llvm dev - Aug 2013 - [LLVMdev] Adding diversity for security (and testing)

Greetings LLVM Devs!

I am a PhD student in the Secure Systems and Software Lab at UC
Irvine. We have been working on adding randomness into code generation
to create a diverse population of binaries. This diversity prevents
code-reuse attacks such as return-oriented-programming (ROP) by
denying the attacker information about the exact code layout. ROP has
been used is several high-profile recent attacks, and has also been
used as a jailbreaking avenue. We believe our transformations would
provide a significant security benefit for LLVM users who choose to
use diversity. For more details see [1] (although we are currently
proposing to upstream only a simplified subset of our work).

We would like to contribute some of our work back to the community,
and are preparing a small patch adding two new features: NOP insertion
and schedule randomization. The NOP insertion pass randomly adds NOPs
after each MachineInstr according to a command-line
parameter. Currently NOP insertion is implemented for X86, and we are
adding support for ARM. The schedule randomizer randomly picks a valid
instruction to schedule at every point, bypassing the scheduling
heuristics. These passes result in a binary which, while slightly
slower, is far more secure against code-reuse attacks. In addition,
schedule randomization may be useful for randomized compiler and
micro-architecture testing.

We would also include a secure random number generator which links
against OpenSSL. This would of course be an optional module disabled
by default, but is necessary so the randomization is cryptographically
secure and useful in security applications.

We are in the process of writing test cases and double checking
formatting to produce a useful patch, but would like to solicit
feedback on our proposed changes before submitting patches for
detailed consideration.

Thanks,
Stephen Crane
Secure Systems and Software Lab
UC Irvine


[1] A. Homescu, S. Neisius, P. Larsen, S. Brunthaler, and M. Franz;
“Profile-guided Automated Software Diversity,” in 2013 International
Symposium on Code Generation and Optimization (CGO 2013), Shenzhen,
China; February 2013.

Hi Stephen, 
> Greetings LLVM Devs!
> 
> I am a PhD student in the Secure Systems and Software Lab at UC
> Irvine. We have been working on adding randomness into code generation
> to create a diverse population of binaries. This diversity prevents
> code-reuse attacks such as return-oriented-programming (ROP) by
> denying the attacker information about the exact code layout. ROP has
> been used is several high-profile recent attacks, and has also been
> used as a jailbreaking avenue. We believe our transformations would
> provide a significant security benefit for LLVM users who choose to
> use diversity. For more details see [1] (although we are currently
> proposing to upstream only a simplified subset of our work).
> 
I think that this is very interesting and I would like LLVM to have a
"randomness” feature.  I think that it is useful for other aspects of
security as well.
> We would like to contribute some of our work back to the community,
> and are preparing a small patch adding two new features: NOP insertion
> and schedule randomization. The NOP insertion pass randomly adds NOPs
> after each MachineInstr according to a command-line
> parameter. Currently NOP insertion is implemented for X86, and we are
> adding support for ARM.
Okay. 
> The schedule randomizer randomly picks a valid
> instruction to schedule at every point, bypassing the scheduling
> heuristics. These passes result in a binary which, while slightly
> slower, is far more secure against code-reuse attacks. In addition,
> schedule randomization may be useful for randomized compiler and
> micro-architecture testing.
> 
Which scheduler did you modify ?  The plan is to disable the SelectionDAG
scheduler and move to the MI Scheduler soon.

Also, have you looked at randomizing register-allocation ?
> We would also include a secure random number generator which links
> against OpenSSL. This would of course be an optional module disabled
> by default, but is necessary so the randomization is cryptographically
> secure and useful in security applications.
I am not sure why you need this feature.   You can provide LLVM with a SEED
value that can be controlled from the command line.  A wrapper (such as a
build-script) can control this value.
 
> 
> We are in the process of writing test cases and double checking
> formatting to produce a useful patch, but would like to solicit
> feedback on our proposed changes before submitting patches for
> detailed consideration.
Please make sure that the LLVM nightly test suite passes with randomization
enabled.
> 
> Thanks,
> Stephen Crane
> Secure Systems and Software Lab
> UC Irvine
> 
> 
> [1] A. Homescu, S. Neisius, P. Larsen, S. Brunthaler, and M. Franz;
> “Profile-guided Automated Software Diversity,” in 2013 International
> Symposium on Code Generation and Optimization (CGO 2013), Shenzhen,
> China; February 2013.
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hi Nadav,

Thanks for your interest!


