[tptacek]

It's not that they're better sandboxed; it's that regardless of whether memory corruption exploits work, in the NaCL security model, if the sandbox doesn't work, you're fucked regardless. So why bother with hardened runtimes?

[haberman]

Your threat model seems to be that anyone who sends input to a sandboxed app should be assumed to have full control of it. But the apps frequently process input without wanting to give the sender of that input access to all its resources.

By your analysis, Gmail could implement a feature that lets a sender run arbitrary JavaScript in the recipient's browser, and this would have no security impact as long as the JavaScript sandbox was not escaped. But in reality this would be a huge breach, because there are valuable things inside the sandbox that attackers should not have access to.

Put another way, this wouldn't help defend Chrome from NaCl, but it would help defend the NaCl app from it's clients. This would be in Google's interest to implement because it would make the platform more attractive to developers.

[tptacek]

That seems like an uncommon scenario for the kinds of things NACL gets used for. These are intrinsically server-controlled applications, when they're applications at all (my real sense of what NACL is about is "another tool to migrate away from things like Flash and plugins; ie: a platform for video codecs).

I see your point. I guess you're saying, there could be a photo editing app in which Alice can send pictures to Bob, and Mallory might send a malicious picture to Alice that coerces her client into betraying all its photos.

[willvarfar]

The buffer overflow in NaCL extension that comes with Chrome e.g. Flash may be just the a stepping stone in a Pwnium entry that escapes the sandbox through yet another webGL validation bug or something.

Why would Google want to not bother applying a belt-and-braces exploit mitigation that costs 0% CPU?

[tptacek]

You're not following my point. NaCL programs are content-controlled. Attackers start out with arbitrary code execution.

[willvarfar]

One NaCL use case is a plugin downloaded by the page

But NaCL is also used to isolate "built in" embeddables e.g. Flash, which I have used as an example of a NaCL plugin that comes with chrome in both the previous posts?

Another example is the voice activation plugin that got them into so much trouble recently...

Imagine you walk past your colleagues computers saying things like "let's google something naughty" loudly...

And now let's extend that to playing an audio snippet that invokes a stack smash? :)

[tptacek]

I'm still not following how hardening Nacl against memory corruption bugs can help Chrome, given that Nacl-enabled Chrome already runs content-controlled Nacl code.

[willvarfar]

Are the pdf viewer or flash players 'content-controlled'?

Are not google-bundled plugins allowed to run even when the user doesn't allow third-party NaCL plugins to run?

Its belt and braces. Why wouldn't chrome bother to enable a pass that costs 0% runtime performance? After all the money and time Google have sunk into other runtime checks like ASan, why wouldn't they also enable this?

It made me think: what other 'exploit mitigations' do Google put into NaCL, even though its sandboxed? So I quickly gooogled to see if they use ASLR inside NaCL, for example. Here's what I found:

https://code.google.com/p/nativeclient/issues/detail?id=2962

So they added ASLR, and they made a nice point:

"for the threat model where a NaCl module might process untrusted input, it would be nice to provide ASLR for the NaCl module so untrusted input won't (easily) be able to take control of the NaCl module (if/when the NaCl module has bugs). (this is a different threat model that the usual one in NaCl, where the NaCl module is untrusted. here we are trying to make sure that a NaCl module, which is executing code on the behalf of some domain/web site, isn't easily pwned, even if the NaCl runtime is itself okay.)"