Re: [PATCH 00/10] Control Flow Enforcement - Part (3)

[Yu-cheng Yu]

On Tue, 2018-06-12 at 09:00 -0700, Andy Lutomirski wrote:
> On Tue, Jun 12, 2018 at 8:06 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> >
> > On Tue, 2018-06-12 at 20:56 +1000, Balbir Singh wrote:
> > >
> > > On 08/06/18 00:37, Yu-cheng Yu wrote:
> > > > This series introduces CET - Shadow stack
> > > >
> > > > At the high level, shadow stack is:
> > > >
> > > >     Allocated from a task's address space with vm_flags VM_SHSTK;
> > > >     Its PTEs must be read-only and dirty;
> > > >     Fixed sized, but the default size can be changed by sys admin.
> > > >
> > > > For a forked child, the shadow stack is duplicated when the next
> > > > shadow stack access takes place.
> > > >
> > > > For a pthread child, a new shadow stack is allocated.
> > > >
> > > > The signal handler uses the same shadow stack as the main program.
> > > >
> > >
> > > Even with sigaltstack()?
> > >
> > >
> > > Balbir Singh.
> >
> > Yes.
> >
> 
> I think we're going to need some provision to add an alternate signal
> stack to handle the case where the shadow stack overflows.

The shadow stack stores only return addresses; its consumption will not
exceed a percentage of (program stack size + sigaltstack size) before
those overflow.  When that happens, there is usually very little we can
do.  So we set a default shadow stack size that supports certain nested
calls and allow sys admin to adjust it.