Re: [PATCH v11 01/12] add support for Clang's Shadow Call Stack (SCS)

[Sami Tolvanen]

On Wed, Apr 22, 2020 at 06:39:47PM +0100, Will Deacon wrote:
> On Mon, Apr 20, 2020 at 02:18:30PM -0700, Sami Tolvanen wrote:
> > On Mon, Apr 20, 2020 at 06:17:28PM +0100, Will Deacon wrote:
> > > > +	 * The shadow call stack is aligned to SCS_SIZE, and grows
> > > > +	 * upwards, so we can mask out the low bits to extract the base
> > > > +	 * when the task is not running.
> > > > +	 */
> > > > +	return (void *)((unsigned long)task_scs(tsk) & ~(SCS_SIZE - 1));
> > > 
> > > Could we avoid forcing this alignment it we stored the SCS pointer as a
> > > (base,offset) pair instead? That might be friendlier on the allocations
> > > later on.
> > 
> > The idea is to avoid storing the current task's shadow stack address in
> > memory, which is why I would rather not store the base address either.
> 
> What I mean is that, instead of storing the current shadow stack pointer,
> we instead store a base and an offset. We can still clear the base, as you
> do with the pointer today, and I don't see that the offset is useful to
> an attacker on its own.

I see what you mean. However, even if we store the base address +
the offset, we still need aligned allocation if we want to clear
the address. This would basically just move __scs_base() logic to
cpu_switch_to() / scs_save().

> But more generally, is it really worthwhile to do this clearing at all? Can
> you (or Kees?) provide some justification for it, please? We don't do it
> for anything else, e.g. the pointer authentication keys, so something
> feels amiss here.

Like Kees pointed out, this makes it slightly harder to locate the
current task's shadow stack pointer. I realize there are other useful
targets in thread_info, but we would rather not make this any easier
than necessary. Is your primary concern here the cost of doing this,
or just that it doesn't sufficiently improve security?

Sami