation. The gaps should actually be enforced.

So perhaps 3 things, simply exposed as off-by-default sysctl options
(no need for special treatment on 32-bit):

I'm certainly not an mm-developer, but this looks to me like we're
pushing the work of creating efficient, random mappings out to
userspace.  :-/

a) configurable minimum gap size in pages (for protection against
linear and small {under,over}flows) b) configurable minimum gap size
based on a ratio to allocation size (for making the heap sparse to
mitigate heap sprays, especially when mixed with fine-grained
randomization - for example 2x would add a 2M gap after a 1M mapping)

mmm, this looks like an information leak.  Best to set a range of pages
and pick a random number within that range for each call.

c) configurable maximum random gap size (the random gap would be in
addition to the enforced minimums)

The randomization could just be considered an extra with minor
benefits rather than the whole feature. A full fine-grained
randomization implementation would need a higher-level form of
randomization than gaps in the kernel along with cooperation from
userspace allocators. This would make sense as one part of it though.

Ok, so here's an idea.  This idea could be used in conjunction with
random gaps, or on it's own.  It would be enhanced by userspace random
load order.

The benefit is that with 32bit address space, and no random gapping,
it's still not wasting much space.

Given a memory space, break it up into X bands such that there are 2*X
possible addresses.

  |A     B|C     D|E     F|G     H| ... |2*X-2  2*X-1|
  |--> <--|--> <--|--> <--|--> <--| ... |-->      <--|
min                                                  max

For each call to mmap, we randomly pick a value within [0 - 2*X).
Assuming A=0 in the diagram above, even values grow up, odd values grow
down.  Gradually consuming the single gap in the middle of each band.

How many bands to use would depend on:
  * 32/64bit
  * Average number of mmap calls
  * largest single mmap call usually seen
  * if using random gaps and range used

If the free gap in a chosen band is too small for the request, pick
again among the other bands.

Again, I'm not an mm dev, so I might be totally smoking crack on this
one...

thx,

Jason.

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