On 08/26/2013 11:51 AM, Nadav Rotem wrote:
> Which scheduler did you modify ? The plan is to disable the 
> SelectionDAG scheduler and move to the MI Scheduler soon. Also, have 
> you looked at randomizing register-allocation ?
Yes, we modified the SelectionDAG scheduler. This was before the MI 
scheduler was around, but we will look into porting our ideas over to 
the new scheduler.

Register allocation randomization is in fact another of our existing 
transformations. We thought we would propose just a few simple 
transforms initially, but we can certainly include register 
randomization as well if there is enough interest.
>> We would also include a secure random number generator which links
>> against OpenSSL. This would of course be an optional module disabled
>> by default, but is necessary so the randomization is cryptographically
>> secure and useful in security applications.
> I am not sure why you need this feature.   You can provide LLVM with a SEED
value that can be controlled from the command line.  A wrapper (such as a
build-script) can control this value.
We do in fact seed the RNG with a command line parameter (we reuse the 
-frandom-seed param that gcc implemented). However, we need some 
reproducible, cryptographically secure source of randomness for the each 
random decision made during our transformations. We have found that the 
system randomness (/dev/random) is insufficient for this purpose since 
reproducible builds (given the secret seed) are preferable. The only way 
to provide this reproducible stream of randomness is to have a 
process-specific RNG, which we implement on top of OpenSSL for simplicity.
> Please make sure that the LLVM nightly test suite passes with 
> randomization enabled.
Of course. Our patched version currently passes the existing test suite 
on x86_64, and after adding additional tests we will certainly make sure 
that the final patch passes the latest test suite.


Thanks,
Stephen

On Aug 26, 2013, at 11:39 AM, Stephen Crane <sjcrane at uci.edu>
wrote:
> I am a PhD student in the Secure Systems and Software Lab at UC
> Irvine. We have been working on adding randomness into code generation
> to create a diverse population of binaries. This diversity prevents
> code-reuse attacks such as return-oriented-programming (ROP) by
> denying the attacker information about the exact code layout.
How is the "diverse population" of binaries generated and delivered?  
The tradition
software development model is to qualify one “golden master” which is then
duplicated to all customers.

-Nick

> > We would also include a secure random number generator which links
> > against OpenSSL. This would of course be an optional module disabled
> > by default, but is necessary so the randomization is cryptographically
> > secure and useful in security applications.
>
> I am not sure why you need this feature.   You can provide LLVM with a
> SEED value that can be controlled from the command line.  A wrapper (such
> as a build-script) can control this value.

(disclaimer:  I was a member of Stephen's research group and worked on this
project.)

To my knowledge, the pseudorandom number generator in LLVM is not
cryptographically secure.  In this use case, the intent is to make it
difficult to get the random number seed or the generator's state at any
point in the random number stream by looking at the output.  If the state
of the generator can be compromised, then an attacker can predict the
output of the generator by playing it forward.  If an attacker can play the
generator forward, then the attacker can reproduce the rest of the
executable, including the randomized components that are no longer random
to the attacker.  Reproducing the executable means that diversification
isn't going to work because the attacker can plan around it.

For reproducibility, such as for debugging, a pure software generator is a
good idea.  This also prevents blocking read operations in the generator,
slowing down the compiler.  Software generators can be optimized for speed.
 These are reasons to avoid /dev/random et al.

Personally, I think it is necessary to go for the strongest random number
generator possible.  Cryptographically secure pseudorandom number
generators have good properties that make them resistant to compromise.  In
the case of the generator I think Stephen is proposing -- a generator based
on running AES-128 in Counter Mode and described in one of NIST's documents
-- the security comes from strong crypto building blocks, while still
suitable for embedding in a  compiler.
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On Aug 26, 2013, at 2:39 PM, Stephen Crane <sjcrane at uci.edu> wrote:
> We have been working on adding randomness into code generation
> to create a diverse population of binaries. This diversity prevents
> code-reuse attacks such as return-oriented-programming (ROP) by
> denying the attacker information about the exact code layout. 
Putting on my security hat (as opposed to my lurking-on-compiler-mailing-lists
hat), note that artificial software heterogeneity doesn't actually prevent
ROP, it makes it harder in a qualitatively similar way to ASLR. With ASLR, the
attacker needs to discover a single memory address in order to construct a
return-oriented program. What you're proposing requires reading out more of
the address space.

