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

Ingo Molnar
2017-05-08T14:02:30Z

* Kees Cook <keescook@chromium.org> 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