Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

[Ingo Molnar]

* Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> > And yes, I realize that there were other such bugs and that such bugs might 
> > occur in the future - but why not push the overhead of the security check to 
> > the kernel build phase? I.e. I'm wondering how well we could do static 
> > analysis during kernel build - would a limited mode of Sparse be good enough 
> > for that? Or we could add a new static checker to tools/, built from first 
> > principles and used primarily for extended syntactical checking.
> 
> Static analysis is just not going to cover all cases. We've had vulnerabilities 
> where interrupt handlers left KERNEL_DS set, for example. [...]

Got any commit ID of that bug - was it because a function executed by the 
interrupt handler leaked KERNEL_DS?

> [...] If there are performance concerns, let's put this behind a CONFIG. 2-5 
> instructions is not an issue for most people that want this coverage.

That doesn't really _solve_ the performance concerns, it just forces most people 
to enable it by creating a 'security or performance' false dichotomy ...

> [...] and it still won't catch everything. Bug-finding is different from making 
> a bug class just unexploitable at all. As we've done before, it's the difference 
> between trying to find format string attacks vs just removing %n from the format 
> parser.

No, it does not make it unexploitable, it could still be exploitable if the 
runtime check is buggy or if there's kernel execution outside of the regular 
system call paths - there's plenty of such hardware functionality on x86 for 
example.

Thanks,

	Ingo