Re: [PATCH v13 00/12] add support for Clang's Shadow Call Stack

[Sami Tolvanen]

On Mon, Apr 27, 2020 at 10:50:34PM +0200, Ard Biesheuvel wrote:
> > OK, so one thing that came up in an offline discussion about SCS is
> > the way it interacts with the vmap'ed stack.
> >
> > The vmap'ed stack is great for robustness, but it only works if things
> > don't explode for other reasons in the mean time. This means the
> > ordinary-to-shadow-call-stack size ratio should be chosen such that it
> > is *really* unlikely you could ever overflow the shadow call stack and
> > corrupt another task's call stack before hitting the vmap stack's
> > guard region.
> >
> > Alternatively, I wonder if there is a way we could let the SCS and
> > ordinary stack share the [bottom of] the vmap'ed region. That would
> > give rather nasty results if the ordinary stack overflows into the
> > SCS, but for cases where we really recurse out of control, we could
> > catch this occurrence on either stack, whichever one occurs first. And
> > the nastiness -when it does occur- will not corrupt any state beyond
> > the stack of the current task.
> 
> Hmm, I guess that would make it quite hard to keep the SCS address
> secret though :-(

Yes, and the stack potentially overflowing into the SCS sort of defeats
the purpose. I'm fine with increasing the SCS size to something safer,
but using a vmapped shadow stack seems like the correct solution to this
problem, at least on devices where allocating a full page isn't an issue.

Sami