A recent paper at Oakland proposed a "just-in-time code reuse" attack
that repeatedly uses a memory disclosure and JITs an attack based on the results
[1]. So while it does make the attackers job harder, it doesn't prevent such
attacks.

1. Kevin Z. Snow, Fabian Monrose, Lucas Davi, Alexandra Dmitrienko, Christopher
Liebchen, and Ahmad-Reza Sadeghi. Just-In-Time Code Resule: On the Effectiveness
of Find-Grained Address Space Layout Randomization. In Proceedings of the IEEE
Symposium on Security and Privacy ("Oakland") 2013.
<http://cs.unc.edu/~fabian/papers/oakland2013.pdf>

-- 
Stephen Checkoway

Hi Nick,

On 08/26/2013 02:01 PM, Nick Kledzik wrote:
> How is the "diverse population" of binaries generated and
delivered?
> The tradition software development model is to qualify one “golden 
> master” which is then duplicated to all customers. -Nick 
Yes indeed. Adding diversity at compilation requires that the code 
producer create a population of variants. However, by introducing 
diversity at compile-time, we have much greater freedom in transforming 
the end result with lower performance impact. In addition, producing 
variants during distribution allows the distributor to use diversity to 
provide a certain amount of watermarking and protection against 
client-side tampering (jailbreaking, etc).

We initially forsee that this could be especially used where security 
was of the utmost concern. Open-source end users often have the option 
of compiling from source, and could create their own diversified 
versions, especially for criticial services. We think this would be an 
ideal situation to begin adopting diversity for security.

Of course, in the testing use-case, creating various versions is fairly 
trivial. These versions could then be compared for testing for 
micro-architectural and compiler corner cases, as well as performance.

Finally, for wide-spread adoption, we are currently researching ways to 
cache the bulk of the compilation effort and create many variants for 
distribution in a cost-effective manner using cloud compilation. In 
addition, error reporting can be normalized with knowledge of the secret 
random seed by regeneration of the particular binary.

Hope this helps to explain our ideas in more clarity.

- stephen

On 08/26/2013 10:16 PM, Stephen Checkoway wrote:
> Putting on my security hat (as opposed to my 
> lurking-on-compiler-mailing-lists hat), note that artificial software 
> heterogeneity doesn't actually prevent ROP, it makes it harder in a 
> qualitatively similar way to ASLR. With ASLR, the attacker needs to 
> discover a single memory address in order to construct a 
> return-oriented program. What you're proposing requires reading out 
> more of the address space. A recent paper at Oakland proposed a 
> "just-in-time code reuse" attack that repeatedly uses a memory 
> disclosure and JITs an attack based on the results [1]. So while it 
> does make the attackers job harder, it doesn't prevent such attacks. 
> 1. Kevin Z. Snow, Fabian Monrose, Lucas Davi, Alexandra Dmitrienko, 
> Christopher Liebchen, and Ahmad-Reza Sadeghi. Just-In-Time Code 
> Resule: On the Effectiveness of Find-Grained Address Space Layout 
> Randomization. In Proceedings of the IEEE Symposium on Security and 
> Privacy ("Oakland") 2013. 
> <http://cs.unc.edu/~fabian/papers/oakland2013.pdf> 
Hi Stephen,

Great to see a fellow security researcher lurking on this list as well!

You raise a very valid point here. I quite agree that diversity is not 
guaranteed to prevent ROP. Very sorry -- I wasn't as careful in my 
original wording as I certainly should have been. In practice, we 
believe introducing variation significantly raises the bar for 
attackers, which is a big step in the right direction. For many 
applications this may be more than sufficient. However, diversity may 
not be good enough for all applications, particularly in the presence of 
possible remote memory disclosures.

