Re: [PATCH v12 01/12] add support for Clang's Shadow Call Stack (SCS)

Kees Cook
2020-04-23T18:09:33Z

On Wed, Apr 22, 2020 at 07:00:40PM +0100, Will Deacon wrote:
> On Wed, Apr 22, 2020 at 10:54:45AM -0700, Kees Cook wrote:
> > On Mon, Apr 20, 2020 at 07:14:42PM -0700, Sami Tolvanen wrote:
> > > +void scs_release(struct task_struct *tsk)
> > > +{
> > > + void *s;
> > > +
> > > + s = __scs_base(tsk);
> > > + if (!s)
> > > + return;
> > > +
> > > + WARN_ON(scs_corrupted(tsk));
> > > +
> >
> > I'd like to have task_set_scs(tsk, NULL) retained here, to avoid need to
> > depend on the released task memory getting scrubbed at a later time.
>
> Hmm, doesn't it get zeroed almost immediately by kmem_cache_free() if
> INIT_ON_FREE_DEFAULT_ON is set? That seems much better than special-casing
> SCS, as there's a tonne of other useful stuff kicking around in the
> task_struct and treating this specially feels odd to me.

That's going to be an uncommon config except for the most paranoid of
system builders. :) Having this get wiped particular thing wiped is just
a decent best practice for what is otherwise treated as a "secret", just
like crypto routines wipe their secrets before free().

--
Kees Cook