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

[Kees Cook]

On Wed, Apr 17, 2019 at 10:17 AM Theodore Ts'o <tytso@xxxxxxx> wrote:
>
> On Wed, Apr 17, 2019 at 09:28:35AM +0000, David Laight wrote:
> >
> > If you can guarantee back to back requests on the PRNG then it is probably
> > possible to recalculate its state from 'bits of state'/5 calls.
> > Depend on the PRNG this might be computationally expensive.
> > For some PRNG it will be absolutely trivial.
> > ...
> > Stirring in a little bit of entropy doesn't help much either.
> > The entropy bits are effectively initial state bits.
> > Add 4 in with each request and 128 outputs gives 640 linear
> > equations in the (128 + 4 * 128) unknowns - still solvable.
>
> This is basically a scenario where the attacker has already taken
> control of Ring 3 execution and the question is how hard is it for
> them to perform privilege escalation attack to ring 0, right?  I'm
> sure the security folks will think I'm defeatist, but my personal rule
> of thumb is if the attacker has ring 3 control, you've already lost
> --- I figure there are so many zero days that getting ring 0 control
> is a foregone conclusion.  :-(

I think this attitude comes from Linux traditionally having had such a
weak line between ring 3 and ring 0. That's what we're trying to fix,
generally speaking. :)

> So that basically means if we want to protect against this, we're
> going to do something which involves Real Crypto (tm).  Whether that's
> RDRAND, or using Chacha20, etc., or something that has some attack
> resistance, such as "half MD5", etc., but emminently crackable by
> brute force, is essentially a overhead vs. security argument, and what
> it is we are willing to pay.

I wonder how a separate per-cpu state combined with frequent reseeding
would compare to chacha20 (or RDRAND)?

Another point to consider is that this weakness depends on a separate
bug existing, which is becoming less and less likely, given the
always-init options now available. I don't think we should try to
over-engineer this too much. Best-effort here seems fine. Using a
stack leak when the stack is randomized may also prove difficult, so
there's some chicken-and-egg problems with the proposed threat...

-- 
Kees Cook