Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

[Ingo Molnar <mingo@kernel.org>]

* Kees Cook <keescook@chromium.org> wrote:

> On Sat, May 11, 2019 at 03:45:19PM -0700, Andy Lutomirski wrote:
> > ISTM maybe a better first step would be to make get_random_bytes() be
> > much faster? :)
> 
> I'm not opposed to that, but I want to make sure we don't break it for
> "real" crypto uses...

I'm quite sure Andy implied that.

> I still think just using something very simply like rdtsc would be good 
> enough.
>
> This isn't meant to be a perfect defense: it's meant to disrupt the 
> ability to trivially predict (usually another thread's) stack offset. 

But aren't most local kernel exploit attacks against the current task? 
Are there any statistics about this?

> And any sufficiently well-positioned local attacker can defeat this no 
> matter what the entropy source, given how small the number of bits 
> actually ends up being, assuming they can just keep launching whatever 
> they're trying to attack. (They can just hold still and try the same 
> offset until the randomness aligns: but that comes back to us also 
> needing a brute-force exec deterance, which is a separate subject...)
> 
> The entropy source bikeshedding doesn't seem helpful given how few bits 
> we're dealing with.

The low number of bits is still useful in terms of increasing the 
probability of crashing the system if the attacker cannot guess the stack 
offset.

With 5 bits there's a ~96.9% chance of crashing the system in an attempt, 
the exploit cannot be used for a range of attacks, including spear 
attacks and fast-spreading worms, right? A crashed and inaccessible 
system also increases the odds of leaving around unfinished attack code 
and leaking a zero-day attack.

Thanks,

	Ingo