- stephen

On 26 August 2013 11:39, Stephen Crane <sjcrane at uci.edu> wrote:
> Greetings LLVM Devs!
>
> I am a PhD student in the Secure Systems and Software Lab at UC
> Irvine. We have been working on adding randomness into code generation
> to create a diverse population of binaries. This diversity prevents
> code-reuse attacks such as return-oriented-programming (ROP) by
> denying the attacker information about the exact code layout. ROP has
> been used is several high-profile recent attacks, and has also been
> used as a jailbreaking avenue. We believe our transformations would
> provide a significant security benefit for LLVM users who choose to
> use diversity. For more details see [1] (although we are currently
> proposing to upstream only a simplified subset of our work).
>
> We would like to contribute some of our work back to the community,
> and are preparing a small patch adding two new features: NOP insertion
> and schedule randomization. The NOP insertion pass randomly adds NOPs
> after each MachineInstr according to a command-line
> parameter. Currently NOP insertion is implemented for X86, and we are
> adding support for ARM. The schedule randomizer randomly picks a valid
> instruction to schedule at every point, bypassing the scheduling
> heuristics. These passes result in a binary which, while slightly
> slower, is far more secure against code-reuse attacks. In addition,
> schedule randomization may be useful for randomized compiler and
> micro-architecture testing.
>
> We would also include a secure random number generator which links
> against OpenSSL. This would of course be an optional module disabled
> by default, but is necessary so the randomization is cryptographically
> secure and useful in security applications.
>
> We are in the process of writing test cases and double checking
> formatting to produce a useful patch, but would like to solicit
> feedback on our proposed changes before submitting patches for
> detailed consideration.
>
Thanks. This is really interesting -- it's hard to find good protections
against ROP attacks, and this is promising. However, I'm going to start
with the "bad news" first. I have a few questions as to whether this
is
useful for our (llvm's) users in the real world:

1. I'm concerned about the deployment problem. I realize that being in the
compiler means you can transform the program in more exciting ways, but it
gives you a much worse deployment story than something which modifies the
program on disk like "prelink".

2. Does this actually fill a gap in our protections? How do we ever get
into the situation where the user is able to deploy a ROP attack against
us, without tripping asan or ubsan or something caught by our warnings or
the static analyzer or any of the other protections offered by clang and
llvm? It may suffice that there exists a niche which can't afford the
performance penalty from asan or other things, but then we'll need to
discuss what the performance impact is.

3. Reproducible builds are a must. GCC has a -frandom-seed=text flag and
you should use that here too. Given the same random seed, the compiler
shall produce the same output.

Ultimately my concern here derives from the fact that I do use clang
warnings today and I do use asan and ubsan today. If this ROP-protection
were added, I don't immediately see how I would use it. Just being I'm
not
inside the target audience doesn't mean the target audience doesn't
exist,
but I am asking for a little justification.

And one issue for us, the llvm developers. If we're going to accept this
feature, own it and maintain it, how can we test it? We know how to test
for correctness and measure performance, we even understand what
protections -fstack-protector or ASAN offer and can write spot-tests for
them, but how can we measure the effectiveness of probability-based
ROP-protections? We don't need an answer to this right now, but over time
we'll be maintaining it, and it seems plausible that we could accept a
patch which accidentally diminishes the actual security provided (image us
maintaining a random number generator -- it's possible to add a weakness
which isn't easily noticed).

There's a "good news" side to this too. Over lunch I talked with
one of our
security guys, and he's excited. He tells me that diversity for
ROP-protection is entirely the way to go, and speculates that it ought to
be deployable by other vendors. Furthermore, we expect that rotating the
register allocation is going to be especially painful on the exploit
authors.

Looking forward to seeing patches!

