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

[Yu-cheng Yu]

On Tue, 2018-06-12 at 10:24 -0700, Yu-cheng Yu wrote:
> On Tue, 2018-06-12 at 09:31 -0700, Andy Lutomirski wrote:
> > On Tue, Jun 12, 2018 at 9:24 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> > >
> > > 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.
> > >
> > 
> > Of course there's something you can do: add a sigaltstack-like stack
> > switching mechanism.  Have a reserve shadow stack and, when a signal
> > is delivered (possibly guarded by other conditions like "did the
> > shadow stack overflow"), switch to a new shadow stack and maybe write
> > a special token to the new shadow stack that says "signal delivery
> > jumped here and will restore to the previous shadow stack and
> > such-and-such address on return".
> 
> If (shstk size == (stack size + sigaltstack size)), then shstk will not
> overflow before program stack overflows and sigaltstack also overflows.
> 
> Let me think about this.

The reserve shadow stack will help only when the shstk overflows but
signal stack/sigaltstack still has room and we can deliver a signal.  If
the shstk is large enough to cover any nested calls that will overflow
both the program stack and sigaltstack then we don't need a reserve
shstk.

We can estimate how big the shstk needs to be; in the worst case it
should not be greater than (program stack size + sigaltstack size).  The
default shstk size we choose pass all signal tests in GLIBC.  In case
there is a need to increase it for a very large RLIMIT_STACK or very
large sigaltstack, the sys admin can increase the default shstk size.

Yu-cheng