Friday, 10 June 2016

Are Dynamic Control-Flow Integrity Schemes Worth Deploying?

Most exploits against C/C++ code today rely on hijacking CPU-level control flow to execute the attacker's code. Researchers have developed schemes to defeat such attacks based on the idea of control flow integrity: characterize a program's "valid control flow", and prevent deviations from valid control flow at run time. There are lots of CFI schemes, employing combinations of static and dynamic techniques. Some of them don't even call themselves CFI, but I don't have a better term for the general definition I'm using here. Phrased in this general way, it includes control-transfer instrumentation (CCFIR etc), pointer obfuscation, shadow stacks, and even DEP and ASLR.

Vendors of C/C++ software need to consider whether to deploy CFI (and if so, which scheme). It's a cost/benefit analysis. The possible benefit is that many bugs may become significantly more difficult --- or even impossible --- to exploit. The costs are complexity and run-time overhead.

A key question when evaluating the benefit is, how difficult will it be for CFI-aware attackers to craft exploits that bypass CFI? That has two sub-questions: how often is it possible to weaponize a memory-safety bug that's exploited via control-flow hijacking today, with an exploit that is permitted by the CFI scheme? And, crucially, will it be possible to package such exploitation techniques so that weaponizing common C/C++ bugs into CFI-proof exploits becomes cheap? A very interesting paper at Oakland this year, and related work by other authors, suggests that the answer to the first sub-question is "very often" and the answer to the second sub-question is "don't bet against it".

Coincidentally, Intel has just unveiled a proposal to add some CFI features to their CPUs. It's a combination of shadow stacks with dynamic checking that the targets of indirect jumps/calls are explicitly marked as valid indirect destinations. Unlike some more precise CFI schemes, you only get one-bit target identification; a given program point is a valid destination for all indirect transfers or none.

So will CFI be worth deploying? It's hard to say. If you're offered a turnkey solution that "just works" with negligible cost, there may be no reason not to use it. However, complexity has a cost, and we've seen that sometimes complex security measures can even backfire. The tail end of Intel's document is rather terrifying; it tries to enumerate the interactions of their CFI feature with all the various execution modes that Intel currently supports, and leaves me with the impression that they're generally heading over the complexity event horizon.

Personally I'm skeptical that CFI will retain value over the long term. The Oakland DOP paper is compelling, and I think we generally have lots of evidence that once an attacker has a memory safety bug to work on, betting against the attacker's ingenuity is a loser's game. In an arms race between dynamic CFI (and its logical extension to dynamic data-flow integrity) and attackers, attackers will probably win, not least because every time you raise the CFI bar you'll pay with increased complexity and overhead. I suggest that if you do deploy CFI, you should do so in a way that lets you pull it out if the cost-benefit equation changes. Baking it into the CPU does not have that property...

One solution, of course, is to reduce the usage of C/C++ by writing code in a language whose static invariants are strong enough to give you CFI, and much stronger forms of integrity, "for free". Thanks to Rust, the old objections that memory-safe languages were slow, tied to run-time support and cost you control over resources don't apply anymore. Let's do it.

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