[RFC] ASLR Whitepaper and Candidate Final Patch

[Robert Watson]

On Sun, 20 Jul 2014, Shawn Webb wrote:

>> - It is yet undetermined what the performance effect will be, and it is not 
>> clear (but seems likely from past measurements) if there will be a 
>> performance hit even when ASLR is off. -Apparently there are applications 
>> that will segfault (?).
>
> So I have an old Dell Latitude E6500 that I bought at Defcon a year or
> so ago that I'm doing testing on. Even though it's quite an underpowered
> laptop, I'm running ZFS on it for BE support (in case one of our changes
> kills it). I'll run unixbench on it a few times to benchmark the ASLR
> patch. I'll test these three scenarios:
>    1) ASLR compiled in and enabled;
>    2) ASLR compiled in and disabled;
>    3) ASLR compiled out (GENERIC kernel).
>
> In each of these three scenarios, I'll have the kernel debugging features 
> (WITNESS, INVARIANTS, etc.) turned off to better simulate a production 
> system and to remove just one more variable in the tests.
>
> I'll run unixbench ten times under each scenario and I'll compute averages.
>
> Since this is an older laptop (and it's running ZFS), these tests will take 
> a couple days. I'll have an answer for you soon.

Hi Shawn:

Great news that this work is coming to fruition -- ASLR is long overdue.

Are you having any luck with performance measurements?  Unixbench seems like a 
good starting point, but I wonder if it would be useful to look, in 
particular, at memory-mapping intensive workloads that might be affected as a 
result of changes in kernel VM data-structure use, or greater fragmentation of 
the address space.  I'm not sure I have a specific application here in mind -- 
in the past I might have pointed out tools such as ElectricFence that tend to 
increase fragmentation themselves.

Also, could you say a little more about the effects that the change might have 
on transparent superpage use -- other than suitable alignment of large 
mappings, it's not clear to me what effect it might have.

I wonder if some equipment in the FreeBSD Netperf cluster might be used to 
help with performance characterisation -- in particular, very recent high-end 
server hardware, and also, lower-end embedded-style systems that have markedly 
different virtual-memory implementations in hardware and software.  Often 
those two classes of systems see markedly different performance-change 
characteristics as a result of greater cache-centrism and instruction-level 
parallelism in the higher-end designs that can mask increases in instruction 
count.

I think someone has already commented that Peter Holm's help might be 
enlisted; you have have seen his 'stress2' suite, which could help with 
stability testing.

Thanks,

Robert