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

[Yu-cheng Yu]

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.

> Also, I have a couple of other questions after reading the
> documentation some more:
> 
> 1. Why on Earth does INCSSP only take an 8-bit number of frames to
> skip?  It seems to me that code that calls setjmp() and then calls
> longjmp() while nested more than 256 function call levels will crash.

GLIBC takes care of more than 256 functions calls.

> 2. The mnemonic RSTORSSP makes no sense to me.  RSTORSSP is a stack
> *switch* operation not a stack *restore* operation, unless I'm
> seriously misunderstanding.

The intention is to switch shadow stacks with tokens.  RSTORSSP restores
to a previous shadow stack address from a restore token.

> 3. Is there anything resembling clear documentation of the format of
> the shadow stack?  That is, what types of values might be found on the
> shadow stack and what do they all mean?

Only return addresses and restore tokens can be on a user-mode shadow
stack.  The restore token has the incoming shadow stack address plus one
bit indicating 64/32-bit mode.

I will put this into Documentation/x86/intel_cet.txt.

> 
> 4. Usually Intel doesn't submit upstream Linux patches for ISA
> extensions until the ISA is documented for real.  CET does not appear
> to be documented for real.  Could Intel kindly release something that
> at least claims to be authoritative documentation?
> 
> --Andy