Nick
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On 8/28/13 4:37 PM, Nick Lewycky wrote:
> On 26 August 2013 11:39, Stephen Crane <sjcrane at uci.edu 
> <mailto:sjcrane at uci.edu>> wrote:
>
>     Greetings LLVM Devs!
>
>     I am a PhD student in the Secure Systems and Software Lab at UC
>     Irvine. We have been working on adding randomness into code generation
>     to create a diverse population of binaries. This diversity prevents
>     code-reuse attacks such as return-oriented-programming (ROP) by
>     denying the attacker information about the exact code layout. ROP has
>     been used is several high-profile recent attacks, and has also been
>     used as a jailbreaking avenue. We believe our transformations would
>     provide a significant security benefit for LLVM users who choose to
>     use diversity. For more details see [1] (although we are currently
>     proposing to upstream only a simplified subset of our work).
>
>     We would like to contribute some of our work back to the community,
>     and are preparing a small patch adding two new features: NOP insertion
>     and schedule randomization. The NOP insertion pass randomly adds NOPs
>     after each MachineInstr according to a command-line
>     parameter. Currently NOP insertion is implemented for X86, and we are
>     adding support for ARM. The schedule randomizer randomly picks a valid
>     instruction to schedule at every point, bypassing the scheduling
>     heuristics. These passes result in a binary which, while slightly
>     slower, is far more secure against code-reuse attacks. In addition,
>     schedule randomization may be useful for randomized compiler and
>     micro-architecture testing.
>
>     We would also include a secure random number generator which links
>     against OpenSSL. This would of course be an optional module disabled
>     by default, but is necessary so the randomization is cryptographically
>     secure and useful in security applications.
>
>     We are in the process of writing test cases and double checking
>     formatting to produce a useful patch, but would like to solicit
>     feedback on our proposed changes before submitting patches for
>     detailed consideration.
>
>
> Thanks. This is really interesting -- it's hard to find good 
> protections against ROP attacks, and this is promising. However, I'm 
> going to start with the "bad news" first. I have a few questions
as to
> whether this is useful for our (llvm's) users in the real world:
>
> 1. I'm concerned about the deployment problem. I realize that being in 
> the compiler means you can transform the program in more exciting 
> ways, but it gives you a much worse deployment story than something 
> which modifies the program on disk like "prelink".
>
> 2. Does this actually fill a gap in our protections? How do we ever 
> get into the situation where the user is able to deploy a ROP attack 
> against us, without tripping asan or ubsan or something caught by our 
> warnings or the static analyzer or any of the other protections 
> offered by clang and llvm? It may suffice that there exists a niche 
> which can't afford the performance penalty from asan or other things, 
> but then we'll need to discuss what the performance impact is.
Tools like asan and ubsan don't detect ever type of problem.  From my 
understanding of asan, it only performs checks on loads and stores and 
delays reuse of virtual address space for memory allocations, so it is 
not guaranteed to catch all use-after-free and array bounds errors.  To 
catch everything, you would need to use a tool like Softbound + CETS.  
Even then, you need to exercise all the program paths which I suspect 
does not happen in practice.

Static analysis tools don't find every possible memory safety bug, 
either.  I believe it's theoretically impossible for them to do so.

Run-time protections are attractive because they can guarantee that some 
bad behavior does not happen in any execution of the program.
>
> 3. Reproducible builds are a must. GCC has a -frandom-seed=text flag 
> and you should use that here too. Given the same random seed, the 
> compiler shall produce the same output.
>
> Ultimately my concern here derives from the fact that I do use clang 
> warnings today and I do use asan and ubsan today. If this 
> ROP-protection were added, I don't immediately see how I would use it. 
> Just being I'm not inside the target audience doesn't mean the
target
> audience doesn't exist, but I am asking for a little justification.
>
> And one issue for us, the llvm developers. If we're going to accept 
> this feature, own it and maintain it, how can we test it? We know how 
> to test for correctness and measure performance, we even understand 
> what protections -fstack-protector or ASAN offer and can write 
> spot-tests for them, but how can we measure the effectiveness of 
> probability-based ROP-protections? We don't need an answer to this 
> right now, but over time we'll be maintaining it, and it seems 
> plausible that we could accept a patch which accidentally diminishes 
> the actual security provided (image us maintaining a random number 
> generator -- it's possible to add a weakness which isn't easily
noticed).


>
> There's a "good news" side to this too. Over lunch I talked
with one
> of our security guys, and he's excited. He tells me that diversity for 
> ROP-protection is entirely the way to go, and speculates that it ought 
> to be deployable by other vendors. Furthermore, we expect that 
> rotating the register allocation is going to be especially painful on 
> the exploit authors.
Is there a reason why your security guy thinks that diversity is a 
better protection than control-flow integrity (CFI)?  Recent work on CFI 
[1] indicates that even very conservative call graphs remove nearly all 
gadgets from the list of valid function targets (hence they cannot be 
used).  That implementation has an average overhead of 3.6% and a max 
overhead of 8.6%.  A more flexible implementation [2] that already works 
for LLVM has average overhead of 10%. There's more recent work on CFI 
(including at least one paper that uses LLVM) published this month at 
Usenix Security, but I haven't had time to read it yet.

Control-flow integrity should be effective in stopping ROP attacks, and 
we should be able to deploy it without using inter-procedural analysis 
(although inter-procedural analysis can improve the call graph results 
to make the program even more secure).

I think diversity is a nice thing to have to provide defense in depth, 
but I currently think that CFI will provide the most bang for the buck.

-- John T.

[1] http://www.cs.berkeley.edu/~dawnsong/papers/Oakland2013-CCFIR-CR.pdf
[2] http://www.eecs.harvard.edu/~greg/papers/cfiDataSandboxing.pdf
>
> Looking forward to seeing patches!
>
> Nick
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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On 08/28/2013 02:37 PM, Nick Lewycky wrote:
> 1. I'm concerned about the deployment problem. I realize that being in 
> the compiler means you can transform the program in more exciting 
> ways, but it gives you a much worse deployment story than something 
> which modifies the program on disk like "prelink".
Yes, definitely. Deployment is an issue which users will need to 
consider when deciding whether to use compiler-based diversity. However 
there are certainly use-cases which could benefit, especially in small 
deployments. Additionally, the benefits may outweigh the increase in 
deployment costs for larger deployments.
> 2. Does this actually fill a gap in our protections? How do we ever 
> get into the situation where the user is able to deploy a ROP attack 
> against us, without tripping asan or ubsan or something caught by our 
> warnings or the static analyzer or any of the other protections 
> offered by clang and llvm? It may suffice that there exists a niche 
> which can't afford the performance penalty from asan or other things, 
> but then we'll need to discuss what the performance impact is.
Briefly looking at ASAN again, I saw a performance penalty of 2x 
mentioned. Diversity could act as both defense in depth, and as a 
lower-impact defense for performance critical code.
> 3. Reproducible builds are a must. GCC has a -frandom-seed=text flag 
> and you should use that here too. Given the same random seed, the 
> compiler shall produce the same output.
Completely agree, and that's exactly what we have done.
> And one issue for us, the llvm developers. If we're going to accept 
> this feature, own it and maintain it, how can we test it? We know how 
> to test for correctness and measure performance, we even understand 
> what protections -fstack-protector or ASAN offer and can write 
> spot-tests for them, but how can we measure the effectiveness of 
> probability-based ROP-protections? We don't need an answer to this 
> right now, but over time we'll be maintaining it, and it seems 
> plausible that we could accept a patch which accidentally diminishes 
> the actual security provided (image us maintaining a random number 
> generator -- it's possible to add a weakness which isn't easily
noticed).
Unfortunately testing for security is a difficult problem. We have 
currently been evaluating our work by comparing a population of 
randomized variants to make sure that no gadgets are the same across 
variants. While this is reasonable for research evaluation, I'm not sure 
that would be practical (or useful) for the compiler test suite. To 
prevent regressions, I think the test suite will need to test at least 3 
major areas: making sure that RNG results are consistent and 
deterministic, testing that NOPs are actually inserted and scheduling 
decisions randomized, and verifying that different seeds result in 
entirely different binaries with approximately equal amounts of 
diversifications.

Speaking of maintenance, we plan to continue work on this in our lab. We 
can certainly maintain these features at least for the next year and a 
half, and possibly longer.
> There's a "good news" side to this too. Over lunch I talked
with one
> of our security guys, and he's excited. He tells me that diversity for 
> ROP-protection is entirely the way to go, and speculates that it ought 
> to be deployable by other vendors. Furthermore, we expect that 
> rotating the register allocation is going to be especially painful on 
> the exploit authors.
As mentioned, we have an implementation of register randomization, 
although it will require some reworking to be ready for prime-time. We 
can definitely look into getting that polished.

- stephen